Fingerprint Web Application Framework (OTG-INFO-009)

Summary
Web framework[*] fingerprinting is an important subtask of the information gathering process. Knowing the type of framework can automatically give a great advantage if such a framework has already been tested by the penetration tester. It is not only the known vulnerabilities in unpatched versions but specific misconfigurations in the framework and known file structure which makes the fingerprinting process so important.

Several different vendors and versions of web frameworks are widely used. Information about it significantly helps in the testing process, and can also help in changing the course of the test. Such information can be derived by careful analysis of certain common locations. Most of the web frameworks have several markers in those locations which help an attacker to spot them. This is basically what all automatic tools do, they look for a marker from a predefined location and then compare it to the database of known signatures. For better accuracy several markers are usually used.

[*] Please note that in this article we make no difference between Web Application Frameworks (WAF) and Content Management Systems (CMS). This has been done to make it convenient to fingerprint both of them in one chapter.Further we reference both of the categories as web frameworks.

Test Objectives
To define type of used web framework so to have a better understanding of the penetration testing methodology.

Black Box testing
There are several most common locations to look in in order to define the current framework:
 * HTTP headers
 * Cookies
 * HTML source code
 * Specific files and folders

Let's look closer at those approaches.

HTTP headers
The most basic form of identifying a web framework is to look at the X-Powered-By field in the HTTP response header. Many tools can be used to fingerprint a target. The simplest one is netcat utility. Consider the following HTTP Request-Response: $ nc 127.0.0.1 80 HEAD / HTTP/1.0

HTTP/1.1 200 OK Server: nginx/1.0.14 Date: Sat, 07 Sep 2013 08:19:15 GMT Content-Type: text/html;charset=ISO-8859-1 Connection: close Vary: Accept-Encoding X-Powered-By: Mono

From the X-Powered-By field, we understand that the web application framework is likely to be Mono.

However, although simplicity and quickness of such an approach, this methodology doesn't work in 100% of cases. It is possible to easily disable X-Powered-By header by a proper configuration. There are also several techniques that allow a web site to obfuscate HTTP headers (see an example in chapter) So in the same example we could either miss the X-Powered-By header or obtain an answer like the following: HTTP/1.1 200 OK Server: nginx/1.0.14 Date: Sat, 07 Sep 2013 08:19:15 GMT Content-Type: text/html;charset=ISO-8859-1 Connection: close Vary: Accept-Encoding X-Powered-By: Blood, sweat and tears

Sometimes there are more HTTP-headers which point at the certain web framework. In the following example according to the information from HTTP-request one can see that X-Powered-By header contains PHP version. However, X-Generator header points out the used framework is actually Swiftlet, which helps a penetration tester to expand his attack vectors. When performing fingerprinting, always carefully inspect every HTTP-header for such leaks. HTTP/1.1 200 OK Server: nginx/1.4.1 Date: Sat, 07 Sep 2013 09:22:52 GMT Content-Type: text/html Connection: keep-alive Vary: Accept-Encoding X-Powered-By: PHP/5.4.16-1~dotdeb.1 Expires: Thu, 19 Nov 1981 08:52:00 GMT Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0 Pragma: no-cache X-Generator: Swiftlet

Cookies
Another similar and somehow more reliable way to determine the current web framework are framework-specific cookies. Consider the following HTTP-request:



As we can see, cookie CAKEPHP has automatically been set, which gives us the information about the framework being used. List of common cookies names is presented in chapter. Limitations are the same - it is possible to change the name of the cookie. For example, for the selected CakePHP framework this could be done by the following configuration (excerpt from core.php):

/** Configure::write('Session.cookie', 'CAKEPHP');
 * The name of CakePHP's session cookie.
 * Note the guidelines for Session names states: "The session name references
 * the session id in cookies and URLs. It should contain only alphanumeric
 * characters."
 * @link http://php.net/session_name
 * @link http://php.net/session_name

However from the experience, these changes are less likely to be made than, for example, X-Powered-By header so this approach can be considered as more reliable one.

HTML source code
This technique is based on finding certain patterns in the HTML page source code. Often one can find much information which helps a tester to recognize a specific web framework. Ones of the common markers are HTML comments which directly lead to framework disclosure. More often certain framework-specific paths can be found, i.e. links to framework-specific css and/or js folders. Finally, specific script variables might also point to a certain framework. From the screenshot below one can easily learn the used framework and its version by the mentioned markers: comment, specific paths and script variables can all help an attacker to quickly determine an instance of ZK framework.



More frequently such information is placed between, in tags or at the end of the page. Nevertheless, it is recommended to check the whole document since it can be useful for other purposes such as inspection of other useful comments and hidden fields. Sometimes, however, web developers do not care much about hiding tracks of the used framework. It's still possible to stumble upon something like that at the bottom of the page:



Specific files and folders
Apart from information gathered from HTML sources, there is another approach which greatly helps an attacker to determine the framework with high accuracy. Every framework has its own specific file and folder structure on the server. We pointed out that one can see the specific path from the HTML page source but sometimes they are not explicitly presented there and still reside on the server. In order to uncover them a technique known as dirbusting is used. Dirbusting is brute forcing target with predictable folder and file names and monitoring HTTP-responses thus enumerating server contents. This information can be used both for finding default files and attacking them in next stages and for fingerprinting the web framework. Dirbusting can be done in several ways, example below shows successful dirbusting attack against a WordPress-powered target with the help of defined list and Intruder functionality of Burp Suite.



We can see that for some WordPress-specific folders (for instance, /wp-includes/, /wp-admin/ and /wp-content/) HTTP-reponses are 403 (Forbidden), 302 (Found, redirection to wp-login.php) and 200 (OK) respectively. This is a good indicator that the target is WordPress-powered. The same way it is possible to dirbust different framework plugin folders and their versions. On the screenshot below one can see a typical CHANGELOG file of a Drupal plugin, which provides information on the framework being used and discloses vulnerable plugin version.



Tip: before starting dirbusting (which is a useful operation anyway), it is recommended to check robots.txt file first. Sometimes framework specific folders and other sensitive information can be found there as well! Example of such robots file is presented on a screenshot below.



Gray Box testing
Please address yourself to watching Monty Python's Flying Circus, I'm sure you'll like it!

Specific files and folders
These are different for each specific framework. It is recommended to install the corresponding framework during penetration tests in order to have better understanding of what infrastructure is presented and which files might be left on the server. However, several good pre-made lists already exist, one good example of them - FuzzDB wordlists of predictable files/folders (http://code.google.com/p/fuzzdb/)

Tools
A list of general and well-known tools is presented below. There are also a lot of other utilities, as well as framework-based fingerprinting tools.

WhatWeb
Website: http://www.morningstarsecurity.com/research/whatweb Currently one of the best fingerprinting tools on the market. Included in a default Kali Linux build. Language: Ruby Matches for fingerprinting are made with:
 * Text strings (case sensitive)
 * Regular expressions
 * Google Hack Database queries (limited set of keywords)
 * MD5 hashes
 * URL recognition
 * HTML tag patterns
 * Custom ruby code for passive and aggressive operations

Sample output is presented on a screenshot below:



BlindElephant
Website: https://community.qualys.com/community/blindelephant This great tool works on the principle of static file checksum based version difference thus providing a very high quality of fingerprinting. Language: Python

Sample output of a successful fingerprint: pentester$ python BlindElephant.py http://my_target drupal Loaded /Library/Python/2.7/site-packages/blindelephant/dbs/drupal.pkl with 145 versions, 478 differentiating paths, and 434 version groups. Starting BlindElephant fingerprint for version of drupal at http://my_target

Hit http://my_target/CHANGELOG.txt File produced no match. Error: Retrieved file doesn't match known fingerprint. 527b085a3717bd691d47713dff74acf4

Hit http://my_target/INSTALL.txt File produced no match. Error: Retrieved file doesn't match known fingerprint. 14dfc133e4101be6f0ef5c64566da4a4

Hit http://my_target/misc/drupal.js Possible versions based on result: 7.12, 7.13, 7.14

Hit http://my_target/MAINTAINERS.txt File produced no match. Error: Retrieved file doesn't match known fingerprint. 36b740941a19912f3fdbfcca7caa08ca

Hit http://my_target/themes/garland/style.css Possible versions based on result: 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 7.10, 7.11, 7.12, 7.13, 7.14

...

Fingerprinting resulted in: 7.14

Best Guess: 7.14

Wappalyzer
Website: http://wappalyzer.com Wapplyzer is a Firefox/Chrome plug-in. It works only on regular expression matching and doesn't need anything other than the page to be loaded on browser. It works completely at the browser level and gives results in the form of icons. Although sometimes it has false positives, this is very handy to have notion of what technologies were used to construct a target website immediately after browsing a page. Sample output of a plug-in is presented on a screenshot below.



Vulnerability References
Whitepapers
 * Saumil Shah: "An Introduction to HTTP fingerprinting" - http://www.net-square.com/httprint_paper.html
 * Anant Shrivastava : "Web Application Finger Printing" - http://anantshri.info/articles/web_app_finger_printing.html

Remediation
General advice: use several tools described above and check scan logs in order to better understand what exactly helps an attacker to disclose your web framework. By performing multiple scans after changes you've done to hide framework tracks, it's possible to achieve better security level and to make sure of undetectability by automatic scans. Below you may find specific recommendations by framework marker location and some additional interesting approaches.

HTTP headers
Check your configuration and disable/obfuscate all HTTP-headers which disclose information about used technologies. An interesting article about HTTP-headers obfuscation using Netscaler: http://grahamhosking.blogspot.ru/2013/07/obfuscating-http-header-using-netscaler.html

Cookies
It is recommended to change cookie names by making proper changes in corresponding config files.

HTML source code
Check manually the contents of your HTML code and remove everything what explicitly points to the framework. General guidelines:
 * Make sure you didn't leave any visual markers disclosing used framework
 * Remove any unnecessary comments (copyrights, bug information, specific framework comments)
 * Remove META and generator tags
 * Use your own css/js and do not store those in a framework-specific folders
 * Make sure you're not using default scripts on the page; if using them is needed, obfuscate them.

Specific files and folders
RewriteCond %{REQUEST_URI} /wp-login\.php$ [OR] RewriteCond %{REQUEST_URI} /wp-admin/$ RewriteRule $ /http://your_website [R=404,L] However, these are not the only ways to restrict access. In order to automate this process, certain framework-specific plugins exist. One example for WordPress is StealthLogin (http://wordpress.org/plugins/stealth-login-page)
 * Remove any unnecessary/unused files on the server. This implies text files disclosing information about versions and installation too.
 * Restrict access to other files in order to achieve 404-response when accessing them from outside. This can be done, for example, by modifying htaccess file and adding RewriteCond / RewriteRule there. Example of such restriction for two common WordPress folders is presented below.

Additional approaches

 * Checksum management
 * Purpose of this approach is to beat checksum-based scanners and don't let them to disclose files by their hashes. Generally, there are two approaches in checksum management:
 * Change the location of where those files are placed (i.e. move them to another folder, or rename existing)
 * Modify their contents - even slight modification results in a completely different hash sum, so adding a single byte in the end of the file should not be a big problem.
 * Controlled chaos
 * A funny and effective method which essense is adding bogus files and folders from other frameworks in order to fool scanners and confuse an attacker. But be careful not to overwrite exisiting files and folders and to break the current framework!