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Testing for Bypassing Authentication Schema (OTG-AUTHN-004)

Revision as of 16:13, 11 November 2006 by Gfedon (talk | contribs) (References)

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OWASP Testing Guide v2 Table of Contents

Brief Summary

While most most application require authentication for gaining access to private information or tasks, not every authentication method is able to provide adequate security level.

Negligence, ignorance or simple understatement of the security threats often result in authentication schemes that can be easily bypassed by simply skipping the login page and directly calling an internal page that is supposed to be accessed only after authentication has been performed.

In addition to this it is often possible to bypass compulsory authentication tampering with requests and tricking the application into thinking that we're already authenticated either by modifying the given URL parameter or by manipulating form or by counterfeiting sessions.

Description of the Issue

Problems related to Authentication Schema could be found at different stages of software development life cycle (SDLC), like design, development and deployment phase.

Examples of design errors include a wrong definition of application parts to be protected, the choice of not applying strong encryption protocols for securing authentication data exchange, and many more.

Problems in development phase are for example the incorrect implementation of input validation functionalities, or not following the security best practices for the specific language.

In addition there are issues during application setup (Installation and configuration activities) due to a lack in required technical skills, or due to poor documentation available.

Black Box testing and example

There are several methods to bypass the authentication schema in use by a web application:

  • Direct page request
  • Parameter Modification
  • Session ID Prediction
  • Sql Injection

Direct page request

Several web applications implement access control only inside the login page, otherwise if a user requests directly a different page in the designed protected area, the authentication schema could be bypassed.


Parameter Modification

Another problem related to authentication design is to let the application verify a succesful login upon fixed value parameters. Therefore a user could modify these parameters to gain access to the protected areas without providing valid credentials. 
[email protected] /home $nc 80                    
GET /page.asp?authenticated=yes HTTP/1.0                    
HTTP/1.1 200 OK                                             
Date: Sat, 11 Nov 2006 10:22:44 GMT                         
Server: Apache                                              
Connection: close                                           
Content-Type: text/html; charset=iso-8859-1                 
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">          
<H1>You Are Auhtenticated</H1>                              


Session ID Prediction

Many web applications manage authentication using session identification values(SESSION ID). Therefore if Session ID generation is predictable a malicious user could be able to find a valid session ID and gain unauthorized access to the application, impersonating a previously authenticated user.

In the following figure values inside cookies increase linearly, so could be easy for an attacker to guess a valid session ID.


In the following figure values inside cookies change only partially, so it's possible to restrict a bruteforce attack to the defined fields shown below.


Sql Injection (HTML Form Authentication)

SQL Injection is a widely known range of techniques. We are not going to describe this technique into detail, because it's possible to find many dedicated paragraphs in Owasp Testing Guide for either common and advanced readers.


The following figure shows that with simple sql injection it's possible to bypass the authentication form. The Example is obfuscated because it's part of OWASP WebGoat suite, and to let the reader experiment by himself without knowing the solution.


Gray Box testing and example

In the case an attacker has been able to retrieve the application source code by exploiting a previosly discovered vulnerability (e.g. directory traversal), or from a web repository (Open Source Applications), could be possible to perform refined attacks against the implementation of the authentication process.

In the following example (PHPBB 2.0.13 - Authentication Bypass Vulnerability), at line 5 unserialize() function parse user supplied cookie and set values inside $row array. At line 10 user md5 password hash stored inside the backend database is compared to the one supplied.

1.  if ( isset($HTTP_COOKIE_VARS[$cookiename . '_sid']) ||
2.  {
3.  $sessiondata = isset( $HTTP_COOKIE_VARS[$cookiename . '_data'] ) ?
5.  unserialize(stripslashes($HTTP_COOKIE_VARS[$cookiename . '_data'])) : array();
7.  $sessionmethod = SESSION_METHOD_COOKIE;
8.  }
10. if( md5($password) == $row['user_password'] && $row['user_active'] )
12. {
13. $autologin = ( isset($HTTP_POST_VARS['autologin']) ) ? TRUE : 0;
14. }

In PHP a comparison between a string value and a boolean value (1 - "TRUE") is always "TRUE", so supplying the following string (important part is "b:1") to the userialize() function is possible to bypass the authentication control:



David Endler - Session ID Brute Force Exploitation and Prediction -


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