Session Fixation is an attack that permits an attacker to hijack a valid user session. The attack explores a limitation in the way the web application manages the session ID, more specifically the vulnerable web application. When authenticating a user, it doesn’t assign a new session ID, making it possible to use an existent session ID. The attack consists of inducing a user to authenticate himself with a known session ID, and then hijacking the user-validated session by the knowledge of the used session ID. The attacker has to provide a legitimate Web application session ID and try to make the victim's browser use it.
The session fixation attack is a class of Session Hijacking, which steals the established session between the client and the Web Server after the user logs in. Instead, the Session Fixation attack fixes an established session on the victim's browser, so the attack starts before the user logs in.
There are several techniques to execute the attack; it depends on how the Web application deals with session tokens. Below are some of the most common techniques:
• Session token in the URL argument: The Session ID is sent to the victim in a hyperlink and the victim accesses the site through the malicious URL.
• Session token in a hidden form field: In this method, the victim must be tricked to authenticate in the target Web Server, using a login form developed for the attacker. The form could be hosted in the evil web server or directly in html formatted e-mail.
• Session ID in a cookie:
o Client-side script
Most browsers support the execution of client-side scripting. In this case, the aggressor could use attacks of code injection as the XSS (Cross-site scripting) attack to insert a malicious code in the hyperlink sent to the victim and fix a Session ID in its cookie. Using the function document.cookie, the browser which executes the command becomes capable of fixing values inside of the cookie that it will use to keep a session between the client and the Web Application.
o <META> tag
<META> tag also is considered a code injection attack, however, different from the XSS attack where undesirable scripts can be disabled, or the execution can be denied. The attack using this method becomes much more efficient because it's impossible to disable the processing of these tags in the browsers.
o HTTP header response
This method explores the server response to fix the Session ID in the victim's browser. Including the parameter Set-Cookie in the HTTP header response, the attacker is able to insert the value of Session ID in the cookie and sends it to the victim's browser.
The example below explains a simple form, the process of the attack, and the expected results.
(1)The attacker has to establish a legitimate connection with the web server which (2) issues a session ID or, the attacker can create a new session with the proposed session ID, then, (3) the attacker has to send a link with the established session ID to the victim, she has to click on the link sent from the attacker accessing the site, (4) the Web Server saw that session was already established and a new one need not to be created, (5) the victim provides his credentials to the Web Server, (6) knowing the session ID, the attacker can access the user's account.
Figure 1. Simple example of Session Fixation attack.
As well as client-side scripting, the code injection must be made in the URL that will be sent to the victim.
http://website.kon/<meta http-equiv=Set-Cookie content=”sessionid=abcd”>
HTTP header response
The insertion of the value of the SessionID into the cookie manipulating the server response can be made, intercepting the packages exchanged between the client and the Web Application inserting the Set-Cookie parameter.
Figure 2. Set-Cookie in the HTTP header response