Project: WASC Threat Classification

Threat Type: Attack

Reference ID: WASC-37


Session Fixation

Session Fixation is an attack technique that forces a user’s session ID to an explicit value. Depending on the functionality of the target web site, a number of techniques can be utilized to “fix” the session ID value. These techniques range from Cross-site Scripting exploits to peppering the web site with previously made HTTP requests. After a user’s session ID has been fixed, the attacker will wait for that user to login. Once the user does so, the attacker uses the predefined session ID value to assume the same online identity.


Generally speaking there are two types of session management systems when it comes to ID values. The first type is “permissive” systems that allow web browsers to specify any ID. The second type is “strict” systems that only accept server-side-generated values. With permissive systems, arbitrary session IDs are maintained without contact with the web site. Strict systems require the attacker to maintain the “trap-session”, with periodic web site contact, preventing inactivity timeouts.


Without active protection against Session Fixation, the attack can be mounted against any web site that uses sessions to identify authenticated users. Web sites using sessions IDs are normally cookie-based, but URLs and hidden form fields are used as well. Unfortunately, cookie-based sessions are the easiest to attack. Most of the currently identified attack methods are aimed toward the fixation of cookies.

In contrast to stealing a users’ session IDs after they have logged into a web site, Session Fixation provides a much wider window of opportunity. The active part of the attack takes place before a user logs in.