Session Fixation

Session fixation is a type of security vulnerability in web applications. It occurs when an attacker is able to fixate (set) the session ID (SID) of another user and then hijack the user's session after the user logs in. This vulnerability can be exploited to gain unauthorized access to the system.

Web applications often use sessions to maintain user state and track identity across multiple requests. In a session fixation attack, the attacker exploits weaknesses in the session management system of the web application.

The attacker forces a victim's browser to use a specific session ID. This can be done in various ways, such as by persuading the victim to click on a link that contains a specific session ID as a parameter, or through other means like cross-site scripting. Once the victim logs in using the fixated session ID, the attacker, who knows this ID, can then use it to hijack the session, gaining access to the victim's account and privileges.

If the session ID is passed through the URL, it can be easily fixated by an attacker. If the application accepts session IDs from query parameters or form fields, it can be exploited. A critical vulnerability arises when the application doesn't regenerate a new session ID at login. This means the pre-login and post-login session is the same, which is what the attacker exploits.

Session fixation can lead to unauthorized access and control over a user's account, potentially leading to data theft, privacy breaches, and other malicious activities.