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Preventing Session Hijacking

What is Session Hijacking
Session Hijacking is an attack that when an attacker takes control of an already validated user.
When an application authenticates a user it assigns a random string called sessionID to the validated user and that sessionID is traversed every time there's an interaction between the logged in user and the application. Generally the sessionID remains valid till the user logs out. After that the particular sessionID becomes invalid. So sessionIDs are very important concept in session based application. SessionIDs itself can be captured by following ways:
  • Prediction
  • Capture
  • Fixation
If an adversary takes hold of the sessionID he can impersonate the real valid user and the application won't have any way to differentiate between the real user and the attacker. This is the general concept of Session Hijacking.

Prevention to Session Hijacking
A combination of following methods can prevent the problem:
  • Use of a long random number or string as the session key. This reduces the risk that an attacker could simply guess a valid session key through trial and error or brute force attacks to prevent Prediction.
  • The sensitive application (such as login page) must use secure communication (such as SSL) to avoid sniffing of session identifiers and user credentials to mitigate Capture.
  • Also the session identifiers can be captured if the application has the XSS (Cross Site Scripting) flaw. So the application must be not vulnerable to XSS to prevent Capture.
  • The application must expire the session identifiers after logout and regenerate new session identifiers after the user ‘authentication only’. This will mitigate the Fixation problem.
  • Register some other variables like some secret random no. like hash of user agent details of the browser can be also fruitful. As there is very rare chance that the user will move to another browser while performing an action while currently logged in to the application. Store that hash of the browser along with sessionID onto the server and match it with every incoming request of the user. If matches proceed , if not, there's high chance that somebody other has logged in currently. Invalidate the session.
  • Mark the cookies as 'HTTPOnly' so that they can't be accessed through XSS.
Although there's no single and sufficient methods to mitigate the problem, above methods and an aware user can save themselves from the attack.

Comments

Chintan Dave said…
Dude, apart from prediction, capture (i.e. hijacking) and fixation, there is one more arena that could be explored - you missed out brute forcing :P :P.

If it sounds irritating, you dont have any option, but to bear with this comment :D
July 15, 2009 at 12:55 PM
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