ASP.Net Insufficient Session Timeout
A common security concern found in ASP.Net applications is Insufficient Session Timeout. In this article, the focus is not on the ASP.Net session that is not effectively terminated, but rather the forms authentication cookie that is still valid after logout.
How to Test
- User is currently logged into the application.
- User captures the ASPAuth cookie (name may be different in different applications).
- Cookie can be captured using a browser plugin or a proxy used for request interception.
- User saves the captured cookie for later use.
- User logs out of the application.
- User requests a page on the application, passing the previously captured authentication cookie.
- The page is processed and access is granted.
Typical Logout Options
- The application calls FormsAuthentication.Signout()
- The application sets the Cookie.Expires property to a previous DateTime.
Cookie Still Works!!
Following the user process above, the cookie still provides access to the application as if the logout never occurred. So what is the deal? The key is that unlike a true “session” which is maintained on the server, the forms authentication cookie is self contained. It does not have a server side component to stay in sync with. Among other things, the authentication cookie has your username or ID, possibly roles, and an expiration date. When the cookie is received by the server it will be decrypted (please tell me you are using protection = all) and the data extracted. If the cookie’s internal expiration date has not passed, the cookie is accepted and processed as a valid cookie.
So what did FormsAuthentation.Signout() do?
If you look under the hood of the .Net framework, it has been a few years but I doubt much has changed, you will see that FormsAuthentication.Signout() really just removes the cookie from the browser. There is no code to perform any server function, it merely asks the browser to remove it by clearing the value and back-dating the expires property. While this does work to remove the cookie from the browser, it doesn’t have any effect on a copy of the original cookie you may have captured. The only sure way to really make the cookie inactive (before the internal timeout occurs) would be to change your machine key in the web.config file. This is not a reasonable solution.
Possible Mitigations
You should be protecting your cookie by setting the httpOnly and Secure properties. HttpOnly tells the browser not to allow javascript to have access to the cookie value. This is an important step to protect the cookie from theft via cross-site scripting. The secure flag tells the browser to only send the authentication cookie over HTTPS, making it much more difficult for an attacker to intercept the cookie as it is sent to the server.
Set a short timeout (15 minutes) on the cookie to decrease the window an attacker has to obtain the cookie.
You could attempt to build a tracking system to manage the authentication cookie on the server to disable it before its time has expired. Maybe something for another post.
Understand how the application is used to determine how risky this issue may be. If the application is not used on shared/public systems and the cookie is protected as mentioned above, the attack surface is significantly decreased.
Final Thoughts
If you are facing this type of finding and it is a forms authentication cookie issue, not the Asp.Net session cookie, take the time to understand the risk. Make sure you understand the settings you have and the priority and sensitivity of the application to properly understand “your” risk level. Don’t rely on third party risk ratings to determine how serious the flaw is. In many situations, this may be a low priority, however in the right app, this could be a high priority.
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