Security for Websites - Breaking Sessions to Hack Into a Machine
by Colm Murphy - Technical Director, Espion - Tuesday, 20 June 2006.
Another way to that is used to compromise a Session ID is to attempt to predict it. Prediction occurs when an attacker realises that a pattern exists between session IDs. For example, some web based systems increment the session ID each time a user logs on. Knowing one session ID allows malicious users to identify the previous and next ones. Others use a brute force attack. This is a simple yet potentially effective method for determining a session identifier. A brute force attack occurs when a malicious user repeatedly tries numerous session identifiers until they happen upon a valid one. Although it is not complicated, it can be highly effective.

So what can you do to mitigate these attacks?

1. Always use strong encryption during transmission. Failure to encrypt the session identifier could render the online system insecure. In addition, for cookie based sessions, set the SSL-only attribute to "true" for a little added security. This will reduce the chance that an XSS attack could capture the session ID because the pages on the unencrypted section of the site will not be able to read the cookie.

2. Expire sessions quickly. Force the user to log out after a short period of inactivity. This way, an abandoned session will only be live for a short duration and thus will reduce the chance that an attacker could happen upon an active session. It is also wise to avoid persistent logins. Persistent logins typically leave a session identifier (or worse, login and password information) in a cookie that resides in the user's cache. This substantially increases the opportunity that an attacker has to get a valid SID.

3. Never make the Session ID viewable. This is a major problem with the GET method. GET variables are always present in the path string of the browser. Use the POST or cookie method instead or cycle the SID out with a new one frequently.

4. Always select a strong session identifier. Many attacks occur because the SID is too short or easily predicted. The identifier should be pseudo-random, retrieved from a seeded random number generator. For example, using a 32 character session identifier that contains the letters A-Z, a-z and 0-9 would have 2.27e57 possible IDs. This is equivalent to a 190 bit password. For example, using a 32 character session identifier that contains the letters A-Z, a-z and 0-9 is equivalent to a 190 bit password and is sufficiently strong for most web applications in use today.

5. Always double check critical operations. The server should re-authenticate anytime the user attempts to perform a critical operation. For example, if a user wishes to change their password, they should be forced to provide their original password first.

6. Always log out the user securely. Perform the logout operation such that the server state will inactivate the session as opposed to relying on the client to delete session information. Delete the session ID on logout. Some applications even force the browser to close down completely, thus ensuring stripping down the session and ensuring the deletion of the session ID.

7. Always prevent client-side page caching on pages that display sensitive information. Use HTTP to set the page expiration such that the page is not cached. Setting a page expiration that is in the past will cause the browser to discard the page contents from the cache.

8. Always require that users re-authenticate themselves after a specified period even if their session is still active. This will place an upper limit in the length of time that a successful session hijack can last. Otherwise, an attacker could keep a connection opened for an extremely long amount of time after a successful attack occurs.

9. It is possible to perform other kinds of sanity checking. For example, use web client string analysis, SSL client certificate checks and some level of IP address checking to provide basic assurance that clients are who they say they are.

All in all, web applications rely on good session management to stay secure. If you follow some of the steps outlined in this article and be aware of the risks, you are well on your way to leveraging the full benefits of web applications.

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