ASP.NET Session Cookie Crypto Attack Exploiting

If the Linux CVE-2010-3081: 64bit Linux Kernel Root Exploit didn’t get you, then this little birdy might. It seems the implementation of the AES encryption algorithm which protects the integrity of the Session Cookies in ASP.NET has a weakness which could enable an attacker to hijack sessions – Which bank? The idea behind the use of AES is to ensure that the crypt’d data hasn’t been tampered with – and hence decryptable, but unfortunately the flawed implementation of the use of AES and how it handles errors gives out some much needed clues for an attacker to pursue.

From TheThreatPost article:

In this case, ASP.NET’s implementation of AES has a bug in the way that it deals with errors when the encrypted data in a cookie has been modified. If the ciphertext has been changed, the vulnerable application will generate an error, which will give an attacker some information about the way that the application’s decryption process works. More errors means more data. And looking at enough of those errors can give the attacker enough data to make the number of bytes that he needs to guess to find the encryption key small enough that it’s actually possible.

There is a Microsoft Security Advisory (2416728) which gives some workarounds until a proper fix is made available. What’s really concerning is this little tidbitt from Thai Duong about Using their tool the Padding Oracle Exploit Tool or POET:

“It’s worth noting that the attack is 100 [per cent] reliable, [that is], one can be sure that once they run the attack, they can exploit the target. It’s just a matter of time. If the attacker is lucky, then he can own any ASP.NET website in seconds. The average time for the attack to complete is 30 minutes. The longest time it ever takes is less than 50 minutes.”

What’s really interesting is seeing the video of the exploit in action on dotnetnuke (don’t close your eyes). ScottGu has blogged about this exploit which goes into far more detail than I can, but if you’re keen there’s a nice document on using the Padding Oracle exploit and includes discussions regarding the JSF View state, cracking CAPTCHA schemes as well as some juicy details on CBC-R.