Background info and boring history shit: https://scott.arciszewski.me/research/view/php-framework-timing-attacks-object-injection Vulnerability: 1. Remote timing attack 2. PHP Object Injection 3. Possibly, as a result of 2, remote code execution Affects: - CodeIgniter (<= 2.1.4) - Kohana (<= 3.2.3, 3.3.2) Some PHP frameworks (CodeIgniter and Kohana, for sure), give you the option of storing $_SESSION data in a cookie, then appending a hash of the session data and an unknown key. For example: a:2:{s:1:"a";s:10:"douchelord";s:8:"is_admin";b:0;}5c2d88368e2c154398e77269c3af3edd Vulnerability 1 - Remote Timing Attack https://github.com/EllisLab/CodeIgniter/blob/master/system/libraries/Session.php#L159 https://github.com/kohana/core/blob/3.3/master/classes/Kohana/Cookie.php#L74 If you wanted to alter the above serialized blob to read: a:2:{s:1:"a";s:5:"admin";s:8:"is_admin";b:1;} ... in theory, you would need to either know the encryption key stored in the framework's configuration. And remotely brute-forcing an md5/sha1 hash isn't really attractive. Luckily, the behavior of standard string comparison operators (== and ===) is to return false as soon as two bytes do not match. Thus we could discover that... a:2:{s:1:"a";s:5:"admin";s:8:"is_admin";b:1;}ef000000000000000000000000000000 ...takes a little longer than... a:2:{s:1:"a";s:5:"admin";s:8:"is_admin";b:1;}00000000000000000000000000000000 ... and then ... a:2:{s:1:"a";s:5:"admin";s:8:"is_admin";b:1;}ef870000000000000000000000000000 ... will take slightly longer. Theoretically, if we need 100 samples per possible hexit to guess the correct value through this strategy, and there are 32 hexits in a hash, at 10 request per second it will take 51200 samples to crack an md5 hash. If you can guess 50 per second without bringing the target server to its knees, that's about 18 minutes (worst case) to get the end result: a:2:{s:1:"a";s:5:"admin";s:8:"is_admin";b:1;}ef87978c40e2c2871ba58ab55b50854e Vulnerability 2 - PHP Object Injection If you can convince PHP to pass arbitrary data to unserialize(), which is the automatic next step when the timing attack succeeds, then you can inject objects into the code. From what I understand, this can also lead to remote code execution. CodeIgniter already patched this in its 3.0/develop branch. Kohana accepted my pull request for its 3.3/develop branch: https://github.com/kohana/core/pull/492 Note: Kohana uses SHA1, not MD5, so add another 20% to my estimate.