Local Session Poisoning in PHP Part 3: Bypassing Suhosin's Session Encryption

By default Suhosin transparently encrypts session files stored by PHP. This seems to be adequate protection against local session poisoning in a shared hosting environment. But let's take a closer look.

Local Session Poisoning in PHP Part 2: Promiscuous Session Files

FastCGI, suPHP and suExec can all ensure that a PHP script which is called from the web will execute under the user that owns it, as opposed to the user the web server is running as. This seemingly protects against session poisoning by ensuring that a malicious user no longer can open and manipulate session files owned by other users in a shared host.

The hidden pitfall is that while these protection mechanisms protect session files from unauthorized access, they can not prevent a user from authorizing others to access its session files. If all the session files are stored in a common folder it is trivial to trick a web application into loading session variables from a promiscuous session file.

Local Session Poisoning in PHP Part 1: The Basics of Exploitation and How to Secure a Server

Session poisoning is the act of manipulating sessions specific data in PHP. To add, change or remove variables stored in the super global $_SESSION array.

Local session poisoning is enabled by the fact that one web application can manipulate a variable in the $_SESSION array while another web application has no way of knowing how that variable's value came to be, and will interpret the variable according to its own logic. The $_SESSION array can then be manipulated to contain the values needed to spoof a logged in user or exploit a vulnerable function. PHP programmers put far more trust in $_SESSION variables than for example $_GET variables. The $_SESSION array is considered an internal variable, and an internal variable would never contain malicious input, would it?