In this recent article from ServerSide Magazine they look at a security issue that can be hard to detect if you're not sure what you're looking for - session hijacking - and how you can help to prevent it on your site.

A must have for the attacker in a session hijack is the Session Identifier so he can impersonate the attack. Let’s presume for example that you have your website hosted on a shared hosting on which PHP is installed as an Apache module, thing that makes session files belong to the web user, in other words: accessible.

He breaks it out into three potential kinds of session hijack methods - prediction, capture and fixation - with definitions for each. He also makes recommendations of some secure practices to follow to help prevent some of these issues (like not trusting users, using $_COOKIE and $_SESSION correctly and using a security token too along with the session ID).

In this new post to his blog Stefan Esser looks at cross-site request forgeries and how they can be prevented in PHP 5.3 by two things - the request_order directive in your php.ini and by not using $_REQUEST anymore.

Although PHP 5.3 is still in alpha stage and certain features like the PHAR extension or the whole namespace support are still topics of endless discussions it already contains smaller changes that could improve the security of PHP applications a lot. [...] With request_order it is now possible to control in what order $_REQUEST is created and what variable sources are taken into account. This finally allows removing cookie data from $_REQUEST without removing them from $_COOKIE also.

He explains why the use of $_REQUEST can lead to such problems (and security holes) and notes that its use makes overriding an application's GET or POST values as simple as adding a cookie. There's even a method for creating a Denial of Service attack against a site using $_REQUEST like this. He points to an example similar to this that happened with phpMyAdmin a while back.

His recommendation?

Once PHP 5.3 is out it is recommended for hosters to set request_order to "GP" on all the servers running arbitrary PHP applications to protect applications [and] application developers on the other hand should finally move away from using $_REQUEST for user input.