Redirecting Safely

I don't think this is news to the other security people, but as a developer, I had never heard of this issue. As far as I know, none of the developers I regularly work with had heard of it either (though using the code as recommended by the php.net manual will prevent it).

In many PHP applications, there is an admin side and a public side. It is common for developers to have an "admin check" in the code header of the admin side, which essentially evaluates whether a user is authorised to view a resource and either redirects them to the login page, the public page, or lets them through to the resource.

What many developers don't realize is that a browser doesn't have to terminate a connection as soon as it receives a 302 redirect. If you are relying on the following call to keep the riffraff out of your admin pages, you may be vulnerable:

header('Location: login.php');

I can't tell you how many times I've been able to access reports full of user logins, order information, or even account modification functionality, simply by asking my browser to continue loading a page after receiving a redirect. If you need a testing tool to see if you are vulnerable, I whipped up this perl script one day to allow me to simply snag admin pages or post content to them during a penetration test.

In case it isn't obvious, you can prevent this hole simply by halting the flow of the application. If you aren't using more advanced templating and flow control (which I'd recommend anyways), this will work:

header('Location: login.php');
die('Kindly piss off.');

While obvious, this issue is incredibly widespread, and I have to wonder how many developers are unaware of it.