Introduction
When building a website, security is an important point which is, sadly, often forgotten. In this article, I'll talk about one security hole called XSS with magic_quotes_gpc. It consists, in website context, of injecting arbitrary html code into a trusted website. The attack is normally targeted towards the client-side. The exploit is used to steal client-side informations like cookies. PHP has tryed to protect novices users against this kind of injection holes by introducing a "magic" feature : magic_quotes_gpc. This feature will automatically escape single and double quotes in untrusted inherited data: GET, POST and COOKIE variables. Escaping quotes is a way to prevent injections, because the attaker can't leave a quoted string:
// get untrusted data (magic_quotes_gpc is on)
$variable = $_GET['variable'];
// use it in an output
echo '<img src="', $variable, '" />';
// if the quotes contained in $variable are escaped, it
// makes it impossible to leave the src=".." . The
// result would look like:
// <img src="foo\"bar" />
But, magic_quotes_gpc is not enough, there is still a way to steal passwords is many situations using a trick I've though about.
Idea
Let's take an example : an image gallery community.
- The script that displays each images get the image url by GET variables.
- No checks are made to test the validity of the url.
- magic_quotes_gpc is on.
The code would be similar to the first example:
// get untrusted data (magic_quotes_gpc is on)
$image_url = $_GET['image_url'];
// display the image
echo 'The image you requested : <br />';
echo '<img src="', $image_url, '" alt="some nice image" width="400" height="400" />';
In this case, there is no way to inject an efficient javascript that could request and send the cookie, so we will use a trick.
Let's use the url of a php image requesting http authentication : it will pop up an authentication box requesting a re-login for example. The basic inexperienced user would fill it with his community login and try it. The PHP image will recieve the information and just have to store it.
Note that this kind of trick will work on every injections that will modify client requests to get the complete page: images, frames, css file, ...
Conclusion
As you can see, magic_quotes_gpc does not protect your website against malicious injections that can lead to login theft. There is no "magic" solution when talking security and PHP provides a lot of functions that make your tests easier so use them!