There are many ways to exploit a web server and gain access to the file system – read or write (sometimes both). This becomes even easier when one hosts CGIs or other dynamic code – especially when that code includes user based inputs. Recently, I found one of the most elegant exploits that I have seen for this kind of an attack vector, so I wanted to go over it and share some information about how it works and what exactly it exploits.

To setup the background for this scenario, imagine a web server (ex: ‘’) setup with userdirs, which allows CGI execution – not an uncommon situation at all. This means that ‘user1’ will have a directory like ‘public_html’, which will become directly accessible at: ‘’. For example, creating a ‘blah’ folder in ‘/home/user1/public_html’, will create ‘’ on the web.

At some point, ‘user1’ creates a file called ‘x.cgi’, which simply has a GET parameter called ‘file’, and if that parameter is a file that exists, it loads it via an include. Otherwise, it loads a default.html file. Let’s assume that ‘x.cgi’ is a PHP file which looks like this:

Continue Reading →Web Exploit – user modifiable Read and Execute can give you Write access