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: ‘www.example.com’) 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: ‘http://www.example.com/~user1/’. For example, creating a ‘blah’ folder in ‘/home/user1/public_html’, will create ‘http://www.example.com/~user1/blah’ 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:
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#!/usr/bin/php // x.cgi file - used as a "pre-processor" for loading HTML files $file = $_REQUEST['f']; if (file_exists($file) ) { include($file); } else { include('default.html'); } ?> |
Continue Reading →Web Exploit – user modifiable Read and Execute can give you Write access