When I started out in computers a long time ago, I remember you could open files (images, documents, you name it) without having to worry about viruses because, well, everyone knew you could only get viruses from executable files like .exe, .bat, and .com in the Windows world. In the Linux world, if you could actually get your email client to run an executable attached to an email automatically, you probably had the good sense not to. I remember laughing at someone who asked if they had to worry about getting a virus from looking at images sent to them via email. The times, they have changed.

File-based attacks are now commonplace; at the time of this writing, a PDF-based exploit had become public, and Adobe had announced that they would fix it – in about two weeks. In the meantime you can … uhh … not read any PDF files, I guess, if you use Adobe Reader to view files. Luckily, most Linux users do not use Adobe Reader to view PDF's, but many will have Adobe Flash installed (exploitable via SWF files before Flash Player 10.0.12.36 for Linux), or OpenOffice (EMF and WMF files before OpenOffice 2.4.2). The list goes on and on: image files, font files, web pages, you name it. Many major applications have at some point failed to parse the files properly that they were designed to handle, allowing attackers to create files that can execute arbitrary code when opened.

How Fuzzing Works

The basic premise of file or protocol fuzzing is you subtly (or not so subtly) create input that is malformed. This can be done by such methods as simply flipping random bits in a file or protocol stream to creating a full-featured program or test suite that knows how to generate valid (and thus invalid) files or input for various protocols. An excellent example of a file fuzzer is Michael Zalewski's mangleme CGI program. If you install this CGI program on a web server and a client web browser connects to it, the mangle.cgi script creates a randomly generated HTML file that includes a META REFRESH tag, causing the web browser to reload the script (and get a different randomly generated HTML page). This lets you point a web browser at the URL with the mangle.cgi script and wait for the browser to crash (which it will, eventually, in most cases). The use of META REFRESH addresses one of the biggest problems with fuzzing files: getting a program to load a series of files one after the other in an automated fashion so that a human being doesn't have to sit there opening files.

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