Hanno's blog

I recently played around with the possibilities of fuzzing. It's a simple way to find bugs in applications.

What you do: You have some application that parses some kind of file format. You create lots (thousands) of files which have small errors. The simplest approach is to just change random bits. If the app crashes, you've found a bug, it's quite likely that it's a security relevant one. This is especially crucial for apps like mail scanners (antivirus), but pretty much works for every app that parses foreign input. It works especially well on uncommon file formats, because their code is often not well maintained.

My fuzzing tool of choice is zzuf.

I am impressed and a bit shocked how easy it is to find crashers and potential overflows in common, security relevant applications. My last discovery was a crasher in the chm parser of clamav.

About one year ago, Sam Hocevar posted some results on tests with his fuzzing tool zzuf, which showed a large number of crashes in various applications, especially multimedia apps.
Crash bugs on invalid input very often lead to security issues, thus this should be taken seriously.

Now, I took the freedom to have a look how many of the issues found back then were fixed. I used the most current versions in gentoo linux (testing/~x86-system), which tend to be quite up-to-date. I also cross-checked the crashes for other apps, as they often use the same or similar code.
Seems only vlc devs did their homework (Sam Hocevar is part of the vlc team). Interesting enough, even firefox seems to have a gif-crasher since a year.

gstreamer crash by lol-ffplay.mpg lol-gstreamer.m2v lol-mplayer.m2v lol-mplayer.mpg lol-vlc.m2v lol-vlc.mpg
endless loop by lol-ffplay.m2v lol-xine.mpg

mplayer hang by lol-mplayer.wmv,
crash by lol-ffplay.flac lol-mplayer.aac lol-mplayer.mpg lol-mplayer.ogg lol-ogg123.flac lol-vlc.aac lol-xine.aac

xine crash by lol-mplayer.wmv lol-ffplay.m2v lol-ffplay.ogg lol-ffplay.wmv lol-gstreamer.avi lol-ogg123.flac lol-vlc.aac lol-xine.mpg

firefox crash by lol-firefox.gif