In the presentation, I speculated as why it has been able to find so many bugs:
- It breaks the rules, allowing it to find errors in syntax error handling such as bug 350415 and more generally helping the fuzzer avoid having "blind spots".
- It allows state to accumulate by creating and running functions in a loop. (See bug 361346 for an example of a bug that would be hard to find otherwise.)
- It tests for correctness, not just crashes and assertions. (Since I didn't talk about this aspect much during the security-focused Black Hat presentation, I've made it a separate blog post.)
Update (2015): newer versions are available in the funfuzz repository on GitHub