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November 24, 2014

afl-fuzz: crash exploration mode

One of the most labor-intensive portions of any fuzzing project is the work needed to determine if a particular crash poses a security risk. A small minority of all fault conditions will have obvious implications; for example, attempts to write or jump to addresses that clearly come from the input file do not need any debate. But most crashes are more ambiguous: some of the most common issues are NULL pointer dereferences and reads from oddball locations outside the mapped address space. Perhaps they are a manifestation of an underlying vulnerability; or perhaps they are just harmless non-security bugs. Even if you prefer to err on the side of caution and treat them the same, the vendor may not share your view.

If you have to make the call, sifting through such crashes may require spending hours in front of a debugger - or, more likely, rejecting a good chunk of them based on not much more than a hunch. To help triage the findings in a more meaningful way, I decided to add a pretty unique and nifty feature to afl-fuzz: the brand new crash exploration mode, enabled via -C.

The idea is very simple: you take a crashing test case and give it to afl-fuzz as a starting point for the automated run. The fuzzer then uses its usual feedback mechanisms and genetic algorithms to see how far it can get within the instrumented codebase while still keeping the program in the crashing state. Mutations that stop the crash from happening are thrown away; so are the ones that do not alter the execution path in any appreciable way. The occasional mutation that makes the crash happen in a subtly different way will be kept and used to seed subsequent fuzzing rounds later on.

The beauty of this mode is that it very quickly produces a small corpus of related but somewhat different crashes that can be effortlessly compared to pretty accurately estimate the degree of control you have over the faulting address, or to figure out whether you can get past the initial out-of-bounds read by nudging it just the right way (and if the answer is yes, you probably get to see what happens next). It won't necessarily beat thorough code analysis, but it's still pretty cool: it lets you make a far more educated guess without having to put in any work.

As an admittedly trivial example, let's take a suspect but ambiguous crash in unrtf, found by afl-fuzz in its normal mode:

unrtf[7942]: segfault at 450 ip 0805062b sp bf957e60 error 4 in unrtf[8048000+1c000]

When fed to the crash explorer, the fuzzer took just several minutes to notice that by changing {\cb-44901990 in the converted RTF file to printable representations of other negative integers, it could quickly trigger faults at arbitrary addresses of its choice, corresponding mostly-linearly to the integer set:

unrtf[28809]: segfault at 88077782 ip 0805062b sp bff00210 error 4 in unrtf[8048000+1c000]
unrtf[26656]: segfault at 7271250 ip 0805062b sp bf957e60 error 4 in unrtf[8048000+1c000]

Given a bit more time, it would also almost certainly notice that choosing values within the mapped address space get it past the crashing location and permit even more fun. So, automatic exploit writing next?

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