AFL-Fuzz Demo

This is a very brief demo; the AFL-Fuzz docs are really good and well worth a read.

Setup

For simplicity these instructions all run in a known root directory ($AFL_ROOT).

export AFL_ROOT=path/to/some/root

Building AFL-Fuzz

Fortunately AFL is very easy to build and install:

cd $AFL_ROOT
git clone git@github.com:google/AFL.git
cd AFL
make -j`nproc`
DESTDIR=$AFL_ROOT/install make install

You can probably also get it from your system package manager.

Structure of a Fuzzable Program

This is all in the AFL Quickstart Guide, but for reference:

• Write program harnesses that read bytes from stdin or a file and interpret them somehow.
• Include some behaviour that crashes the program (e.g. assertions that can fail, or sanitiser crashes such as undefined behaviour).
• Supply any relevant seed inputs.

Running the Demos

cd $AFL_ROOT
git clone git@github.com:mob-group/afl-demo.git
cd afl-demo
export AFL_PATH=$AFL_ROOT/install/usr/local/lib/afl

# Compile the code using the AFL compiler wrapper
$AFL_ROOT/install/usr/local/bin/afl-gcc \
  -fsanitize=undefined \
  -fsanitize-undefined-trap-on-error \
  sum_n.c -o sum_n

$AFL_ROOT/install/usr/local/bin/afl-gcc \
  versions.c -o versions

# Run the fuzzer
$AFL_ROOT/install/usr/local/bin/afl-fuzz \
  -i inputs \
  -o sum_n_outputs \
  ./sum_n

$AFL_ROOT/install/usr/local/bin/afl-fuzz \
  -i inputs \
  -o versions_outputs \
  ./versions

Interpreting Results

Once you've killed the fuzzer with ctrl-C, you can look at the results in each of the output directories. For each, AFL generates a crashes/ and queue/ directory with input files that you can use directly as input to the previously compiled programs. The ones in queue/ are valid inputs that don't produce a crash.

You'll find that AFL is able to identify quite quickly that large positive and any negative inputs cause sum_n to integer overflow, and that it cannot find an input that distinguishes sum_n and sum_n_v2 at all (as you'd expect!).