Build a Fuzzing Testbench with AFL

American fuzzy lop (AFL) is a free software fuzzer that can be used to detect software bugs. Use these instructions to build and run a testbench with AFL.

Install AFL

Follow the steps in the AFL Quick Start Guide to install AFL on your system.

We assume that AFL is installed at:


Build a testbench with AFL instrumentation

According to AFL’s README, AFL is a “brute-force fuzzer coupled with an exceedingly simple but rock-solid instrumentation-guided genetic algorithm.” You must add instrumentation to the code before running a fuzzer in order to get potentially useful results; otherwise, you might not get any results.

When you build AFL from the previous step, an afl-gcc executable is generated; this works as a companion tool that acts as a drop-in replacement for gcc or clang. Before you build the testbench, make sure you are compiling code with afl-gcc in order to add instrumentation to the code. The script from the scripts/ directory does exactly this when you run it with the -f option.


By default, the script assumes you have installed AFL in the $HOME/work/ directory. If you install AFL in any other directory, you must change the path in this script.


From the AFL directory, run AFL by entering the following:

./afl-fuzz -i testcase_dir -o findings_dir /path/to/program [...params...] @@

AFL assumes that the inputs for the program you wish to fuzz are in the form of files. So, you must create a directory that contains these input files. This is the testcase_dir in the above command.

Since you are fuzzing the testbench, the program here is testbench.

params are the different parameters of the program apart from the input file.

@@: Each file from testcase_dir is substituted in place of this. As AFL continues to run, newly-generated testcases are placed in testcase_dir, and AFL in its further iterations runs with these newly-generated testcases.


Use AFL to fuzz the volume component of the testbench

To fuzz the volume component of the testbench, use topology files as inputs and place the topology files of volume components in an inputs directory:


# Add AFL directory to $PATH

# Go to the testbench directory
cd tools/testbench

# Run the fuzzer
afl-fuzz -i inputs/ -o output/ build_testbench/install/bin/testbench -r 48000 -R 48000 -i zeros_in.raw -o volume_out.raw -b S16_LE -t @@

AFL runs and places problem inputs in the provided output directory (-o option in the above command). The inputs are well-organized into crashes, hangs, etc. Run the testbench with the volume component in gdb to assist in figuring out the error.


AFL README is a good place to learn more about the AFL tool itself as well as the various options it provides.