Chapter 24 Fuzzing with afl-fuzz

1 Overview

American fuzzy lop (“afl-fuzz”) is a fuzzer, a tool for testing software by providing randomly-generated inputs, searching for those inputs which cause the program to crash.

Unlike most fuzzers, afl-fuzz observes the internal behaviour of the program being tested, and adjusts the test cases it generates to trigger unexplored execution paths. As a result, test cases generated by afl-fuzz cover more of the possible behaviours of the tested program than other fuzzers.

This requires that programs to be tested are instrumented to communicate with afl-fuzz. The native-code compiler “ocamlopt” can generate such instrumentation, allowing afl-fuzz to be used against programs written in OCaml.

For more information on afl-fuzz, see the website at

2 Generating instrumentation

The instrumentation that afl-fuzz requires is not generated by default, and must be explicitly enabled, by passing the -afl-instrument option to ocamlopt.

To fuzz a large system without modifying build tools, OCaml’s configure script also accepts the afl-instrument option. If OCaml is configured with afl-instrument, then all programs compiled by ocamlopt will be instrumented.

2.1 Advanced options

In rare cases, it is useful to control the amount of instrumentation generated. By passing the -afl-inst-ratio N argument to ocamlopt with N less than 100, instrumentation can be generated for only N% of branches. (See the afl-fuzz documentation on the parameter AFL_INST_RATIO for the precise effect of this).

3 Example

As an example, we fuzz-test the following program,

let _ =
  let s = read_line () in
  match Array.to_list (Array.init (String.length s) (String.get s)) with
    ['s'; 'e'; 'c'; 'r'; 'e'; 't'; ' '; 'c'; 'o'; 'd'; 'e'] -> failwith "uh oh"
  | _ -> ()

There is a single input (the string “secret code”) which causes this program to crash, but finding it by blind random search is infeasible.

Instead, we compile with afl-fuzz instrumentation enabled:

ocamlopt -afl-instrument -o readline

Next, we run the program under afl-fuzz:

mkdir input
echo asdf > input/testcase
mkdir output
afl-fuzz -m none -i input -o output ./readline

By inspecting instrumentation output, the fuzzer finds the crashing input quickly.

Note: To fuzz-test an OCaml program with afl-fuzz, passing the option -m none is required to disable afl-fuzz’s default 50MB virtual memory limit.