Instrumenting programs for AFL

When source code is available, instrumentation can be injected by a companion tool that works as a drop-in replacement for gcc or clang in any standard build process for third-party code.

The instrumentation has a fairly modest performance impact; in conjunction with other optimizations implemented by afl-fuzz, most programs can be fuzzed as fast or even faster than possible with traditional tools.

The correct way to recompile the target program may vary depending on the specifics of the build process, but a nearly-universal approach would be:

$ CC=/path/to/afl/afl-gcc ./configure
$ make clean all

For C++ programs, you’d would also want to set CXX=/path/to/afl/afl-g++.

The clang wrappers (afl-clang and afl-clang++) can be used in the same way; clang users may also opt to leverage a higher-performance instrumentation mode, as described in llvm_mode/README.llvm.

When testing libraries, you need to find or write a simple program that reads data from stdin or from a file and passes it to the tested library. In such a case, it is essential to link this executable against a static version of the instrumented library, or to make sure that the correct .so file is loaded at runtime (usually by setting LD_LIBRARY_PATH). The simplest option is a static build, usually possible via:

$ CC=/path/to/afl/afl-gcc ./configure --disable-shared

Setting AFL_HARDEN=1 when calling make will cause the CC wrapper to automatically enable code hardening options that make it easier to detect simple memory bugs. Libdislocator, a helper library included with AFL (see libdislocator/README.dislocator) can help uncover heap corruption issues, too.


ASAN users are advised to review Using ASAN with AFL for important caveats.

Instrumenting binary-only apps

When source code is NOT available, the fuzzer offers experimental support for fast, on-the-fly instrumentation of black-box binaries. This is accomplished with a version of QEMU running in the lesser-known “user space emulation” mode.

QEMU is a project separate from AFL, but you can conveniently build the feature by doing:

$ cd qemu_mode
$ ./

For additional instructions and caveats, see qemu_mode/README.qemu.

The mode is approximately 2-5x slower than compile-time instrumentation, is less conductive to parallelization, and may have some other quirks.