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#81 in Testing

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Fuzzing Rust code with AFLplusplus

What is it?

Fuzz testing is a software testing technique used to find security and stability issues by providing pseudo-random data as input to the software. AFLplusplus is a popular, effective, and modern fuzz testing tool based on AFL. This library, afl.rs, allows one to run AFLplusplus on code written in the Rust programming language.


Documentation can be found in the Rust Fuzz Book.

What does it look like?

Screen recording of afl

Screen recording of AFL running on Rust code.


Before starting to fuzz, you should reconfigure your system for optimal performance and better crash detection. This can be done with cargo-afl afl system-config. But this subcommand requires root, so it uses sudo internally. Hence, you might need to enter your password.

By default, the AFL++ CMPLOG feature is activated, which helps to achieve good code coverage. However, it is not beneficial to activate CMPLOG on more than two instances. So if you run multiple AFL++ instances on your fuzzing target, you can disable CMPLOG by specifying the command line parameter '-c -'.

This document will familiarize you with AFL++ features to help in running a successful fuzzing campaign.

By default, the fuzzing config is set when cargo-afl is used to build. If you want to prevent this, just set the environment variable AFL_NO_CFG_FUZZING to 1 when building.