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Introducing fuzzing with libFuzzer and Sanitizers.

This section provides an overview of the fuzzing process and defines common terms. If you are already familiar with libFuzzer and Sanitizers, feel free to skip to Step 1: Build Integration to begin writing fuzzers and integrating with ClusterFuzzLite’s build system.


Fuzzing is a technique where randomized inputs are automatically created and fed as input to a (target) program in order to find bugs in that program. The program that creates the inputs is called a fuzzer. Fuzzing is highly effective at finding bugs missed by manually written tests, code review, or auditing. Fuzzing has found thousands of bugs in mature software such as Chrome, OpenSSL, and Curl. When done well, fuzzing is able to find bugs in virtually any code.


LibFuzzer is a fuzzer (sometimes called a fuzzing engine) that mutates inputs and feeds them to target code in a loop. During execution of the target on the input, libFuzzer observes the coverage of the code under test using instrumentation inserted by the compiler. LibFuzzer uses this coverage feedback to “evolve” progressively more interesting inputs and reach deeper program states, allowing it to find interesting bugs with little developer effort.

Fuzz target

To fuzz target code, you must define a function called a fuzz target with the following API:

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
  DoSomethingInterestingWithMyAPI(Data, Size);
  return 0;  // Non-zero return values are reserved for future use.

Clang’s -fsanitizer=fuzzer option will link this fuzz target function against libFuzzer, producing a fuzzer binary that will fuzz your target code when run. Note that in ClusterFuzzLite, you will not use this flag directly. Instead, you should use the $LIB_FUZZING_ENGINE environment variable, which is discussed in more detail in Step 1: Build Integration.


Sanitizers are tools that detect bugs in code (typically “native code” such as C/C++, Rust, Go, and Swift) and report bugs by crashing. ClusterFuzzLite relies on sanitizers to detect bugs that would otherwise be missed. Sanitizers work by instructing clang to add compile-time instrumentation, so different builds are needed to use different sanitizers.

The sanitizers ClusterFuzzLite uses are:

  • AddressSanitizer (ASan) : For detecting memory safety issues. This is the most important sanitizer to fuzz with. AddressSanitizer also detects memory leaks.
  • UndefinedBehaviorSanitizer (UBSan) : For detecting undefined behavior such as integer overflows.
  • MemorySanitizer (MSan) : For detecting use of uninitialized memory. MSan is the hardest sanitizer to use because an MSan instrumented binary must be entirely instrumented with MSan. If any part of the binary is not instrumented with MSan, MSan will report false positives.

The ClusterFuzzLite codebase uses shorter names for the sanitizers. When referring to a sanitizer as an input to ClusterFuzzLite, ASan is address, UBSan is ubsan and MSan is memory.

Next: Step 1: Build Integration for directions on integrating your project with ClusterFuzzLite’s build system.