harness-writing

A fuzzing harness is the entrypoint function that receives random data from the fuzzer and routes it to your system under test (SUT). The quality of your harness directly determines which code paths get exercised and whether critical bugs are found. A poorly written harness can miss entire subsystems or produce non-reproducible crashes. The harness is the bridge between the fuzzer's random byte generation and your application's API. It must parse raw bytes into meaningful inputs, call target functions, and handle edge cases gracefully. The most important part of any fuzzing setup is the harness—if written poorly, critical parts of your application may not be covered. Apply this technique when: Skip this technique when: Find functions in your codebase that: Good targets are typically: Start with the simplest possible harness that calls your target function: C/C++: Rust: Reject inputs that are too small or too large to be meaningful: Rationale: The fuzzer generates random inputs of all sizes. Your harness must handle empty, tiny, huge, or malformed inputs without causing unexpected issues in the harness itself (crashes in the SUT are fine—that's what we're looking for).