# Coverage-guided fuzzing of parsers with libFuzzer + ASan/UBSan in CI

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | safe-cpp-practices |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | Coverage-guided fuzzing of parsers with libFuzzer + ASan/UBSan in CI |
| Source URL | https://appsec.guide/docs/fuzzing/c-cpp/libfuzzer/ |
| Source publisher / author | Trail of Bits (appsec.guide); debugg.ai |
| Source type | vendor docs |
| Source date (published) | Unknown — not stated |
| Date researched | 2026-10-27 |
| Researched by | tai (agent), model: Claude Opus 4.8, on behalf of retoor |
| Research method | web search synthesis |
| Confidence | High |
| Status | Active |

## Summary

The standard way to harden an untrusted binary parser: a fuzz harness per entry point, built with -fsanitize=fuzzer,address,undefined and -fno-sanitize-recover, run short on every push and long nightly, with a regression corpus of past crashes rerun on every build.

## Key findings

- Build with `-fsanitize=fuzzer,address,undefined` and `-fno-sanitize-recover` so a finding aborts rather than warns.
- One harness per untrusted-input entry point, with bounded input sizes.
- Keep a regression corpus of past crashing inputs and rerun it on every build.
- 'No crashes' only means something if the harness reaches the risky code, so track coverage.
- ASan adds ~2-4x runtime; it is a test tool, not a release one.

## Pros / strengths

Concrete, matches industry practice; the regression-corpus rule is exactly what turns an unreproducible bug into a permanent test.

## Cons / limitations / caveats

libFuzzer is now maintenance-only at LLVM (successor: Centipede); the CI pattern is unaffected but tool choice may shift.

## Relevance to tai

detz adopts the pattern at its scale: a deterministic hostile-input test plus a random-input fuzz in whatsapp_test, run under ASan+UBSan via tools/san-whatsapp.sh, and a captured-frame file as the regression corpus seed.

## Changelog

| Date | Change |
|---|---|
| 2026-10-27 | Initial dossier created (web search synthesis). |