Corpus management

Shared reference for coverage-guided-fuzzing; the per-engine references and the umbrella workflow live alongside this file.

Overview

Pure-reference catalog of corpus-management practices across libFuzzer, AFL++, native Go fuzz, cargo-fuzz, Atheris, Jazzer, and OSS-Fuzz. Consumed by the per-engine references and the fuzz-target-author agent.

When to use

• Bootstrapping a new fuzz target - what should the seed corpus contain?
• Running a long-running fuzz campaign - when to minimise, where to back up, how to share across CI runs.
• Debugging a crash - locating the crash artefact, reproducing it.
• Migrating between fuzzers (libFuzzer ↔ AFL++) - corpus format compatibility.

Corpus components

A fuzzing corpus has three roles:

Directory layout per fuzzer

libFuzzer

Per llvm.org/docs/LibFuzzer.html:

fuzz/
  fuzz_target.cc       # the harness
  seeds/               # initial inputs (read on startup)
  corpus/              # evolved corpus (read + written)
  fuzz_target          # compiled binary

Invocation:

./fuzz_target -max_total_time=3600 corpus/ seeds/

The first directory listed is the output corpus (writable); subsequent directories are read-only seeds.

Crash artefacts saved to current directory as crash-<sha1>, leak-<sha1>, timeout-<sha1> per LLVM docs.

AFL++

fuzz/
  inputs/              # seed corpus
  output/              # AFL++ output (queue/, crashes/, hangs/)
  fuzz_target          # AFL-instrumented binary (afl-clang-fast)

Invocation:

afl-fuzz -i inputs/ -o output/ -- ./fuzz_target @@

Crash format: output/default/crashes/id:<num>,sig:<signal>,src:<id>,op:<mutator>,....

AFL++ corpora are not directly compatible with libFuzzer - the queue files have a different on-disk format. Convert via afl-fuzz -i + afl-cmin round-trip.

Go native (go test -fuzz)

Per go.dev/doc/security/fuzz:

package/
  fuzz_test.go         # contains FuzzXxx functions
  testdata/
    fuzz/
      FuzzXxx/
        seedfile1      # seed inputs (hand-curated)
        seedfile2

Failures auto-write to testdata/fuzz/FuzzXxx/ with a generated filename - they're meant to be committed as regression cases.

cargo-fuzz

fuzz/
  Cargo.toml
  fuzz_targets/
    fuzz_target_1.rs   # the harness (one per fuzz target)
  corpus/
    fuzz_target_1/     # per-target corpus
  artifacts/
    fuzz_target_1/
      crash-<sha1>

Invocation:

cargo fuzz run fuzz_target_1

Atheris (Python)

fuzz/
  fuzz_target.py       # uses atheris.Setup + atheris.Fuzz
  corpus/

Invocation (Atheris uses libFuzzer's CLI under the hood):

python fuzz_target.py corpus/

Jazzer (JVM)

fuzz/
  FuzzTarget.java      # with @FuzzTest annotation
  corpus/

Invocation:

jazzer --cp=target/test-classes \
       --target_class=com.example.FuzzTarget \
       corpus/

OSS-Fuzz

OSS-Fuzz aggregates corpora across Google's infrastructure:

oss-fuzz/
  projects/<project>/
    Dockerfile         # build the fuzzer
    build.sh           # produces $OUT/fuzz_target_1, $OUT/fuzz_target_1_seed_corpus.zip

Per google.github.io/oss-fuzz.

The corpus syncs to gs://<project>-corpus.clusterfuzz-external.appspot.com/.

Seed corpus construction strategies

A seed corpus of 5-50 hand-curated diverse inputs is typically enough to bootstrap. Bigger isn't always better - the fuzzer finds new paths via mutation.

Dictionary files

A dictionary lists tokens the fuzzer prefers when mutating. For a JSON parser:

# fuzz.dict
"{"
"}"
"["
"]"
"true"
"false"
"null"
"\":\""

Invocation: libFuzzer -dict=fuzz.dict or afl-fuzz -x fuzz.dict.

Dictionaries dramatically improve fuzzer effectiveness on structured formats (JSON, XML, protobuf, SQL).

Corpus minimisation

A corpus that grows over time becomes redundant - many inputs hit the same coverage. Minimise periodically:

# libFuzzer merge mode
mkdir minimised/
./fuzz_target -merge=1 minimised/ corpus/

# AFL++ minimisation
afl-cmin -i corpus/ -o minimised/ -- ./fuzz_target @@

# Per-input minimisation (find smallest input triggering same coverage)
afl-tmin -i crash-input -o min-crash-input -- ./fuzz_target @@

Minimisation reduces fuzz cycle time + reproducibility surface.

Crash artefact handling

When the fuzzer finds a crash:

1. Reproduce locally:

   ./fuzz_target crash-<sha1>
   # Triggers the same sanitiser report

2. Minimise the crash input (see above) so the bug report is small.
3. File the bug via the from-CI-failure workflow in bug-report-template (qa-bug-repro plugin) with the minimised crash as an attachment.
4. Add the original (non-minimised) crash to the seed corpus as a regression test - re-runs will catch reintroduction.

CI integration

Long-running fuzz campaigns run continuously; CI runs short "smoke fuzz" campaigns (~5 min):

- name: Smoke fuzz
  run: ./fuzz_target -max_total_time=300 corpus/ seeds/

Persist evolved corpus to a CI cache so coverage accumulates across runs:

- uses: actions/cache@v4
  with:
    path: corpus/
    key: fuzz-corpus-${{ github.sha }}
    restore-keys: fuzz-corpus-

Anti-patterns

Limitations

• Corpus format incompatibility. libFuzzer / AFL++ / cargo-fuzz corpora aren't directly swap-compatible; cross-fuzzer testing needs conversion.
• Corpus rot. Evolved corpora are tied to a specific binary's coverage instrumentation; rebuilding the target may invalidate some coverage information.
• Dictionary maintenance. Dictionaries should evolve with the target's grammar - manual upkeep.
• Corpus storage. Long campaigns produce GB-scale corpora; needs deliberate storage strategy (S3 / GCS / artifacts).
• No semantic seeding for opaque formats. Custom binary formats may need a generator to bootstrap coverage.

References

• LLVM libFuzzer - llvm.org/docs/LibFuzzer.html.
• AFL++ docs - github.com/AFLplusplus/AFLplusplus/blob/stable/docs.
• Go native fuzzing - go.dev/doc/security/fuzz.
• cargo-fuzz - github.com/rust-fuzz/cargo-fuzz.
• OSS-Fuzz - google.github.io/oss-fuzz.
• Sibling references: sanitizer-integration.md.
• Consumed by: libfuzzer.md, afl-plus-plus.md, go-native-fuzzing.md, cargo-fuzz.md, atheris.md, jazzer.md, OSS-Fuzz (google.github.io/oss-fuzz), the coverage-guided-fuzzing umbrella (../SKILL.md).