---

name: ossfuzz type: technique description: "Enrolls a project in OSS-Fuzz, Google's free continuous fuzzing service for open source, and drives it locally. Covers project.yaml, Dockerfile and build.sh setup, the helper scripts, reproducing OSS-Fuzz crash reports, and the acceptance criteria. Use when setting up continuous fuzzing for an open-source project, reproducing an OSS-Fuzz bug report, or testing an OSS-Fuzz build before submitting it."

OSS-Fuzz

OSS-Fuzz is an open-source project developed by Google that provides free distributed infrastructure for continuous fuzz testing. It streamlines the fuzzing process and facilitates simpler modifications. While only select projects are accepted into OSS-Fuzz, the project's core is open-source, allowing anyone to host their own instance for private projects.

Overview

OSS-Fuzz provides a simple CLI framework for building and starting harnesses or calculating their coverage. Additionally, OSS-Fuzz can be used as a service that hosts static web pages generated from fuzzing outputs such as coverage information.

Key Concepts

| Concept | Description | |---------|-------------| | helper.py | CLI script for building images, building fuzzers, and running harnesses locally | | Base Images | Hierarchical Docker images providing build dependencies and compilers | | project.yaml | Configuration file defining project metadata for OSS-Fuzz enrollment | | Dockerfile | Project-specific image with build dependencies | | build.sh | Script that builds fuzzing harnesses for your project | | Criticality Score | Metric used by OSS-Fuzz team to evaluate project acceptance |

When to Apply

Apply this technique when:

• Setting up continuous fuzzing for an open-source project
• Need distributed fuzzing infrastructure without managing servers
• Want coverage reports and bug tracking integrated with fuzzing
• Testing existing OSS-Fuzz harnesses locally
• Reproducing crashes from OSS-Fuzz bug reports

Skip this technique when:

• Project is closed-source (unless hosting your own OSS-Fuzz instance)
• Project doesn't meet OSS-Fuzz's criticality score threshold
• Need proprietary or specialized fuzzing infrastructure
• Fuzzing simple scripts that don't warrant infrastructure

Quick Reference

| Task | Command | |------|---------| | Clone OSS-Fuzz | git clone https://github.com/google/oss-fuzz | | Build project image | uv run --no-project python infra/helper.py build_image --pull <project> | | Build fuzzers with ASan | uv run --no-project python infra/helper.py build_fuzzers --sanitizer=address <project> | | Run specific harness | uv run --no-project python infra/helper.py run_fuzzer <project> <harness> | | Generate coverage report | uv run --no-project python infra/helper.py coverage <project> | | Check helper.py options | uv run --no-project python infra/helper.py --help |

OSS-Fuzz Project Components

OSS-Fuzz provides several publicly available tools and web interfaces:

Bug Tracker

The bug tracker allows you to:

• Check bugs from specific projects (initially visible only to maintainers, later made public)
• Create new issues and comment on existing ones
• Search for similar bugs across all projects to understand issues

Build Status System

The build status system helps track:

• Build statuses of all included projects
• Date of last successful build
• Build failures and their duration

Fuzz Introspector

Fuzz Introspector displays:

• Coverage data for projects enrolled in OSS-Fuzz
• Hit frequency for covered code
• Performance analysis and blocker identification

Read this case study for examples and explanations.

Step-by-Step: Running a Single Harness

You don't need to host the whole OSS-Fuzz platform to use it. The helper script makes it easy to run individual harnesses locally.

Step 1: Clone OSS-Fuzz

git clone https://github.com/google/oss-fuzz
cd oss-fuzz
uv run --no-project python infra/helper.py --help

Step 2: Build Project Image

uv run --no-project python infra/helper.py build_image --pull <project-name>

This downloads and builds the base Docker image for the project.

Step 3: Build Fuzzers with Sanitizers

uv run --no-project python infra/helper.py build_fuzzers --sanitizer=address <project-name>

Sanitizer options:

• --sanitizer=address for AddressSanitizer with LeakSanitizer
• Other sanitizers available (language support varies)

Note: Fuzzers are built to /build/out/<project-name>/ containing the harness executables, dictionaries, corpus, and crash files.

Step 4: Run the Fuzzer

uv run --no-project python infra/helper.py run_fuzzer <project-name> <harness-name> [<fuzzer-args>]

The helper script automatically runs any missed steps if you skip them.

Step 5: Coverage Analysis (Optional)

First, install gsutil (skip gcloud initialization).

uv run --no-project python infra/helper.py build_fuzzers --sanitizer=coverage <project-name>
uv run --no-project python infra/helper.py coverage <project-name>

Use --no-corpus-download to use only local corpus. The command generates and hosts a coverage report locally.

See official OSS-Fuzz documentation for details.

Common Patterns

Pattern: Running irssi Example

Use Case: Testing OSS-Fuzz setup with a simple enrolled project

# Clone and navigate to OSS-Fuzz
git clone https://github.com/google/oss-fuzz
cd oss-fuzz

# Build and run irssi fuzzer
uv run --no-project python infra/helper.py build_image --pull irssi
uv run --no-project python infra/helper.py build_fuzzers --sanitizer=address irssi
uv run --no-project python infra/helper.py run_fuzzer irssi irssi-fuzz

Expected Output:

INFO:__main__:Running: docker run --rm --privileged --shm-size=2g --platform linux/amd64 -i -e FUZZING_ENGINE=libfuzzer -e SANITIZER=address -e RUN_FUZZER_MODE=interactive -e HELPER=True -v /private/tmp/oss-fuzz/build/out/irssi:/out -t gcr.io/oss-fuzz-base/base-runner run_fuzzer irssi-fuzz.
Using seed corpus: irssi-fuzz_seed_corpus.zip
/out/irssi-fuzz -rss_limit_mb=2560 -timeout=25 /tmp/irssi-fuzz_corpus -max_len=2048 < /dev/null
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 1531341664
INFO: Loaded 1 modules   (95687 inline 8-bit counters): 95687 [0x1096c80, 0x10ae247),
INFO: Loaded 1 PC tables (95687 PCs): 95687 [0x10ae248,0x1223eb8),
INFO:      719 files found in /tmp/irssi-fuzz_corpus
INFO: seed corpus: files: 719 min: 1b max: 170106b total: 367969b rss: 48Mb
#720        INITED cov: 409 ft: 1738 corp: 640/163Kb exec/s: 0 rss: 62Mb
#762        REDUCE cov: 409 ft: 1738 corp: 640/163Kb lim: 2048 exec/s: 0 rss: 63Mb L: 236/2048 MS: 2 ShuffleBytes-EraseBytes-

Pattern: Enrolling a New Project

Use Case: Adding your project to OSS-Fuzz (or private instance)

Create three files in projects/<your-project>/:

1. project.yaml - Project metadata:

homepage: "https://github.com/yourorg/yourproject"
language: c++
primary_contact: "your-email@example.com"
main_repo: "https://github.com/yourorg/yourproject"
fuzzing_engines:
  - libfuzzer
sanitizers:
  - address
  - undefined

2. Dockerfile - Build dependencies:

FROM gcr.io/oss-fuzz-base/base-builder
RUN apt-get update && apt-get install -y \
    autoconf \
    automake \
    libtool \
    pkg-config
RUN git clone --depth 1 https://github.com/yourorg/yourproject
WORKDIR yourproject
COPY build.sh $SRC/

3. build.sh - Build harnesses:

#!/bin/bash -eu
./autogen.sh
./configure --disable-shared
make -j$(nproc)

# Build harnesses
$CXX $CXXFLAGS -std=c++11 -I. \
    $SRC/yourproject/fuzz/harness.cc -o $OUT/harness \
    $LIB_FUZZING_ENGINE ./libyourproject.a

# Copy corpus and dictionary if available
cp $SRC/yourproject/fuzz/corpus.zip $OUT/harness_seed_corpus.zip
cp $SRC/yourproject/fuzz/dictionary.dict $OUT/harness.dict

Docker Images in OSS-Fuzz

Harnesses are built and executed in Docker containers. All projects share a runner image, but each project has its own build image.

Image Hierarchy

Images build on each other in this sequence:

1. base_image - Specific Ubuntu version
2. base_clang - Clang compiler; based on base_image
3. base_builder - Build dependencies; based on base_clang
   • Language-specific variants: base_builder_go, etc.
   • See /oss-fuzz/infra/base-images/ for full list
4. Your project Docker image - Project-specific dependencies; based on base_builder or language variant

Runner Images (Used Separately)

• base_runner - Executes harnesses; based on base_clang
• base_runner_debug - With debug tools; based on base_runner

Advanced Usage

Tips and Tricks

| Tip | Why It Helps | |-----|--------------| | Don't manually copy source code | Project Dockerfile likely already pulls latest version | | Check existing projects | Browse oss-fuzz/projects for examples | | Keep harnesses in separate repo | Like curl-fuzzer - cleaner organization | | Use specific compiler versions | Base images provide consistent build environment | | Install dependencies in Dockerfile | May require approval for OSS-Fuzz enrollment |

Criticality Score

OSS-Fuzz uses a criticality score to evaluate project acceptance. See this example for how scoring works.

Projects with lower scores may still be added to private OSS-Fuzz instances.

Hosting Your Own Instance

Since OSS-Fuzz is open-source, you can host your own instance for:

• Private projects not eligible for public OSS-Fuzz
• Projects with lower criticality scores
• Custom fuzzing infrastructure needs

Anti-Patterns

| Anti-Pattern | Problem | Correct Approach | |--------------|---------|------------------| | Manually pulling source in build.sh | Doesn't use latest version | Let Dockerfile handle git clone | | Copying code to OSS-Fuzz repo | Hard to maintain, violates separation | Reference external harness repo | | Ignoring base image versions | Build inconsistencies | Use provided base images and compilers | | Skipping local testing | Wastes CI resources | Use helper.py locally before PR | | Not checking build status | Unnoticed build failures | Monitor build status page regularly |

Tool-Specific Guidance

libFuzzer

OSS-Fuzz primarily uses libFuzzer as the fuzzing engine for C/C++ projects.

Harness signature:

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    // Your fuzzing logic
    return 0;
}