Frequently Asked Questions

Where can I learn more about fuzzing?

We recommend reading libFuzzer tutorial and the other docs in google/fuzzing repository. These and some other resources are listed on the useful links page.

What kind of projects are you accepting?

We accept established projects that have a critical impact on infrastructure and user security. We will consider each request on a case-by-case basis, but some things we keep in mind are:

  • Exposure to remote attacks (e.g. libraries that are used to process untrusted input).
  • Number of users/other projects depending on this project.

We hope to relax this requirement in the future though, so keep an eye out even if we are not able to accept your project at this time!

How can I find potential fuzz targets in my open source project?

You should look for places in your code that:

  • consume un-trusted data from users or from the network.
  • consume complex input data even if it’s ‘trusted’.
  • use an algorithm that has two or more implementations (to verify their equivalence).
  • look for existing fuzz target examples and find similarities.

Where can I store fuzz target sources and the build script if it’s not yet accepted upstream?

Fuzz target sources as well as the build script may temporarily live inside the projects/<your_project> directory in the OSS-Fuzz repository. Note that we do not accept integrations that rely on forked repositories. Refer to the ideal integration guide for the preferred long term solution.

My project is not open source. Can I use OSS-Fuzz?

You cannot use OSS-Fuzz, but you can use ClusterFuzz which OSS-Fuzz is based on. ClusterFuzz is an open-source fuzzing infrastructure that you can deploy in your own environment and run continuously at scale.

OSS-Fuzz is a production instance of ClusterFuzz, plus the code living in OSS-Fuzz repository: build scripts, project.yaml files with contacts, etc.

Why do you use a different issue tracker for reporting bugs in OSS projects?

Security access control is important for the kind of issues that OSS-Fuzz detects, hence why by default issues are only opened on the OSS-Fuzz tracker. You can opt-in to have them on Github as well by adding the file_github_issue attribute to your project.yaml file. Note that this is only for visibility’s purpose, and that the actual details can be found by following the link to the OSS-Fuzz tracker.

Why do you require a Google account for authentication?

Our ClusterFuzz fuzzing infrastructure and issue tracker require a Google account for authentication. Note that an alternate email address associated with a Google account does not work due to appengine api limitations.

Why do you use Docker?

Building fuzzers requires building your project with a fresh Clang compiler and special compiler flags. An easy-to-use Docker image is provided to simplify toolchain distribution. This also simplifies our support for a variety of Linux distributions and provides a reproducible environment for fuzzer building and execution.

How do you handle timeouts and OOMs?

If a single input to a fuzz target requires more than ~25 seconds or more than 2.5GB RAM to process, we report this as a timeout or an OOM (out-of-memory) bug (examples: timeouts, OOMs). This may or may not be considered as a real bug by the project owners, but nevertheless we treat all timeouts and OOMs as bugs since they significantly reduce the efficiency of fuzzing.

Remember that fuzzing is executed with AddressSanitizer or other sanitizers which introduces a certain overhead in RAM and CPU.

We currently do not have a good way to deduplicate timeout or OOM bugs. So, we report only one timeout and only one OOM bug per fuzz target. Once that bug is fixed, we will file another one, and so on.

Currently we do not offer ways to change the memory and time limits.

Can I launch an additional process (e.g. a daemon) from my fuzz target?

No. In order to get all the benefits of in-process, coverage-guided fuzz testing, it is required to run everything inside a single process. Any child processes created outside the main process introduces heavy launch overhead and is not monitored for code coverage.

Another rule of thumb is: “the smaller fuzz target is, the better it is”. It is expected that your project will have many fuzz targets to test different components, instead of a single fuzz target trying to cover everything. Think of fuzz target as a unit test, though it is much more powerful since it helps to test millions of data permutations rather than just one.

What if my fuzz target finds a bug in another project (dependency) ?

Every bug report has a crash stack-trace that shows where the crash happened. Using that, you can debug the root cause and see which category the bug falls in:

  • If this is a bug is due to an incorrect usage of the dependent project’s API in your project, then you need to fix your usage to call the API correctly.
  • If this is a real bug in the dependent project, then you should CC the maintainers of that project on the bug. Once CCed, they will get automatic access to all the information necessary to reproduce the issue. If this project is maintained in OSS-Fuzz, you can search for contacts in the respective project.yaml file.

What if my fuzzer does not find anything?

If your fuzz target is running for many days and does not find bugs or new coverage, it may mean several things:

  • We’ve covered all reachable code. In order to cover more code we need more fuzz targets.
  • The seed corpus is not good enough and the fuzzing engine(s) are not able to go deeper based on the existing seeds. Need to add more seeds.
  • There is some crypto/crc stuff in the code that will prevent any fuzzing engine from going deeper, in which case the crypto should be disabled in fuzzing mode. Examples: openssl, boringssl
  • It is also possible that the fuzzer is running too slow (you may check the speed of your targets at

In either case, look at the coverage reports for your target(s) and figure out why some parts of the code are not covered.

What if my fuzzer does not find new coverage or bugs after a while?

It is common for fuzzers to plateau and stop finding new coverage or bugs. Fuzz Introspector helps you evaluate your fuzzers’ performance. It can help you identify bottlenecks causing your fuzzers to plateau. It provides aggregated and individual fuzzer reachability and coverage reports. Developers can either introduce a new fuzz target or modify an existing one to reach previously unreachable code. Here are case studies where Fuzz Introspector helped developers improve fuzzing of a project. Fuzz Introspector reports are available on the OSS-Fuzz homepage or through this index.

Developers can also use Fuzz Introspector on their local machines. Detailed instructions are available here.

Why are code coverage reports public?

We work with open source projects and try to keep as much information public as possible. We believe that public code coverage reports do not put users at risk, as they do not indicate the presence of bugs or lack thereof.

Why is the coverage command complaining about format compatibility issues?

This may happen if the Docker images fetched locally become out of sync. Make sure you run the following command to pull the most recent images:

$ python infra/ pull_images

Please refer to code coverage for detailed information on code coverage generation.

What happens when I rename a fuzz target ?

If you rename your fuzz targets, the existing bugs for those targets will get closed and fuzzing will start from scratch from a fresh corpora (seed corpus only). Similar corpora will get accumulated over time depending on the number of cpu cycles that original fuzz target has run. If this is not desirable, make sure to copy the accumulated corpora from the original fuzz target (instructions to download here) and restore it to the new GCS location later (instruction to find the new location here).

Does OSS-Fuzz support AFL or honggfuzz or Centipede?

OSS-Fuzz uses the following fuzzing engines:

  1. libFuzzer.
  2. AFL++, an improved and well-maintained version of AFL.
  3. Honggfuzz.
  4. Centipede (Experimental).

Follow the new project guide and OSS-Fuzz will use all its fuzzing engines on your code.

What are the specs on your machines?

OSS-Fuzz builders have 32CPU/28.8GB RAM.

Fuzzing machines only have a single core and fuzz targets should not use more than 2.5GB of RAM.

Are there any restrictions on using test cases / corpora generated by OSS-Fuzz?

No, you can freely use (i.e. share, add to your repo, etc.) the test cases and corpora generated by OSS-Fuzz. OSS-Fuzz infrastructure is fully open source (including ClusterFuzz, various fuzzing engines, and other dependencies). We have no intent to restrict the use of the artifacts produced by OSS-Fuzz.