Skip to content

Web API Fuzz Testing

DETAILS: Tier: Ultimate Offering:, Self-managed, GitLab Dedicated

Web API fuzzing performs fuzz testing of API operation parameters. Fuzz testing sets operation parameters to unexpected values in an effort to cause unexpected behavior and errors in the API backend. This helps you discover bugs and potential security issues that other QA processes may miss.

We recommend that you use fuzz testing in addition to GitLab Secure's other security scanners and your own test processes. If you're using GitLab CI/CD, you can run fuzz tests as part your CI/CD workflow.

For an overview, see Web API Fuzzing.

When Web API fuzzing runs

Web API fuzzing runs in the fuzz stage of the CI/CD pipeline. To ensure API fuzzing scans the latest code, your CI/CD pipeline should deploy changes to a test environment in one of the stages preceding the fuzz stage.

If your pipeline is configured to deploy to the same web server on each run, running a pipeline while another is still running could cause a race condition in which one pipeline overwrites the code from another. The API to scan should be excluded from changes for the duration of a fuzzing scan. The only changes to the API should be from the fuzzing scanner. Any changes made to the API (for example, by users, scheduled tasks, database changes, code changes, other pipelines, or other scanners) during a scan could cause inaccurate results.

You can run a Web API fuzzing scan using the following methods:

Example projects using these methods are available:

Get support or request an improvement

To get support for your particular problem use the getting help channels.

The GitLab issue tracker on is the right place for bugs and feature proposals about API Security and API Fuzzing. Use ~"Category:API Security" label when opening a new issue regarding API fuzzing to ensure it is quickly reviewed by the right people. Refer to our review response SLO to understand when you should receive a response.

Search the issue tracker for similar entries before submitting your own, there's a good chance somebody else had the same issue or feature proposal. Show your support with an emoji reaction or join the discussion.

When experiencing a behavior not working as expected, consider providing contextual information:

  • GitLab version if using a self-managed instance.
  • .gitlab-ci.yml job definition.
  • Full job console output.
  • Scanner log file available as a job artifact named gl-api-security-scanner.log.

WARNING: Sanitize data attached to a support issue. Remove sensitive information, including: credentials, passwords, tokens, keys, and secrets.


  • Assert: Assertions are detection modules used by checks to trigger a fault. Many assertions have configurations. A check can use multiple Assertions. For example, Log Analysis, Response Analysis, and Status Code are common Assertions used together by checks. Checks with multiple Assertions allow them to be turned on and off.
  • Check: Performs a specific type of test, or performed a check for a type of vulnerability. For example, the JSON Fuzzing Check performs fuzz testing of JSON payloads. The API fuzzer is comprised of several checks. Checks can be turned on and off in a profile.
  • Fault: During fuzzing, a failure identified by an Assert is called a fault. Faults are investigated to determine if they are a security vulnerability, a non-security issue, or a false positive. Faults don't have a known vulnerability type until they are investigated. Example vulnerability types are SQL Injection and Denial of Service.
  • Profile: A configuration file has one or more testing profiles, or sub-configurations. You may have a profile for feature branches and another with extra testing for a main branch.