Block SSRF with a Fetch Allowlist
Validate every outbound URL against a host allowlist before fetching so a user-supplied URL cannot reach internal metadata endpoints.
Task
Implement if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked'); to close a known attack surface before the next security review finds it.
When to use this
A penetration test report flags the missing if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked'); as a medium-severity finding on a public endpoint.
Prerequisites
- A running application with the vulnerable endpoint
- Access to security headers or middleware config
Steps
1. Configure if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked'); in the project configuration file with the recommended defaults.
if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked');
Expected output: The configuration is accepted and if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked'); runs without errors on the first invocation.
2. Run the check locally to verify if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked'); produces the expected result before pushing.
pnpm test
Expected output: Local run matches CI behavior; no surprises after push.
3. Add the gate to the CI pipeline so if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked'); blocks merge on failure.
- name: Run add ssrf allowlist fetch
run: pnpm test
Expected output: Pull requests with failing if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked'); are blocked from merging.
Troubleshooting
Problem
The check passes locally but fails in CI
Solution
Pin dependency versions and the runtime to match CI exactly; reproduce with the same Docker image or act.
Problem
Configuration is rejected with a schema error
Solution
Check the tool version matches the config schema; older versions may not support newer fields.
Limitations
- The gate only runs on events that trigger the workflow; locally pushed commits bypass it.
- Configuration drift is possible if someone edits settings outside version control.
Close the attack surface
Start by measuring the problem. Validate every outbound URL against a host allowlist before fetching so a user-supplied URL cannot reach internal metadata endpoints. The most common mistake is adding the gate without first confirming it catches the issue you care about. Run if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error(‘blocked’); against the current codebase and inspect the output before wiring it into CI.
if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error('blocked');Verify and monitor
Once the gate is in place, make failures actionable. A failing if (!ALLOWED_HOSTS.has(new URL(url).hostname)) throw new Error(‘blocked’); should point to the exact file and line, not a generic error. Pin the tool version in the lockfile so a upstream release does not silently change behavior. Review the gate output in every pull request just like a test result.