A feature flag can control rollout, but it must not be the authorization mechanism for a protected action. Audit security-relevant flags by inventorying them, attempting to manipulate client-visible state, and verifying that the backend independently enforces access. Then test configuration exposure, administration, outages, inconsistent evaluations, rollback, and stale code paths.
Which feature flags need priority review?
Start with flags that can change security behavior or expose sensitive operations. OWASP’s Web Security Testing Guide (WSTG), test WSTG-CONF-15, identifies flags affecting authentication, multifactor authentication, authorization, fraud detection, rate limiting, risk-based authentication, account recovery, administration, and security monitoring.
For each flag, record its identifier, owner, purpose, environments, evaluation locations, targeting rules, consumers, and the code paths or services it influences. Mark any flag tied to a security control or sensitive operation for deeper testing. Include flags evaluated in application code and those controlled through a management service; a user interface may reveal only part of the relevant configuration.
Can users bypass a feature flag?
They may be able to manipulate client-side state or call a backend operation without using the interface that a flag hides. That should not grant access. OWASP’s WSTG expected result is that the server enforces authorization independently of client-side flag state: an unauthorized user is denied, for example with 401 Unauthorized or 403 Forbidden, even if the flag is manipulated.
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Test the interface and the protected operation
- Identify a high-risk flag and the action it gates, such as viewing an administrative record or invoking a restricted endpoint.
- Using a low-privilege test identity, observe the normal interface and record the request made when the action is attempted.
- Change the client-side flag value where possible, or otherwise expose the gated interface, using browser developer tools or an intercepting proxy.
- Call the underlying API or backend handler directly with the same low-privilege identity. Do not treat a hidden button or absent page as evidence that access is denied.
- Repeat for every endpoint, service, or message handler that implements the protected action. Record the actual response and compare it with the expected authorization result.
OWASP’s Developer Guide access-control checklist recommends placing access-control checks on the server side, at a gateway, or in serverless functions. The practical test is whether each protected operation rejects an unauthorized identity irrespective of the client’s flag value.
What flag data can leak to clients?
Inspect API responses, JavaScript bundles, source maps where available, and administration interfaces. Look for unreleased feature names, internal service names, employee or test targeting cohorts, and URLs or descriptions that expose implementation details. Such data may disclose how a feature is built or who it targets even if it does not directly grant access.
Return only the flags relevant to the current user and context rather than sending the full configuration to every client. Treat client-delivered flag data as visible to the user; do not put secrets or sensitive configuration values there.
Who can change or publish flags?
Review the complete flag lifecycle: who can create, read, change, approve, and publish flags. Apply least privilege and fine-grained access, and log administrative changes and authorization events. A flag console is part of the security boundary when its settings can affect access or other controls.
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What should happen during outages, inconsistent state, or rollback?
Exercise conditions in which the flag service is unavailable, returns stale data, or evaluates the same flag differently across services or instances. For every security-relevant flag, document the intended fallback and verify it under test. A fallback appropriate for a cosmetic rollout is not automatically appropriate for an authentication or authorization control.
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Check rollback as a paired change: an older code version must not run with a mismatched, permissive security configuration. OWASP WSTG explicitly calls out service failure, inconsistent state, and coupling code rollback with configuration rollback as audit concerns. Confirm what configuration is restored, who can restore it, and whether the affected code and security settings return to a compatible state.
How do you find and retire stale flags?
Search both the codebase and flag-management service for flags whose rollout is complete or which are no longer actively changed. For each one, determine whether the gated code remains reachable. If it does, check that the path remains patched and that authorization still holds; an old or inactive flag does not make its code harmless.
When safe, remove the stale flag and obsolete gated path together. Retest the affected operation after removal so that cleanup does not leave an unintended route or weaken a server-side check.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should the audit be tested and recorded?
OWASP WSTG describes black-box testing, such as comparing behavior across rollout states, replaying requests, and observing timing, and gray-box testing, which includes inspecting the flag-management system and toggling states directly. Combining the approaches helps reveal both externally exploitable behavior and inconsistent internal enforcement.
WSTG lists Burp Suite, ZAP, browser developer tools, and JavaScript bundle analyzers as relevant software tools. These are examples, not a requirement to use a particular product. Choose tools that let the team inspect client responses, replay requests, and examine flag configuration within its authorized test environment.
Keep an audit record with enough detail to reproduce and verify each finding:
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- Flag identifier, owner, security purpose, and affected routes or services.
- Test identity and privilege level, manipulated state, observed response, and expected response.
- Outage, stale-state, inconsistency, and rollback behavior observed.
- Evidence reference, remediation owner, and retest result.
These record fields are practical documentation for the tests above; adapt them to local policy.
What does a secure implementation look like?
Compare the implementation and any proposed remediation across the failure modes, rather than asking only whether the flag is client-side or server-side. The same flag may be evaluated in multiple places, so assess each location and the operation it influences.
| Audit dimension | Question to answer |
|---|---|
| Evaluation location | Is the flag evaluated on the client, the server, or both, and which component makes the access decision? |
| Independent authorization | Does the backend authorize each protected action even when client state is altered? |
| Failure behavior | What happens when the flag service or configuration is unavailable: does the security control fail open or fail closed, and is that behavior intentional? |
| Consistency | Can services or instances evaluate different states, and what is the effect on the protected operation? |
| Configuration exposure | Can clients see irrelevant rules, targeting data, or implementation details? |
| Audit and rollback | Are changes attributable and logged, and can matching configuration be restored with code? |
OWASP’s Application Security Verification Standard (ASVS) 5.0 configuration content provides additional verification context. The linked ASVS content is on the project’s mutable master branch, so teams should check the current version and their own requirements when applying it.
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