FastAPI Api Key has a timing side-channel in verify_key that allows statistical key validity detection

Impact

Timing side-channel vulnerability in verify_key(). The method applied a random delay only on verification failures, allowing an attacker to statistically distinguish valid from invalid API keys by measuring response latencies. With enough repeated requests, an adversary could infer whether a key_id corresponds to a valid key, potentially accelerating brute-force or enumeration attacks.

Affected: all users relying on verify_key() for API key authentication prior to the fix.

Patches

Yes. Users should upgrade to version 1.1.0 (or the version containing this fix). The patch applies a uniform random delay (min_delay to max_delay) to all responses regardless of outcome, eliminating the timing correlation.

Workarounds

• Add an application-level fixed delay or random jitter to all authentication responses (success and failure) before the fix is applied.
• Use rate limiting to reduce the feasibility of statistical timing attacks.

References

• CWE-208: Observable Timing Discrepancy
• Commit: 87b27640f77c5ef86c46311b6b5a7e2887e35b77
• OWASP: https://owasp.org/www-community/attacks/Timing_attack