FastAPI Api Key has a timing side-channel in verify_key that allows statistical key validity detection
Published Jan 21, 2026 · Updated Nov 3, 2026
Original summary
Normalized public record · Updated Nov 3, 2026
FastAPI Api Key has a timing side-channel in verify_key that allows statistical key validity detection
Publisher and source text is retained separately from QFinch interpretation.
QFinch assessment
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.
Potential impact
Timing side-channel vulnerability in verifykey(). 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 keyid corresponds to a valid key, potentially accelerating brute-force or enumeration attacks. Affected: all users relying on verifykey() for API key authentication prior to the fix.
Evidence: public source recordsAffected products
| Affected package, product or range | Fix status |
|---|---|
| Consult the referenced vendor advisory for affected versions. | Consult source advisory |
Matching a version is only one part of assessing exposure. QFinch has not assessed your environment on this public page.
Remediation
Apply the vendor-supported fixed release or mitigation, then verify the affected component is no longer reachable.
- Confirm the affected product and reachable code path exist in your environment.
- Apply the vendor-supported patch or mitigation from the primary advisory.
- Rescan the exact deployed revision and retain verification evidence.
Evidence coverage
- SSVC priority
- LOW
- Confidence
- Not available
- Aliases
- None reported
Scores appear only when corresponding evidence exists. Missing data is never interpreted as zero risk.
Sources and record history
Record history
Published Jan 21, 2026 · Last refreshed Nov 3, 2026