CVE-2026-23996 FastAPI Api Key has a timing side-channel in verify_key that allows statistical key validity detection

What is CVE-2026-23996?

CVE-2026-23996 is a low-severity vulnerability in Athroniaeth fastapi-api-key, published on 21 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

FastAPI Api Key provides a backend-agnostic library that provides an API key system. Version 1.1.0 has a 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. All users relying on verify_key() for API key authentication prior to the fix are affected. Users should upgrade to version 1.1.0 to receive a patch. The patch applies a uniform random delay (min_delay to max_delay) to all responses regardless of outcome, eliminating the timing correlation. Some workarounds are available. Add an application-level fixed delay or random jitter to all authentication responses (success and failure) before the fix is applied and/or use rate limiting to reduce the feasibility of statistical timing attacks.

Weakness: CWE-208

Which versions are affected by CVE-2026-23996?

3 open source packages ship an affected version, including fastapi-api-key. It affects 1 product.

Products include Athroniaeth Fastapi Api Key.

How do I fix CVE-2026-23996?

Upgrade fastapi-api-key to 1.1.0 or later.

Is CVE-2026-23996 being exploited?

There is no known exploitation of CVE-2026-23996 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-95c6-p277-p87g, PYSEC-2026-1358.