CVE-2021-46743: Google Firebase PHP-Jwt

Critical severity, CVSS 9.1. EPSS: 0.8% chance of exploitation in the next 30 days.

In Firebase PHP-JWT before 6.0.0, an algorithm-confusion issue (e.g., RS256 / HS256) exists via the kid (aka Key ID) header, when multiple types of keys are loaded in a key ring. This allows an attacker to forge tokens that validate under the incorrect key. NOTE: this provides a straightforward way to use the PHP-JWT library unsafely, but might not be considered a vulnerability in the library itself.

Affected products

• Google Firebase PHP-Jwt: before 6.0.0 (fixed in 6.0.0)

Published 2022-03-29. Last modified 2026-06-17.