What’s Vulnerable › Vulnerabilities

CVE-2021-46743: Firebase PHP-JWT key/algorithm type confusion

• Severity: critical (CVSS 9.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.8% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2022-03-29 · updated 2026-06-17
• Weakness: CWE-843

Description

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
• Packagist Firebase/Php-Jwt: before 6.0.0; 1.0.0, 2.0.0, v2.1.0, v2.2.0, v3.0.0, v4.0.0 (+8) — fixed in 6.0.0

How to fix

Upgrade to a fixed version: Google Firebase Php-Jwt 6.0.0; Packagist Firebase/Php-Jwt 6.0.0. Check the vendor advisory for mitigations.

References

• https://github.com/firebase/php-jwt/issues/351
• https://github.com/firebase/php-jwt/issues/351

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