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CRITICAL severity

CVE-2021-46743 — php-jwt

CRITICAL

CVE-2021-46743 is a critical-severity (CVSS 9.1) CWE-843 vulnerability in firebase/php-jwt. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for firebase/php-jwt — see the affected versions and patch details below.

Firebase PHP-JWT key/algorithm type confusion

Also known asGHSA-8xf4-w7qw-pjjw
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
1 known
Exploitation data as of Nov 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
0.9%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs60th percentile — riskier than 60% of all scored CVEsHighest risk
0.14%0.57%0.99%1.42%0.6%0.9%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐘firebase/php-jwt

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

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 Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐘Packagistfirebase/php-jwtall versions6.0.0composer require firebase/php-jwt:^6.0.0

Affected Products

1 product · 1 configurations
Application
firebase php-jwtgoogle
< 6.0.0
range
Exploits & PoCs
1

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

In Firebase PHP-JWT before 6.0.0, an algorithm-confusion issue (e.g., RS…

github.comNVDApr 2022

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for firebase/php-jwt, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update firebase/php-jwt to 6.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-46743 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

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.
O3 Security · Impact-Aware SCA

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CVE-2021-46743: php-jwt — Fixed in 6.0.0