Firebase PHP-JWT key/algorithm type confusion

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

9.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

• CVE-2021-46743: 0.92% estimated exploitation activity in the next 30 days; percentile 60.22 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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.

Published by the source

30 Mar 2022, 00:00 UTC

Last modified by the source

21 Feb 2023, 20:03 UTC

Source last checked

26 Oct 2026, 14:05 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-8xf4-w7qw-pjjw

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# Patch Next advisory brief: CVE-2021-46743

Advisory: Firebase PHP-JWT key/algorithm type confusion
IDs: CVE-2021-46743, GHSA-8xf4-w7qw-pjjw
Record: https://patchnext.com/advisories/cve-2021-46743 (revision 1, source checked 26 Oct 2026, 14:05 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: firebase/php-jwt 6.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist firebase/php-jwt: affected < 6.0.0; fixed in 6.0.0

## Suggested inspection (read-only)
- Packagist firebase/php-jwt: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on firebase/php-jwt.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-8xf4-w7qw-pjjw (GitHub-reviewed advisory GHSA-8xf4-w7qw-pjjw lists affected ranges: Packagist firebase/php-jwt >=0 <6.0.0)

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