SimpleJWT has an Unauthenticated Denial of Service via JWE header tampering

Exploitation

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

CVSS

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

EPSS

• CVE-2026-33204: 0.49% estimated exploitation activity in the next 30 days; percentile 42.13 (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.

Summary An unauthenticated attacker can perform a Denial of Service via JWE header tampering when PBES2 algorithms are used. Applications that call JWE::decrypt() on attacker-controlled JWEs using PBES2 algorithms are affected. Details PHP version: PHP 8.4.11 SimpleJWT version: v1.1.0 The relevant portion of the vulnerable implementation is shown below (PBES2.php): The security flaw lies in the lack of input validation when handling JWEs that uses PBES2. A "sanity ceiling" is not set on the iteration count, which is the parameter known in the JWE specification as p2c (RFC7518). The library calls decryptKey() with the untrusted input $headers which then use the PHP function hashpbkdf2() with the user-supplied value $headers['p2c']. This results in an algorithmic complexity denial-of-service (CPU exhaustion) because the PBKDF2 iteration count is fully attacker-controlled. Because the header is processed before successful decryption and authentication, the attack can be triggered using…

Published by the source

18 Mar 2026, 20:16 UTC

Last modified by the source

25 Mar 2026, 18:12 UTC

Source last checked

26 Oct 2026, 14:39 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-xw36-67f8-339x

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# Patch Next advisory brief: CVE-2026-33204

Advisory: SimpleJWT has an Unauthenticated Denial of Service via JWE header tampering
IDs: CVE-2026-33204, GHSA-xw36-67f8-339x
Record: https://patchnext.com/advisories/cve-2026-33204 (revision 1, source checked 26 Oct 2026, 14:39 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: kelvinmo/simplejwt 1.1.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist kelvinmo/simplejwt: affected <= 1.1.0; fixed in 1.1.1

## Suggested inspection (read-only)
- Packagist kelvinmo/simplejwt: 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 kelvinmo/simplejwt.
- 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-xw36-67f8-339x (GitHub-reviewed advisory GHSA-xw36-67f8-339x lists affected ranges: Packagist kelvinmo/simplejwt >=0 <=1.1.0)

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