CVE-2026-33204 SimpleJWT has an Unauthenticated Denial of Service via JWE header tampering
An unauthenticated attacker can cause a denial of service in applications using SimpleJWT by tampering with JWE headers when PBES2 algorithms are enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-33204?
CVE-2026-33204 is a high-severity denial of service vulnerability in kelvinmo simplejwt, published on 20 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
SimpleJWT is a simple JSON web token library written in PHP. Prior to version 1.1.1, 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. This issue has been patched in version 1.1.1.
Weakness: CWE-400
Which versions are affected by CVE-2026-33204?
2 open source packages ship an affected version, including kelvinmo/simplejwt. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
kelvinmo/simplejwt Packagist | All versions | 1.1.1 |
kelvinmo/simplejwt Packagist | All versions | 1.1.1 |
Products include Kelvinmo Simplejwt.
How do I fix CVE-2026-33204?
Upgrade kelvinmo/simplejwt to 1.1.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-33204 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33204 exists.
Sources
Also known as GHSA-xw36-67f8-339x.