CVE-2026-34240 jose vulnerable to untrusted JWK header key acceptance during signature verification
Attackers can forge valid JWT tokens by embedding their own public key in the header, bypassing signature verification in the JOSE library.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-34240?
CVE-2026-34240 is a high-severity authentication bypass vulnerability in appsup-dart jose, published on 31 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
JOSE is a Javascript Object Signing and Encryption (JOSE) library. Prior to version 0.3.5+1, a vulnerability in jose could allow an unauthenticated, remote attacker to forge valid JWS/JWT tokens by using a key embedded in the JOSE header (jwk). The vulnerability exists because key selection could treat header-provided jwk as a verification candidate even when that key was not present in the trusted key store. Since JOSE headers are untrusted input, an attacker could exploit this by creating a token payload, embedding an attacker-controlled public key in the header, and signing with the matching private key. Applications using affected versions for token verification are impacted. This issue has been patched in version 0.3.5+1. A workaround for this issue involves rejecting tokens where header jwk is present unless that jwk matches a key already present in the application's trusted key store.
Weakness: CWE-347
Which versions are affected by CVE-2026-34240?
2 open source packages ship an affected version, including jose. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
jose Pub | All versions | 0.3.5+1 |
jose Pub | All versions | 0.3.5+1 |
Products include Appsup-Dart Jose.
How do I fix CVE-2026-34240?
Upgrade jose to 0.3.5+1 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-34240 being exploited?
There is no known exploitation of CVE-2026-34240 in the wild and no public exploit code.
Sources
Also known as GHSA-vm9r-h74p-hg97.