The vulnerability in the JOSE library permits an attacker to forge signed JSON Web Tokens by embedding a malicious public key in the token header and using the matching private key for signing. This bypasses normal verification because the library mistakenly accepts the header‑provided key as a valid verification candidate, even when that key is not part of the trusted key store. The result is that an unauthenticated remote actor can generate tokens that the application will accept as legitimate, enabling unauthorized privilege escalation or access to protected resources. The weakness is a broken trust model in key selection, identified as CWE‑347.

Affected deployments are those using the JOSE library (appsup‑dart:jose) in versions prior to 0.3.5+1. Ecosystem components that rely on JOSE for validating JWS or JWT payloads are vulnerable until upgraded to the patched release or until the workaround is applied.

The CVSS score of 7.5 indicates high severity, while the EPSS value of less than 1% suggests low current exploitation probability. The vulnerability is not listed in the CISA KEV catalog, indicating no widespread public exploitation to date. An attacker can achieve this by merely crafting a token with an attacker‑controlled public key in the header and providing the signed payload; no special privileges are needed on the target system. Therefore, the risk is significant for any application that accepts JOSE‑signed tokens without independent key validation.