The flaw in erlef’s oidcc library permits an unauthenticated adversary to forge an encrypted ID token or JARM response that lacks a nested signature. The library accepts a JWE that wraps unsigned claims as completely validated, enabling the attacker to craft a token with arbitrary subject, issuer, and audience fields. An attacker possessing the relying party’s public encryption key can therefore create a spoofed token granting denial of legitimate user identity and enabling session hijacking or unauthorized access to protected resources.

vulnerable releases are erlef’s oidcc starting at 3.2.0-beta.1 up to, but not including, 3.9.0. Both variants listed in the CNA affect this range.

The CVSS score of 7.6 indicates high severity, whereas the EPSS score is unavailable, and the vulnerability is not currently listed in CISA’s KEV catalog. The weakness is exploitable remotely through the browser front‑channel path used by JARM; any party able to supply a crafted JWE that can be decrypted with the relying party’s public key can impersonate an arbitrary user without possessing the provider’s signing key, which meets the conditions outlined in OpenID Connect Core 1.0.