The bug in GoSE causes the HmacShaCryptor.Hash method to ignore its input and return the HMAC of the empty message under the key. As a result, every AES‑CBC‑HMAC JSON Web Encryption (JWE) token generated or verified by the library carries a constant authentication tag that is independent of the ciphertext. This nullifies the integrity guarantees of the JWE: an attacker can modify the ciphertext or forge a new token without detection.

The flaw affects the Eclipse KeyPont GoSE library. The vendor list references both Eclipse KeyPont – GOSE entries, but no specific versions are supplied. Users of GoSE for generating or validating AES‑CBC‑HMAC JWEs are therefore potentially exposed, regardless of the particular build of the library.

With a CVSS score of 8.8 the vulnerability is considered high severity. EPSS data is unavailable, and the issue is not listed in CISA’s KEV catalog, but the fact that it removes all integrity protection for JWE tokens makes exploitation straightforward for any application that trusts GoSE. The likely attack vector is via application code that creates or validates JWEs, allowing an attacker to tamper with the ciphertext or substitute a forged token without being detected.