The flaw causes the HMAC-BLAKE2 finalization functions to discard the message when the supplied key is longer than the BLAKE2 block size. As a result, the MAC computed depends solely on the key and not on the authenticated data, allowing an attacker to forge a valid MAC for any message. This undermines the integrity guarantees that HMAC is meant to provide.

wolfSSL libraries, specifically the HMAC‑BLAKE2 API introduced in version 5.9.0. The issue affects any build that uses these functions with keys exceeding the block size. Versions prior to 5.9.0 are not impacted, while the status of later versions is not specified in the data.

The CVSS score of 5.9 places this vulnerability in the medium severity range. EPSS data is unavailable and it is not listed in the CISA KEV catalog, indicating no publicly known exploitation at present. Nevertheless, the ability to forge a MAC can lead to severe consequences for applications that rely on HMAC‑BLAKE2 for authentication, and the attack vector is plausible through any channel that allows an attacker to provide a key longer than the block size and invoke the affected functions.