A controlled CMS message can trigger an 8‑byte deterministic out‑of‑bounds write on the heap when OpenSSL performs CMS decryption. The vulnerability arises from the way the key‑unwrap output buffer is sized when the AES‑WRAP‑PAD algorithm is used. The unwrapped key size reported during query is smaller than the actual number of bytes the unwrap primitive may write, causing a fixed zero write beyond the allocated memory. This overflow corrupts the heap and normally terminates the process, giving an attacker the means to crash any system that accepts the malformed CMS payload.

The flaw exists in the CMS module of OpenSSL. All releases prior to the fix commit that appears in the provided commit history are affected; there is no version list supplied, so any OpenSSL build using CMS decryption without the patch is vulnerable.

Because CMS_decrypt is a public function and no special configuration is required, the flaw is readily exploitable in environments that decrypt arbitrary CMS data. The problem is deterministic, and the vulnerability does not rely on user interaction or privilege escalation. No EPSS value is available, and the vulnerability is not listed in CISA's KEV catalog, but the lack of mitigation in the affected code implies a high likelihood of successful denial‑of‑service attacks. The CVSS score of 7.5 indicates high severity.