An integer underflow in the mlen parameter of crypto_sign_open() causes a buffer overflow while processing a signature. This overflow occurs in the CROSS implementation of the post‑quantum signature algorithm and can corrupt memory used by the process. If an attacker supplies a specially crafted signature, the overflow could result in arbitrary code execution, privilege escalation, or denial of service. The vulnerability is classified as stack‑based buffer overflow (CWE‑121) and general buffer overflow (CWE‑122).

The issue affects all versions of CROSS‑implementation from the CROSS‑signature project that have not incorporated the fix commit fc6b7e7. This includes any deployments that utilize the reference or optimized implementations of the CROSS algorithm prior to applying the patch.

The CVSS score of 6.9 indicates a moderate risk severity. Though the EPSS score is not available, the lack of listing in the CISA KEV catalog suggests that widespread exploitation has not yet been observed. Nevertheless, because the vulnerability is triggered by a crafted input to a cryptographic routine, the likely attack vector would involve an attacker sending malicious signatures over an exposed interface that invokes crypto_sign_open().