The vulnerability is a heap buffer overflow in liboqs’ XMSS and XMSS^MT signature verification code. When a verification call is made with a signature buffer sized for the declared algorithm but a public key whose OID identifies a different XYSS parameter set with a larger signature length, the code re‑parses the OID, selects the larger expected signature length, and indexes into the caller‑supplied buffer. This results in an out‑of‑bounds read (CWE‑125) that consumes data only for internal hash computation. No attacker‑controlled data is returned, so there is no disclosure, but the read may trigger a crash if it crosses into unmapped memory, causing a denial of service.

The affected product is open‑quantum‑safe’s liboqs library. Versions prior to 0.16.0 are vulnerable; the bug is fixed in the 0.16.0 release. All builds of the library that include the XMSS or XMSS^MT interfaces before this version are impacted.

The CVSS score is 5.3, indicating moderate severity. The EPSS score is not available, so the exploitation probability is not quantified, and the vulnerability is not listed in CISA’s KEV catalog. The impact is limited to a crash of the process performing verification, and the vulnerability is exploitable only when an attacker can supply a crafted signature and public key to the verification routine. In configurations where the library processes untrusted inputs—such as network services or cryptographic validation APIs—an attacker could trigger a denial of service. No known exploits have been reported, but the vulnerability could be abused in environments where maintaining service availability is critical.