A specially crafted PKCS#7 or S/MIME signed message can trigger a use‑after‑free during signature verification in OpenSSL's PKCS7_verify function. The vulnerability can lead to crashes, heap corruption, or potentially remote code execution if an attacker controls the message content. The flaw stems from an incorrectly freed caller‑owned BIO when the SignedData digestAlgorithms field is an empty ASN.1 SET, causing subsequent use of that BIO to reference freed memory.

The flaw affects the OpenSSL library itself, including all non‑FIPS builds that use the PKCS#7 APIs. The issue is not present in the FIPS modules in OpenSSL 4.0, 3.6, 3.5, 3.4, or 3.0. Any application that processes PKCS#7 or S/MIME signed messages via the PKCS#7 interface is potentially vulnerable, whereas applications using the CMS APIs are not impacted.

The CVSS score is 9.8, indicating critical severity, and the EPSS is not available, so the exact exploitation likelihood remains uncertain. The vulnerability is listed in no CISA KEV catalog. The attack vector is inferred to be remote, with an attacker delivering a malicious signed message to an OpenSSL‑based application. Successful exploitation requires that the application later frees the affected BIO, leading to a crash or memory corruption which could be leveraged for code execution in some contexts.