The vulnerability arises when OpenSSL’s X.509 extension cache is updated concurrently by multiple threads. During the first simultaneous use of a trusted CA certificate, threads may build the extension cache under a read lock, then install results under a write lock. The thread that installs last frees data that an earlier thread has returned to its caller. The caller may subsequently read freed memory, causing a use‑after‑free crash. This leads to denial of service when the TLS process terminates. The weakness is classified as CWE‑416 and CWE‑825.

Affected is the OpenSSL library from the vendor OpenSSL:OpenSSL. Only version 4.0 of OpenSSL is vulnerable. Versions 3.6, 3.5, 3.4, 3.0, 1.1.1, and 1.0.2 are not affected. The vulnerability can impact any application that links to OpenSSL 4.0 when it verifies a server or client certificate against a shared trusted CA certificate and does so concurrently across multiple TLS connections.

The CVSS score of 7.5 indicates a high‑severity denial of service risk, and the EPSS score is not available, so the current exploitation probability cannot be quantified from the data. The vulnerability is not listed in CISA’s KEV catalog, but because the flaw involves memory corruption that can easily crash a process, it is likely exploitable via any remote TLS client or server connection. The attack vector is inferred to be an unauthenticated, remote TLS connection that triggers concurrent chain verification involving the same trusted CA certificate. An attacker who can connect multiple times to the target can trigger the crash, resulting in service interruption.