Zephyr sockets created with IPPROTO_TLS_1_3 can negotiate TLS 1.2 when both versions are enabled in Kconfig. Because the socket-level protocol selection is not passed to mbedTLS, the ClientHello advertises both TLS 1.2 and 1.3, allowing the peer to establish a TLS 1.2 connection. Applications that assume that IPPROTO_TLS_1_3 enforces TLS 1.3 may silently operate over TLS 1.2 and be exposed to TLS 1.2 weaknesses such as weaker cipher suites and downgrade attacks.

Zephyr RTOS (Zephyr project) is affected. The issue occurs when sockets are created with IPPROTO_TLS_1_3 while the Zephyr configuration enables both TLS 1.2 and 1.3 in Kconfig. No specific version range is listed in the advisory; any build that satisfies these conditions is potentially vulnerable.

The CVSS score of 5.3 indicates a medium severity vulnerability. EPSS is not available, but the vulnerability is not listed in CISA KEV, suggesting limited known exploitation. A likely attack vector is a remote client that connects to the TLS 1.3 socket and forces a TLS 1.2 session, thereby lowering the security of the connection. As the downgrade can occur without authentication, the risk is primarily to confidentiality and integrity of data transmitted over the affected sockets.