Zephyr sockets created with IPPROTO_TLS_1_3 can negotiate TLS 1.2 when both TLS versions are enabled in Kconfig. Because the socket-level protocol selection is not propagated 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 IPPROTO_TLS_1_3 enforces TLS 1.3 may silently operate over TLS 1.2 and be exposed to weaker cipher suites and downgrade attacks.

Zephyr RTOS 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; any build that satisfies these conditions is potentially vulnerable.

The CVSS score of 5.3 indicates medium severity. The EPSS score is <1%, a low but non‑zero probability that the flaw could be exploited in the wild. The vulnerability is not listed in the CISA KEV catalog. The likely attack vector is a remote client that connects to a Zephyr TLS 1.3 socket and forces a TLS 1.2 session, thereby downgrading the connection. This can expose transmitted data to confidentiality or integrity risks associated with TLS 1.2 weaknesses.