In Zephyr RTOS, when sockets are created with the IPPROTO_TLS_1_3 protocol, the intended enforcement of TLS 1.3 is not guaranteed because the protocol version selection is not correctly communicated to the underlying mbedTLS library. Consequently, the ClientHello message advertises both TLS 1.2 and TLS 1.3, allowing the peer to negotiate a TLS 1.2 connection. This behavior results in an algorithm downgrade vulnerability where TLS 1.2 is used despite the application's expectation of TLS 1.3, potentially exposing the connection to vulnerabilities specific to TLS 1.2. The issue arises when both TLS versions are enabled in the Kconfig configuration. The vendor has not provided an official patch or remediation at this time.

The vulnerability allows a downgrade from TLS 1.3 to TLS 1.2 during the TLS handshake, which may expose applications to known weaknesses in TLS 1.2. Confidentiality could be partially impacted due to the use of a less secure protocol version. There is no indication of integrity or availability impact. No known exploits are reported in the wild.

No official patch or fix has been published yet. As a workaround, restrict the TLS_CIPHERSUITE_LIST socket option to only include TLS 1.3 cipher suites to prevent negotiation of TLS 1.2 connections. Monitor the vendor advisory for updates and official fixes.