The vulnerability in Zephyr RTOS arises because sockets created with IPPROTO_TLS_1_3 do not enforce TLS 1.3 exclusively. The socket-level protocol selection is not communicated to mbedTLS, which leads to the ClientHello message advertising both TLS 1.2 and TLS 1.3. This allows the peer to negotiate a TLS 1.2 connection, undermining the intended security guarantees of TLS 1.3. The issue occurs when both TLS 1.2 and TLS 1.3 are enabled in the Kconfig configuration. The lack of enforcement means applications assuming TLS 1.3 is used may be exposed to vulnerabilities inherent to TLS 1.2. The recommended workaround is to limit the TLS_CIPHERSUITE_LIST socket option to TLS 1.3-only cipher suites to prevent fallback to TLS 1.2.

Applications using Zephyr sockets with IPPROTO_TLS_1_3 may unintentionally negotiate TLS 1.2 connections, exposing them to known weaknesses of TLS 1.2. This reduces the confidentiality guarantees expected from TLS 1.3 and may allow attackers to exploit vulnerabilities specific to TLS 1.2. There is no indication of integrity or availability impact. No known exploits in the wild have been reported.

No official patch or fix has been published yet. As a workaround, restrict the TLS_CIPHERSUITE_LIST socket option to only TLS 1.3 cipher suites to prevent negotiation of TLS 1.2 connections. Monitor the vendor advisory for updates regarding an official fix or patch. Patch status is not yet confirmed — check the vendor advisory for current remediation guidance.