CVE-2026-1677: Zephyrproject Zephyr

Medium severity, CVSS 5.3. EPSS: 0.2% chance of exploitation in the next 30 days.

Zephyr sockets created with `IPPROTO_TLS_1_3` can still negotiate a TLS 1.2 connection when both TLS versions are enabled in Kconfig, because the socket-level protocol selection is not propagated to mbedTLS (e.g. via `mbedtls_ssl_conf_min_tls_version`). The ClientHello advertises both versions and the peer can establish TLS 1.2, so applications that assumed `IPPROTO_TLS_1_3` enforces TLS 1.3 may silently use TLS 1.2 and remain exposed to TLS 1.2-specific weaknesses. As a workaround, the `TLS_CIPHERSUITE_LIST` socket option can be restricted to TLS 1.3-only cipher suites.

Affected products

• Zephyrproject Zephyr: up to and including 4.3.0

Published 2026-05-11. Last modified 2026-07-08.