Go TLS KeyUpdate Flaw Enables Unauthenticated Resource-Exhaustion Attacks

A denial-of-service vulnerability in Go’s crypto/tls package, tracked as CVE-2026-56862 and GO-2026-6090, allows a remote, unauthenticated client to exhaust server resources by repeatedly sending TLS 1.3 KeyUpdate messages. The implementation incorrectly treats handshake messages as advancing connection state even after the handshake completes, allowing them to bypass the non-advancing record limit and trigger indefinite key derivation. Red Hat rates the flaw Important, with a CVSS v3.1 score of 7.5, citing high availability impact without confidentiality or integrity impact.

The fix stops treating post-handshake handshake messages as state-advancing. Affected versions include Go releases before 1.25.13, the 1.26 release line before 1.26.6, and 1.27 prereleases before 1.27.0-rc.3; organizations should upgrade affected toolchains and rebuild and redeploy services that embed the vulnerable implementation. Red Hat identifies affected Kafka exporter packages and several components marked “Will not fix,” while flightctl fixes for RHEM 1.1 and 1.2 on RHEL 9 and 10 were released on September 16. The listed Service Telemetry Framework 1.5 component is not affected because the vulnerable code is outside its execution path.