CVE-2026-56862: Go Standard Library Crypto/tls

High severity, CVSS 7.5. EPSS: 0.6% chance of exploitation in the next 30 days.

Handshake messages, such as KeyUpdate, are always considered as state-advancing, regardless of whether a handshake has been completed or not. As a result, a malicious client can keep sending KeyUpdate messages to force the server to keep performing key derivation operations indefinitely.

Affected products

• Go Standard Library Crypto/tls: before 1.25.13 (fixed in 1.25.13); from 1.26.0-0, before 1.26.6 (fixed in 1.26.6); from 1.27.0-0, before 1.27.0-rc.3 (fixed in 1.27.0-rc.3)

Published 2026-08-13. Last modified 2026-09-03.