TLS 1.3 handshake messages incorrectly accepted across encryption level boundaries

Rustls accepted TLS 1.3 handshake messages sent at the wrong encryption level when they followed a key-changing message in the same record. For example, a plaintext `EncryptedExtensions` message packed into the same record as the `ServerHello` was accepted. RFC 8446 section 5.1 requires that handshake messages do not span key changes, and that implementations terminate the connection with an "unexpected_message" alert if they do. The handshake transcript is still authenticated, so a network-position attacker cannot use this to alter or complete a handshake; the practical effect is that a peer could send handshake messages that should be encrypted in plaintext without rustls rejecting the connection. This is functionally the same bug as Go's [GO-2026-4340](https://pkg.go.dev/vuln/GO-2026-4340) (CVE-2025-61730).

Source published

14 Sep 2026 12:00:00 UTC

Source modified

12 Oct 2026 18:00:10 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

OSV · upstream source license applies

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N