Rustls TLS 1.3 handshake message validation bypass across encryption levels

Severity: medium · CVSS 5.3 · Published 2026-10-05

Rustls, a TLS library used in many applications, incorrectly accepts TLS 1.3 handshake messages sent at the wrong encryption level when they appear in the same record as a key-changing message. While the handshake remains cryptographically authenticated, an attacker positioned on the network path could inject plaintext messages that bypass encryption requirements, potentially allowing protocol manipulation.

Technologies: rustls (crates.io), Rustls. Vendors: crates.io, Rustls.

Quick answers

Is This vulnerability exploited in the wild?
No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.
How severe is This vulnerability?
Medium severity, with a CVSS score of 5.3, published 5 October 2026.
Which products and versions are affected?
rustls (crates.io) >= 0.23.13, < 0.23.45; Rustls 0.23.13 through 0.23.44.
Is there a fix for This vulnerability?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is This vulnerability attacked?
It is attacked over the network, in a software library.
Am I affected?
Check whether you run rustls (crates.io) or Rustls at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability exists in Rustls's record deframing logic, which fails to enforce RFC 8446 Section 5.1's requirement that handshake messages must align with record boundaries across key changes. An attacker can pack a plaintext EncryptedExtensions message in the same record as a ServerHello, which rustls will process after installing handshake keys, treating the plaintext message as valid. The handshake transcript remains authenticated, preventing message modification, but messages that should be encrypted are incorrectly accepted in plaintext.

Affected products

  • Rustls Rustls 0.23.13 through 0.23.44

Identifiers

  • GHSA-2mjx-qc3c-rqvc
  • RUSTSEC-2026-0285

Timeline

  • 2026-10-05: disclosed

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