GHSA-pmc3-p9hx-jq96: uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

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1. GitHub Advisory Database
2. GitHub Reviewed
3. GHSA-pmc3-p9hx-jq96

uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

Moderate severity GitHub Reviewed Published Apr 23, 2025 in refraction-networking/utls • Updated Apr 23, 2025

Package

gomod github.com/refraction-networking/utls (Go)

Affected versions

< 1.7.0

Description

Description

Before version 1.7.0, utls did not implement the TLS 1.3 downgrade protection mechanism specified in RFC 8446 Section 4.1.3 when using a utls ClientHello spec. This allowed an active network adversary to downgrade TLS 1.3 connections initiated by a utls client to a lower TLS version (e.g., TLS 1.2) by modifying the ClientHello message to exclude the SupportedVersions extension, causing the server to respond with a TLS 1.2 ServerHello (along with a downgrade canary in the ServerHello random field). Because utls did not check the downgrade canary in the ServerHello random field, clients would accept the downgraded connection without detecting the attack. This attack could also be used by an active network attacker to fingerprint utls connections.

Fix Commit or Pull Request

refraction-networking/utls#337, specifically refraction-networking/utls@f889276

References

• refraction-networking/utls#181

References

• GHSA-pmc3-p9hx-jq96
• refraction-networking/utls#181
• refraction-networking/utls#337
• refraction-networking/utls@f889276

Published to the GitHub Advisory Database

Apr 23, 2025

Last updated

Apr 23, 2025

EPSS score