uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

中 6.5CVE-2026-26994 · GHSA-pmc3-p9hx-jq96 · GO-2025-3638 · 20/02/2026 公開 · 17/06/2026 更新

要点

CVE-2026-26994 は github.com/refraction-networking/utls, utls (refraction-networking) に影響する深刻度「中」 (CVSS 6.5) の脆弱性です。
今後30日間に悪用される確率（EPSS）：0.30%（全脆弱性の 23.0% より高い）。
対処：1.7.0 以降に更新してください。

パッケージ	影響を受けるバージョン	修正済みバージョン
github.com/refraction-networking/utls Go	`≥ 1.0.0, < 1.7.0< 1.7.0`	`1.7.0`
utls refraction-networking	`< 1.7.0`	`1.7.0`

概要

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@f8892761e2a4d29054264651d3a86fda83bc83f9

References

https://github.com/refraction-networking/utls/issues/181

NVD の説明

uTLS is a fork of crypto/tls, created to customize ClientHello for fingerprinting resistance while still using it for the handshake. In versions 1.6.7 and below, 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. This issue has been fixed in version 1.7.0.