GHSA-pmc3-p9hx-jq96

uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

Source: GitHub Advisory Database (reviewed) Moderate CVE-2026-26994

Packages

Ecosystem	Package	Affected	Fixed in
Go	github.com/refraction-networking/utls	< 1.7.0	1.7.0

Details

As written in the advisory (Markdown source, shown as plain text).

### 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

References

External links from the advisory. They lead to third-party sites.