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Vulnerability · Published Feb 20, 2026 · Assigned by GitHub_M

CVE-2026-26994uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

Medium 6.5

In plain English

A medium-severity security flaw in utls from refraction-networking. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 23% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.5Medium
Exploit odds (30 days)0.3%top 77.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12golang-refraction-networking-utlsno fix listedDEBIAN-CVE-2026-26994
Debian:13golang-refraction-networking-utlsno fix listedDEBIAN-CVE-2026-26994
Debian:14golang-refraction-networking-utlsno fix listedDEBIAN-CVE-2026-26994
Gogithub.com/refraction-networking/utls1.7.0GHSA-pmc3-p9hx-jq96
Gogithub.com/refraction-networking/utls1.7.0GO-2025-3638
Ubuntu:22.04:LTSgolang-refraction-networking-utlsno fix listedUBUNTU-CVE-2026-26994
Ubuntu:24.04:LTSgolang-refraction-networking-utlsno fix listedUBUNTU-CVE-2026-26994
Ubuntu:25.10golang-refraction-networking-utlsno fix listedUBUNTU-CVE-2026-26994
Ubuntu:26.04:LTSgolang-refraction-networking-utlsno fix listedUBUNTU-CVE-2026-26994
Ubuntu:26.10golang-refraction-networking-utlsno fix listedUBUNTU-CVE-2026-26994

Affected products (as reported)

VendorProductVersions
refraction-networking utls < 1.7.0

Weakness: CWE-693 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

References