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

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.

Published: 2026-02-20 Last update: 2026-02-20 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-26994 is rated Low Risk (26.6/100): CVSS Medium severity, with low exploitation likelihood (EPSS 0.01%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-26994

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-02-20 — 0.01% —

Full EPSS history (1 record total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-26994

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
6.5 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 2.5 [email protected]

Weakness enumeration for CVE-2026-26994

GitHub Security Advisory for CVE-2026-26994

GHSA-pmc3-p9hx-jq96 · Severity: medium · Ecosystem: go — uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

OS Trackers for CVE-2026-26994

vendor priority summary link
debian not yet assigned CVE-2026-26994 not yet assigned priority: Debian including 1 source packages (golang-refraction-networking-utls), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): open 5. https://security-tracker.debian.org/tracker/CVE-2026-26994
ubuntu medium CVE-2026-26994 medium priority: Ubuntu including 1 source packages (golang-refraction-networking-utls), 4 status rows across 4 suites (jammy, noble, questing, upstream): needs-triage 4. https://ubuntu.com/security/CVE-2026-26994

Affected software / configurations for CVE-2026-26994

Vendor Product Version Raw CPE
refraction-networking utls < 1.7.0 cpe:2.3:a:refraction-networking:utls:*:*:*:*:*:go:*:*

References for CVE-2026-26994

cvelogic Threat Intelligence