Connect your agent
AdvisoriesCriticalCheck your lockfilePluginHow it worksPricing Connect your agent

Advisory record · GHSA-pmc3-p9hx-jq96

uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

CVSS 6.5 MediumEPSS 0.0030 · 23rd percentile

Published by the GitHub Advisory Database at 23 Apr 2025, 14:43 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

PackageEcosystemAffectedFixed in
github.com/refraction-networking/utlsGo<1.7.01.7.0
Identifiers
GHSA-pmc3-p9hx-jq96 · CVE-2026-26994 · GO-2025-3638
Weakness (CWE)
CWE-693
Tags
Published
23 Apr 2025
Updated
10 Sept 2026

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

uTLS (github.com/refraction-networking/utls) versions before 1.7.0 do not check the TLS 1.3 downgrade canary in the ServerHello random field when a utls ClientHello spec is used. An active network attacker can alter the ClientHello to remove the SupportedVersions extension, forcing a downgrade to TLS 1.2 that the client accepts without detecting it, and the attack can also be used to fingerprint utls connections. Upgrade to version 1.7.0 or later.

Written 1 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

ScoreRatingCVSSVectorSource
6.5 Medium 3.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N CNA (GitHub_M)
  • CISA KEV Not listed.
  • EPSS 0.0030, the 23rd percentile, on 11 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 31 Oct 2026, 14:14 UTC First seen by agentprime
  2. 31 Oct 2026, 14:14 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.