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


- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 20 Feb 2026
- **Updated:** 20 Feb 2026
- **Also known as:** DEBIAN-CVE-2026-26994, GHSA-pmc3-p9hx-jq96, GO-2025-3638, UBUNTU-CVE-2026-26994

## What is CVE-2026-26994?

CVE-2026-26994 is a medium-severity vulnerability in refraction-networking utls, published on 20 Feb 2026. 

### 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.

Weakness: CWE-693 (Protection Mechanism Failure)

## Which versions are affected by CVE-2026-26994?

3 open source packages ship an affected version, including github.com/refraction-networking/utls. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/refraction-networking/utls` | Go | All versions | 1.7.0 |
| `github.com/refraction-networking/utls` | Go | All versions | 1.7.0 |
| `github.com/refraction-networking/utls` | Go | From 1.0.0 | 1.7.0 |

Products: Refraction-Networking Utls.

## How do I fix CVE-2026-26994?

Upgrade github.com/refraction-networking/utls to 1.7.0 or later.

## Is CVE-2026-26994 being exploited?

There is no known exploitation of CVE-2026-26994 in the wild and no public exploit code.

## Other vulnerabilities in refraction-networking utls

- [CVE-2026-27017](https://insights.sbomify.com/CVE-2026-27017/): uTLS has a Chrome Parrot Fingerprint Vulnerability due to GREASE ECH Cipher Suite Mismatch (low 2.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-26994 and fix it if it is affected.

What it is: CVE-2026-26994 is a medium-severity vulnerability in refraction-networking utls, published on 20 Feb 2026. 
Exploitation: There is no known exploitation of CVE-2026-26994 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/refraction-networking/utls (Go): affected all versions; fixed in 1.7.0
- github.com/refraction-networking/utls (Go): affected all versions; fixed in 1.7.0
- github.com/refraction-networking/utls (Go): affected from 1.0.0; fixed in 1.7.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-26994/ (Markdown: https://insights.sbomify.com/CVE-2026-26994/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-26994
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-26994
- OSV: https://osv.dev/vulnerability/CVE-2026-26994

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-26994/
Summaries are generated and may contain mistakes; the official description above is the record.