# CVE-2026-27017: uTLS has a Chrome Parrot Fingerprint Vulnerability due to GREASE ECH Cipher Suite Mismatch

> uTLS library fails to mimic Chrome's GREASE ECH cipher suite selection, allowing fingerprinting detection of non-browser clients.

- **Severity:** Low 2.3 (`CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 20 Feb 2026
- **Updated:** 20 Feb 2026
- **Also known as:** GHSA-7m29-f4hw-g2vx, GO-2026-4509, UBUNTU-CVE-2026-27017

## What is CVE-2026-27017?

CVE-2026-27017 is a low-severity vulnerability in refraction-networking utls, published on 20 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

uTLS is a fork of crypto/tls, created to customize ClientHello for fingerprinting resistance while still using it for the handshake. Versions 1.6.0 through 1.8.0 contain a fingerprint mismatch with Chrome when using GREASE ECH, related to cipher suite selection. When Chrome selects the preferred cipher suite in the outer ClientHello and for ECH, it does so consistently based on hardware support—for example, if it prefers AES for the outer cipher suite, it also uses AES for ECH. However, the Chrome parrot in uTLS hardcodes AES preference for outer cipher suites but selects the ECH cipher suite randomly between AES and ChaCha20. This creates a 50% chance of selecting ChaCha20 for ECH while using AES for the outer cipher suite, a combination impossible in Chrome. This issue only affects GREASE ECH; in real ECH, Chrome selects the first valid cipher suite when AES is preferred, which uTLS handles correctly. This issue has been fixed in version 1.8.1.

Weakness: CWE-1240 (Use of a Cryptographic Primitive with a Risky Implementation)

## Which versions are affected by CVE-2026-27017?

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 | From 1.6.0 | 1.8.1 |
| `github.com/refraction-networking/utls` | Go | From 1.6.0 | 1.8.1 |
| `github.com/refraction-networking/utls` | Go | From 1.6.0 | 1.8.1 |

Products: Refraction-Networking Utls.

## How do I fix CVE-2026-27017?

Upgrade github.com/refraction-networking/utls to 1.8.1 or later.

## Is CVE-2026-27017 being exploited?

There is no known exploitation of CVE-2026-27017 in the wild and no public exploit code.

## Other vulnerabilities in refraction-networking utls

- [CVE-2026-26994](https://insights.sbomify.com/CVE-2026-26994/): uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-27017 and fix it if it is affected.

What it is: CVE-2026-27017 is a low-severity vulnerability in refraction-networking utls, published on 20 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-27017 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/refraction-networking/utls (Go): affected from 1.6.0; fixed in 1.8.1
- github.com/refraction-networking/utls (Go): affected from 1.6.0; fixed in 1.8.1
- github.com/refraction-networking/utls (Go): affected from 1.6.0; fixed in 1.8.1

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-2026-27017/ (Markdown: https://insights.sbomify.com/CVE-2026-27017/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-27017
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-27017
- OSV: https://osv.dev/vulnerability/CVE-2026-27017

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-27017/
Summaries are generated and may contain mistakes; the official description above is the record.