GitHub Advisoryadvisory · Go

GHSA-7m29-f4hw-g2vx uTLS has a fingerprint vulnerability from GREASE ECH mismatch for Chrome parrots

LOW Published 2026-02-18 Modified 2026-02-27 Official page ↗
Aliases
CVE-2026-27017
Ecosystem
Go
Packages
github.com/refraction-networking/utls
Severity
Low
CVSS vector
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
Weaknesses
CWE-1240
Reviewed
2026-02-18

Affected packages

• Go github.com/refraction-networking/utls: >= 1.6.0, < 1.8.1 → fixed in 1.8.1

Description

There is a fingerprint mismatch with Chrome when using GREASE ECH, having to do with ciphersuite selection. When Chrome selects the preferred ciphersuite in the outer ClientHello and the ciphersuite for ECH, it does so consistently based on hardware support. That means, for example, if it prefers AES for the outer ciphersuite, it would also use AES for ECH. The Chrome parrot in utls hardcodes AES preference for outer ciphersuites but selects the ECH ciphersuite randomly between AES and ChaCha20. So there is a 50% chance of selecting ChaCha20 for ECH while using AES for the outer ciphersuite, which is impossible in Chrome. This is only a problem in GREASE ECH, since in real ECH Chrome selects the first valid ciphersuite when AES is preferred, which is the same in utls. So no change is done there. Affected symbols: `HelloChrome_120`, `HelloChrome_120_PQ`, `HelloChrome_131`, `HelloChrome_133` Fix commit: 24bd1e05a788c1add7f3037f4532ea552b2cee07 Thanks to telegram @acgdaily for reporting this issue.

Related

Alias · CVE (1)

  • CVE-2026-27017 uTLS is a fork of crypto/tls, created to customize ClientHello for fingerprinting resistance while still using it for the handshake. vulnerability

Weakness · CWE (1)

  • CWE-1240 Use of a Cryptographic Primitive with a Risky Implementation weakness

References