CVE-2026-27017

uTLS has a Chrome Parrot Fingerprint Vulnerability due to GREASE ECH Cipher Suite Mismatch

CVSS 2.3 LOWEPSS 0.2%CWE-1240

In short

uTLS versions 1.6.0 to 1.8.0 mimic Chrome's TLS fingerprint inconsistently when using GREASE ECH: they randomly pick between AES and ChaCha20 for encrypted client hello while always using AES for the outer connection, creating a detectable mismatch that Chrome never produces. This breaks the fingerprinting resistance that uTLS is designed to provide.

Technical detail

The vulnerability exists in uTLS's Chrome parrot implementation for GREASE ECH (Encrypted Client Hello), where cipher suite selection for the ECH inner handshake is randomized (50% AES, 50% ChaCha20) rather than being deterministically derived from the outer cipher suite preference, allowing passive TLS fingerprinting attacks to distinguish uTLS from genuine Chrome browsers. The issue requires GREASE ECH to be negotiated during the TLS handshake; real ECH is unaffected because uTLS correctly selects the first valid cipher suite when AES is preferred. Remediation is available in version 1.8.1.

Summary generated and translated by AI from the 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.

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

Affected products

refraction-networking · utls

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References

https://github.com/refraction-networking/utls/security/advisories/GHSA-7m29-f4hw-g2vx