uTLS has a Chrome Parrot Fingerprint Vulnerability due to GREASE ECH Cipher Suite Mismatch
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.
Affected packages
Versions sourced from the GitHub Security Advisory.
| Package | Affected versions | Patched versions |
|---|---|---|
github.com/refraction-networking/utlsGo | >= 1.6.0, < 1.8.1 | 1.8.1 |
Affected products
1- Range: >= 1.6.0, < 1.8.1
Patches
124bd1e05a788fix: use AES in GREASE ECH for Chrome fingerprint
1 file changed · +0 −4
u_ech.go+0 −4 modified@@ -300,10 +300,6 @@ func BoringGREASEECH() *GREASEEncryptedClientHelloExtension { KdfId: dicttls.HKDF_SHA256, AeadId: dicttls.AEAD_AES_128_GCM, }, - { - KdfId: dicttls.HKDF_SHA256, - AeadId: dicttls.AEAD_CHACHA20_POLY1305, - }, }, CandidatePayloadLens: []uint16{128, 160, 192, 224}, // +16: 144, 176, 208, 240 }
Vulnerability mechanics
Generated by null/stub on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.
References
6- github.com/advisories/GHSA-7m29-f4hw-g2vxghsaADVISORY
- nvd.nist.gov/vuln/detail/CVE-2026-27017ghsaADVISORY
- github.com/refraction-networking/utls/commit/24bd1e05a788c1add7f3037f4532ea552b2cee07ghsaWEB
- github.com/refraction-networking/utls/releases/tag/v1.8.1ghsaWEB
- github.com/refraction-networking/utls/security/advisories/GHSA-7m29-f4hw-g2vxghsax_refsource_CONFIRMWEB
- pkg.go.dev/vuln/GO-2026-4509ghsaWEB
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