CVE-2026-27017
mediumpriority · track · 21EPSS 0.002 · 7 %ileEUUSEPSS · scored 2026-10-27
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
Technical severity, 0–10. One row per scoring source.
base score 0–10 · higher = more severe · hover a vector to decode⚠ sources disagree · low–medium · 0.6–5.3
ssvc
A stakeholder decision — act, attend, or track — from exploitation and impact.
weaknesses
The CWE weakness classes this flaw belongs to.
affected products (3)
Products and version ranges known to be affected.
- Go github.com/refraction-networking/utls1.6.0 → 1.8.1osvUSpkg:golang/github.com/refraction-networking/utls
advisories (2)
Vendor and ecosystem advisories describing this.
- GHSA-7m29-f4hw-g2vxuTLS has a fingerprint vulnerability from GREASE ECH mismatch for Chrome parrots2026-02-18ghsa · osvUS
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.
- GO-2026-4509Fingerprint vulnerability in uTLS from GREASE ECH mismatch for Chrome parrots in github.com/refraction-networking/utls2026-02-24vendor · osvUS
Fingerprint vulnerability in uTLS from GREASE ECH mismatch for Chrome parrots in github.com/refraction-networking/utls
also known as (5)
Other names and identifiers for this entity.
sources
Which sources assert this record, and when.
references (4)
External citations — patches, advisories, analysis.
- https://github.com/refraction-networking/utls/releasesPatchcnnvd
- https://github.com/refraction-networking/utls/security/advisories/GHSA-7m29-f4hw-g2vxcnnvd
- https://github.com/refraction-networking/utls/security/advisories/GHSA-7m29-f4hw-g2vxenisa_euvd
- https://github.com/refraction-networking/utls/security/advisories/GHSA-7m29-f4hw-g2vxPatchVendor Advisorynvd
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