{"cve":"CVE-2026-27017","nvd":{"published":"2026-02-20T03:16:01.797","modified":"2026-06-17T10:26:32.577","status":"Analyzed","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_v4":{"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","score":2.3},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N","score":5.3},"cwes":["CWE-1240"],"references":["https://github.com/refraction-networking/utls/security/advisories/GHSA-7m29-f4hw-g2vx"],"products":["refraction-networking:utls"],"source":"nvd","synced_at":"2026-10-19T22:23:35.260773158+00:00"},"epss":{"cve":"CVE-2026-27017","score":0.00175,"percentile":0.07004,"date":"2026-11-01"},"kev":null,"osv":[{"id":"GHSA-7m29-f4hw-g2vx","url":"https://osv.dev/vulnerability/GHSA-7m29-f4hw-g2vx","summary":"uTLS has a fingerprint vulnerability from GREASE ECH mismatch for Chrome parrots","modified":"2026-09-10T03:50:34.315551939Z","packages":[{"ecosystem":"Go","name":"github.com/refraction-networking/utls","purl":"pkg:golang/github.com/refraction-networking/utls","ranges":[">=1.6.0 <1.8.1"],"fixed":["1.8.1"]}]},{"id":"GO-2026-4509","url":"https://osv.dev/vulnerability/GO-2026-4509","summary":"Fingerprint vulnerability in uTLS from GREASE ECH mismatch for Chrome parrots in github.com/refraction-networking/utls","modified":"2026-02-24T23:41:14.415506Z","packages":[{"ecosystem":"Go","name":"github.com/refraction-networking/utls","purl":"pkg:golang/github.com/refraction-networking/utls","ranges":[">=1.6.0 <1.8.1"],"fixed":["1.8.1"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}