{"cve":"CVE-2026-26994","nvd":{"published":"2026-02-20T03:16:01.330","modified":"2026-06-17T10:26:30.400","status":"Analyzed","description":"uTLS is a fork of crypto/tls, created to customize ClientHello for fingerprinting resistance while still using it for the handshake. In versions 1.6.7 and below, uTLS did not implement the TLS 1.3 downgrade protection mechanism specified in RFC 8446 Section 4.1.3 when using a uTLS ClientHello spec. This allowed an active network adversary to downgrade TLS 1.3 connections initiated by a uTLS client to a lower TLS version (e.g., TLS 1.2) by modifying the ClientHello message to exclude the SupportedVersions extension, causing the server to respond with a TLS 1.2 ServerHello (along with a downgrade canary in the ServerHello random field). Because uTLS did not check the downgrade canary in the ServerHello random field, clients would accept the downgraded connection without detecting the attack. This attack could also be used by an active network attacker to fingerprint uTLS connections. This issue has been fixed in version 1.7.0.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","score":6.5},"cwes":["CWE-693"],"references":["https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9","https://github.com/refraction-networking/utls/issues/181","https://github.com/refraction-networking/utls/pull/337","https://github.com/refraction-networking/utls/security/advisories/GHSA-pmc3-p9hx-jq96"],"products":["refraction-networking:utls"],"source":"nvd","synced_at":"2026-10-19T22:23:35.260773158+00:00"},"epss":{"cve":"CVE-2026-26994","score":0.00302,"percentile":0.22946,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-pmc3-p9hx-jq96","url":"https://osv.dev/vulnerability/GHSA-pmc3-p9hx-jq96","summary":"uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries","modified":"2026-09-10T03:50:23.931003647Z","packages":[{"ecosystem":"Go","name":"github.com/refraction-networking/utls","purl":"pkg:golang/github.com/refraction-networking/utls","ranges":["<1.7.0"],"fixed":["1.7.0"]}]},{"id":"GO-2025-3638","url":"https://osv.dev/vulnerability/GO-2025-3638","summary":"ServerHellos are accepted without checking TLS 1.3 downgrade canaries in github.com/refraction-networking/utls","modified":"2026-02-18T23:56:59.171282Z","packages":[{"ecosystem":"Go","name":"github.com/refraction-networking/utls","purl":"pkg:golang/github.com/refraction-networking/utls","ranges":[">=1.0.0 <1.7.0"],"fixed":["1.7.0"]}]}],"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."]}