CVE-2026-26994: Github.com/refraction networking/utls UTLS ServerHellos Are Accepted Without Vulnerability
MODERATECVE-2026-26994 is a MODERATE (CVSS 6.5) utls serverhellos are accepted without vulnerability (CWE-693: Protection Mechanism Failure) affecting Github.com/refraction networking/utls libraries in the Go ecosystem. The issue involves utls serverhellos are accepted without checking tls 1.3 downgrade canaries. Remediation is available by upgrading affected packages to version 1.7.0.
CVE-2026-26994 at a Glance
Security Impact
CVSS: 6.5
The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N).
How to Fix CVE-2026-26994
To remediate this vulnerability, upgrade all affected Github.com/refraction networking/utls dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.
Upgrade github.com/refraction-networking/utls to 1.7.0Run the following command in your project directory to verify that all resolved dependencies use the patched release:
inspect dependencies for github.com/refraction-networking/utlsHow to Check if You Are Affected
To determine whether your application is affected by CVE-2026-26994, inspect your project dependencies for Github.com/refraction networking/utls:
inspect dependencies for github.com/refraction-networking/utlsAnalyze the terminal output to locate any version of Github.com/refraction networking/utls in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.7.0), your application is affected.
Note: Verify declared build manifests and deployed runtime container environments.
Technical Details
Vulnerability Mechanism & Technical Analysis
This vulnerability is classified under CWE-693: Protection Mechanism Failure, where the product does not use or incorrectly uses a protection mechanism that provides sufficient defense against directed attacks against the product. In Github.com/refraction networking/utls, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.
Source Advisory Statement
CVE-2026-26994 Affected Github.com/refraction networking/utls Versions
1 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
github.com/refraction-networking/utls
|
Go |
< 1.7.0
|
1.7.0
|
References
Frequently Asked Questions
What is CVE-2026-26994?
Which Github.com/refraction networking/utls versions are affected?
What version fixes CVE-2026-26994?
Is github.com/refraction-networking/utls affected?
What is the CVSS score of CVE-2026-26994?
What CWE is associated with CVE-2026-26994?
How can I check Go dependencies for CVE-2026-26994?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-pmc3-p9hx-jq96",
"modified" : "2026-02-20T16:51:12Z",
"published" : "2025-04-23T14:43:44Z",
"aliases" : [ "CVE-2026-26994" ],
"summary" : "uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries",
"details" : "### Description\nBefore version 1.7.0, 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.\n\n### Fix Commit or Pull Request\n\nrefraction-networking/utls#337, specifically refraction-networking/utls@f8892761e2a4d29054264651d3a86fda83bc83f9\n\n### References\n\n- https://github.com/refraction-networking/utls/issues/181",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/refraction-networking/utls"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "1.7.0"
} ]
} ]
} ],
"references" : [ {
"type" : "WEB",
"url" : "https://github.com/refraction-networking/utls/security/advisories/GHSA-pmc3-p9hx-jq96"
}, {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-26994"
}, {
"type" : "WEB",
"url" : "https://github.com/refraction-networking/utls/issues/181"
}, {
"type" : "WEB",
"url" : "https://github.com/refraction-networking/utls/pull/337"
}, {
"type" : "WEB",
"url" : "https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/refraction-networking/utls"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-693" ],
"severity" : "MODERATE",
"github_reviewed" : true,
"github_reviewed_at" : "2025-04-23T14:43:44Z",
"nvd_published_at" : "2026-02-20T03:16:01Z"
}
}