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Vulnerability Details :

CVE-2026-26994

Summary
Assigner-GitHub_M
Assigner Org ID-a0819718-46f1-4df5-94e2-005712e83aaa
Published At-20 Feb, 2026 | 02:50
Updated At-20 Feb, 2026 | 15:12
Rejected At-
Credits

uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

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.

Vendors
-
Not available
Products
-
Metrics (CVSS)
VersionBase scoreBase severityVector
Weaknesses
Attack Patterns
Solution/Workaround
References
HyperlinkResource Type
EPSS History
Score
Latest Score
-
N/A
No data available for selected date range
Percentile
Latest Percentile
-
N/A
No data available for selected date range
Stakeholder-Specific Vulnerability Categorization (SSVC)
▼Common Vulnerabilities and Exposures (CVE)
cve.org
Assigner:GitHub_M
Assigner Org ID:a0819718-46f1-4df5-94e2-005712e83aaa
Published At:20 Feb, 2026 | 02:50
Updated At:20 Feb, 2026 | 15:12
Rejected At:
▼CVE Numbering Authority (CNA)
uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

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.

Affected Products
Vendor
refraction-networking
Product
utls
Versions
Affected
  • < 1.7.0
Problem Types
TypeCWE IDDescription
CWECWE-693CWE-693: Protection Mechanism Failure
Type: CWE
CWE ID: CWE-693
Description: CWE-693: Protection Mechanism Failure
Metrics
VersionBase scoreBase severityVector
3.16.5MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Version: 3.1
Base score: 6.5
Base severity: MEDIUM
Vector:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Metrics Other Info
Impacts
CAPEC IDDescription
Solutions
Configurations
Workarounds
Exploits
Credits
Timeline
EventDate
Replaced By
Rejected Reason
References
HyperlinkResource
https://github.com/refraction-networking/utls/security/advisories/GHSA-pmc3-p9hx-jq96
x_refsource_CONFIRM
https://github.com/refraction-networking/utls/issues/181
x_refsource_MISC
https://github.com/refraction-networking/utls/pull/337
x_refsource_MISC
https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9
x_refsource_MISC
Hyperlink: https://github.com/refraction-networking/utls/security/advisories/GHSA-pmc3-p9hx-jq96
Resource:
x_refsource_CONFIRM
Hyperlink: https://github.com/refraction-networking/utls/issues/181
Resource:
x_refsource_MISC
Hyperlink: https://github.com/refraction-networking/utls/pull/337
Resource:
x_refsource_MISC
Hyperlink: https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9
Resource:
x_refsource_MISC
▼Authorized Data Publishers (ADP)
CISA ADP Vulnrichment
Affected Products
Metrics
VersionBase scoreBase severityVector
Metrics Other Info
Impacts
CAPEC IDDescription
Solutions
Configurations
Workarounds
Exploits
Credits
Timeline
EventDate
Replaced By
Rejected Reason
References
HyperlinkResource
Information is not available yet
▼National Vulnerability Database (NVD)
nvd.nist.gov
Source:security-advisories@github.com
Published At:20 Feb, 2026 | 03:16
Updated At:20 Feb, 2026 | 19:20

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.

CISA Catalog
Date AddedDue DateVulnerability NameRequired Action
N/A
Date Added: N/A
Due Date: N/A
Vulnerability Name: N/A
Required Action: N/A
Metrics
TypeVersionBase scoreBase severityVector
Secondary3.16.5MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Type: Secondary
Version: 3.1
Base score: 6.5
Base severity: MEDIUM
Vector:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
CPE Matches
refraction-networking
refraction-networking
>>utls>>Versions before 1.7.0(exclusive)
cpe:2.3:a:refraction-networking:utls:*:*:*:*:*:go:*:*
Weaknesses
CWE IDTypeSource
CWE-693Primarysecurity-advisories@github.com
CWE ID: CWE-693
Type: Primary
Source: security-advisories@github.com
Evaluator Description
Evaluator Impact

Evaluator Solution

Vendor Statements
References
HyperlinkSourceResource
https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9security-advisories@github.com
Patch
https://github.com/refraction-networking/utls/issues/181security-advisories@github.com
Issue Tracking
https://github.com/refraction-networking/utls/pull/337security-advisories@github.com
Issue Tracking
Patch
https://github.com/refraction-networking/utls/security/advisories/GHSA-pmc3-p9hx-jq96security-advisories@github.com
Patch
Vendor Advisory
Hyperlink: https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9
Source: security-advisories@github.com
Resource:
Patch
Hyperlink: https://github.com/refraction-networking/utls/issues/181
Source: security-advisories@github.com
Resource:
Issue Tracking
Hyperlink: https://github.com/refraction-networking/utls/pull/337
Source: security-advisories@github.com
Resource:
Issue Tracking
Patch
Hyperlink: https://github.com/refraction-networking/utls/security/advisories/GHSA-pmc3-p9hx-jq96
Source: security-advisories@github.com
Resource:
Patch
Vendor Advisory

Change History

0
Information is not available yet

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