TLS
CVE-2026-26994
MEDIUM
GHSA-pmc3-p9hx-jq96
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Primary rating from GitHub Advisory.
CVSS VectorGitHub Advisory
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Lifecycle Timeline
3DescriptionGitHub Advisory
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.
AnalysisAI
uTLS versions 1.6.7 and below fail to validate TLS 1.3 downgrade protection mechanisms, allowing network attackers to force ClientHello modifications that cause servers to respond with lower TLS versions while bypassing detection checks. An active attacker can exploit this to downgrade encrypted connections to TLS 1.2 or earlier, potentially exposing traffic to known cryptographic weaknesses. Affected users of uTLS, Red Hat, and other TLS implementations should update to patched versions immediately.
Technical ContextAI
Classified as CWE-693 (Protection Mechanism Failure). Affects Utls. 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 Supporte
RemediationAI
A vendor patch is available — apply it immediately. Fixed in version 1.7.0.. Restrict network access to the affected service where possible.
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Same weakness CWE-693 – Protection Mechanism Failure
View allVendor StatusVendor
SUSE
Severity: MediumShare
External POC / Exploit Code
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