Bug 2441269 (CVE-2026-26994)

Summary: CVE-2026-26994 utls: uTLS: TLS 1.3 downgrade protection bypass allows information disclosure and fingerprinting
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in uTLS. An active network attacker could exploit this vulnerability by manipulating the initial connection message (ClientHello) during the TLS handshake. This manipulation forces a downgrade from the more secure TLS 1.3 protocol to an older, less secure version like TLS 1.2. As a result, the client would accept the weaker connection without detecting the attack, potentially leading to information disclosure or allowing the attacker to identify uTLS connections.
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Description OSIDB Bzimport 2026-02-20 04:01:39 UTC 
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.