The vulnerability CVE-2026-26994 affects uTLS, a fork of Go's crypto/tls library designed to customize the TLS ClientHello message for fingerprinting resistance. In versions 1.6.7 and below, uTLS fails to implement the mandatory TLS 1.3 downgrade protection mechanism defined in RFC 8446 Section 4.1.3. Normally, TLS 1.3 clients include a SupportedVersions extension in the ClientHello message, and servers respond with a ServerHello indicating the negotiated version. To prevent downgrade attacks, TLS 1.3 servers embed a 'downgrade canary' in the ServerHello random field when responding with a lower TLS version, signaling a forced downgrade attempt. However, uTLS clients did not check this downgrade canary, allowing an active network adversary to strip the SupportedVersions extension from the ClientHello, forcing the server to downgrade to TLS 1.2. The client accepts this downgraded connection without detection, exposing the session to vulnerabilities inherent in older TLS versions. Additionally, this manipulation allows attackers to fingerprint uTLS clients based on their behavior under downgrade conditions. The vulnerability does not require authentication or user interaction and can be exploited remotely by a man-in-the-middle attacker. The issue was addressed in uTLS version 1.7.0 by implementing proper downgrade canary verification.

The primary impact of this vulnerability is the potential for man-in-the-middle attackers to downgrade TLS 1.3 connections to older, less secure TLS versions such as TLS 1.2. This downgrade can expose encrypted communications to known weaknesses in earlier TLS versions, including weaker cipher suites and vulnerabilities like BEAST or Lucky13. Confidentiality and integrity of data transmitted over the downgraded connection may be compromised, increasing the risk of data interception or modification. Furthermore, the ability to fingerprint uTLS clients can aid attackers in identifying and targeting specific clients or users, potentially facilitating further attacks or surveillance. Organizations relying on uTLS for secure communications may face increased risk of data breaches and loss of trust. Although no active exploits are currently known, the vulnerability's presence in widely used versions means it could be targeted in the future, especially in environments where TLS 1.3 is critical for security compliance.

Organizations using uTLS should immediately upgrade to version 1.7.0 or later, where the downgrade protection mechanism is properly implemented. For environments where immediate upgrade is not feasible, deploying network-level protections such as TLS interception detection and anomaly-based intrusion detection systems can help identify downgrade attempts. Additionally, enforcing strict TLS version policies on servers to refuse connections below TLS 1.3 can reduce exposure. Monitoring network traffic for missing SupportedVersions extensions or unexpected ServerHello downgrade canaries may provide early warning of exploitation attempts. Developers integrating uTLS should audit their TLS handshake implementations to ensure compliance with RFC 8446 and proper downgrade detection. Finally, educating security teams about this specific downgrade attack vector can improve incident response readiness.