refraction-networking utls up to 1.6.x RFC 8446 protection mechanism

Summaryinfo

A vulnerability, which was classified as critical, has been found in refraction-networking utls up to 1.6.x. This vulnerability affects unknown code of the component RFC 8446. The manipulation leads to protection mechanism. This vulnerability is referenced as CVE-2026-26994. Remote exploitation of the attack is possible. No exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in refraction-networking utls up to 1.6.x. It has been rated as critical. This issue affects an unknown functionality of the component RFC 8446. The manipulation with an unknown input leads to a protection mechanism vulnerability. Using CWE to declare the problem leads to CWE-693. The product does not use or incorrectly uses a protection mechanism that provides sufficient defense against directed attacks against the product. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-26994 since 02/17/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. The technical details are unknown and an exploit is not publicly available.

The vulnerability scanner Nessus provides a plugin with the ID 299752 (Linux Distros Unpatched Vulnerability : CVE-2026-26994), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 1.7.0 eliminates this vulnerability. Applying the patch f8892761e2a4d29054264651d3a86fda83bc83f9 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at Tenable (299752). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Vendor

• refraction-networking

Name

• utls

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6

License

• open-source

Website

• Product: https://github.com/refraction-networking/utls/

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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