Bug 2441267 (CVE-2026-27017)

Summary: CVE-2026-27017 utls: uTLS: Fingerprint mismatch allows client identification during GREASE ECH
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: low    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in uTLS. When using GREASE Encrypted ClientHello (ECH), uTLS versions 1.6.0 through 1.8.0 may exhibit a fingerprint mismatch with Chrome. This occurs due to an inconsistent selection of cipher suites between the outer ClientHello and the ECH, potentially allowing a remote observer to distinguish the uTLS client from a standard Chrome browser. This could lead to a low impact information disclosure regarding the client's identity.
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Description OSIDB Bzimport 2026-02-20 04:01:31 UTC 
uTLS is a fork of crypto/tls, created to customize ClientHello for fingerprinting resistance while still using it for the handshake. Versions 1.6.0 through 1.8.0 contain a fingerprint mismatch with Chrome when using GREASE ECH, related to cipher suite selection. When Chrome selects the preferred cipher suite in the outer ClientHello and for ECH, it does so consistently based on hardware support—for example, if it prefers AES for the outer cipher suite, it also uses AES for ECH. However, the Chrome parrot in uTLS hardcodes AES preference for outer cipher suites but selects the ECH cipher suite randomly between AES and ChaCha20. This creates a 50% chance of selecting ChaCha20 for ECH while using AES for the outer cipher suite, a combination impossible in Chrome. This issue only affects GREASE ECH; in real ECH, Chrome selects the first valid cipher suite when AES is preferred, which uTLS handles correctly. This issue has been fixed in version 1.8.1.