The vulnerability CVE-2026-27017 affects the uTLS library, a fork of Go's crypto/tls designed to customize the ClientHello message for fingerprinting resistance while maintaining standard TLS handshake behavior. Versions 1.6.0 through 1.8.0 contain a logic flaw in cipher suite selection when handling GREASE ECH (Encrypted ClientHello) extensions. Chrome's implementation consistently selects cipher suites based on hardware support, ensuring the outer ClientHello and ECH ClientHello cipher suites align (e.g., both AES preferred). However, uTLS hardcodes AES preference for the outer ClientHello but randomly chooses between AES and ChaCha20 for the ECH ClientHello cipher suite, resulting in a 50% chance of an inconsistent cipher suite pairing (AES outer, ChaCha20 ECH) that Chrome never produces. This inconsistency can cause fingerprint mismatches, undermining the fingerprinting resistance goals of uTLS. The issue is limited to GREASE ECH, a mechanism used for testing and protocol robustness, and does not affect real ECH, where uTLS correctly selects cipher suites. The vulnerability is classified under CWE-1240 (Use of a Cryptographic Primitive with a Risky Implementation). It has a CVSS 4.0 base score of 2.3, reflecting low severity due to limited impact and exploitation complexity. No known exploits have been reported. The issue was addressed in uTLS version 1.8.1 by aligning cipher suite selection logic with Chrome's behavior.

The primary impact of this vulnerability is a reduced effectiveness of fingerprinting resistance in TLS client implementations using vulnerable uTLS versions. The inconsistent cipher suite selection can lead to detectable fingerprint mismatches when interacting with servers or middleboxes expecting Chrome-like behavior, potentially allowing adversaries to identify or differentiate clients that rely on uTLS for privacy. However, this does not directly compromise confidentiality, integrity, or availability of communications. The vulnerability does not enable remote code execution or data leakage. Since it affects GREASE ECH, which is a testing mechanism rather than widely deployed real ECH, the practical impact is limited. Organizations deploying uTLS in privacy-sensitive applications or anti-fingerprinting tools may experience reduced anonymity or increased fingerprintability until patched. No known active exploitation or widespread attacks have been observed, minimizing immediate risk.

To mitigate this vulnerability, organizations and developers should upgrade uTLS to version 1.8.1 or later, where the cipher suite selection logic for GREASE ECH has been corrected to match Chrome's behavior. For projects that cannot immediately upgrade, a temporary workaround is to avoid using GREASE ECH in client configurations or disable fingerprinting features that rely on this mechanism. Additionally, thorough testing should be conducted to verify that TLS client fingerprints align with expected patterns to prevent inadvertent fingerprint leaks. Monitoring TLS client behavior and network traffic for anomalies related to cipher suite mismatches can help detect potential fingerprinting inconsistencies. Finally, maintain awareness of updates from refraction-networking and related cryptographic libraries to promptly apply security patches.