CVE-2026-27017

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.

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.

Fingerprint vulnerability in uTLS from GREASE ECH mismatch for Chrome parrots in github.com/refraction-networking/utls

There is a fingerprint mismatch with Chrome when using GREASE ECH, having to do with ciphersuite selection. When Chrome selects the preferred ciphersuite in the outer ClientHello and the ciphersuite for ECH, it does so consistently based on hardware support. That means, for example, if it prefers AES for the outer ciphersuite, it would also use AES for ECH. The Chrome parrot in utls hardcodes AES preference for outer ciphersuites but selects the ECH ciphersuite randomly between AES and ChaCha20. So there is a 50% chance of selecting ChaCha20 for ECH while using AES for the outer ciphersuite, which is impossible in Chrome. This is only a problem in GREASE ECH, since in real ECH Chrome selects the first valid ciphersuite when AES is preferred, which is the same in utls. So no change is done there. Affected symbols: `HelloChrome_120`, `HelloChrome_120_PQ`, `HelloChrome_131`, `HelloChrome_133` Fix commit: 24bd1e05a788c1add7f3037f4532ea552b2cee07 Thanks to telegram @acgdaily for reporting this issue.

uTLS es un 'fork' de crypto/tls, creado para personalizar ClientHello para que resista la identificación de huellas digitales mientras se sigue utilizando para el protocolo de enlace. Las versiones 1.6.0 a 1.8.0 contienen una falta de coincidencia de huella digital con Chrome cuando se usa GREASE ECH, relacionada con la selección de la suite de cifrado. Cuando Chrome selecciona la suite de cifrado preferida en el ClientHello externo y para ECH, lo hace de forma consistente basándose en el soporte de hardware; por ejemplo, si prefiere AES para la suite de cifrado externa, también usa AES para ECH. Sin embargo, el 'loro' de Chrome en uTLS codifica de forma rígida la preferencia de AES para las suites de cifrado externas, pero selecciona la suite de cifrado ECH aleatoriamente entre AES y ChaCha20. Esto hace que haya un 50% de probabilidad de que se seleccione ChaCha20 para ECH mientras se usa AES para la suite de cifrado externa, una combinación imposible en Chrome. Este problema solo afecta a GREASE ECH; en ECH real, Chrome selecciona la primera suite de cifrado válida cuando se prefiere AES, lo cual uTLS maneja correctamente. Este problema ha sido solucionado en la versión 1.8.1.