2.3

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-27017

CVE-2026-27017: uTLS has a fingerprint vulnerability from GREASE ECH mismatch for Chrome parrots

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.

(GitHub Advisory)

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CVE-2026-27017

CVE-2026-27017:

2.3

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
github.com/refraction-networking/utls	go	>= 1.6.0, < 1.8.1	1.8.1

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability, as described, is a fingerprinting issue in uTLS's Chrome parrot implementation due to a mismatch in cipher suite selection for GREASE ECH. The provided fix commit 24bd1e05a788c1add7f3037f4532ea552b2cee07 clearly shows a change in the u_ech.go file. Specifically, within the BoringGREASEECH function, the AEAD_CHACHA20_POLY1305 cipher suite is removed. Before the patch, this function would randomly select between AEAD_AES_128_GCM and AEAD_CHACHA20_POLY1305 for the ECH extension. However, the Chrome parrot implementation hardcodes a preference for AES in the outer ClientHello. A real Chrome browser, when preferring AES, would use it for both the outer ClientHello and the ECH extension. The vulnerable version of BoringGREASEECH could create a scenario where AES is used for the outer ClientHello and ChaCha20 for the ECH, a combination that a real Chrome browser would not produce. This makes the uTLS client identifiable. The patch corrects this by ensuring that only AES is used for the GREASE ECH, aligning with the outer ClientHello's preference and mimicking Chrome's behavior correctly. Therefore, the BoringGREASEECH function is the source of the vulnerability.

Vulnerable functions

BoringGREASEECH

u_ech.go

The function `BoringGREASEECH` in `u_ech.go` was vulnerable because it included both AES and ChaCha20 cipher suites for GREASE ECH, while the outer ClientHello for Chrome parrots hardcoded a preference for AES. This created a 50% chance of a cipher suite mismatch between the outer and inner ClientHello, which is not possible in a real Chrome browser. This allows for fingerprinting of the uTLS client. The patch removes the ChaCha20 option, ensuring consistency.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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