How severe is CVE-2026-27017?

CVE-2026-27017 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity none, and availability none.

Is CVE-2026-27017 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (5th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-27017?

CVE-2026-27017 affects Refraction-networking Utls. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-27017?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-27017 have an EU (EUVD) identifier?

Yes. CVE-2026-27017 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-8031.

When was CVE-2026-27017 published?

CVE-2026-27017 was published on 2026-02-20 and last updated on 2026-06-17.

CVE-2026-27017

CVE-2026-27017 is a medium-severity vulnerability in Refraction-networking Utls with a CVSS 3.x base score of 5.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-1240.

Key facts

Severity: Medium (CVSS 3.x base score 5.3)
CVSS v4: 2.3
EPSS exploit prediction: 0% (5th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-8031
Weakness: CWE-1240
Affected product: Refraction-networking Utls
Published: 2026-02-20
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is 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.
How severe is CVE-2026-27017?: CVE-2026-27017 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity none, and availability none.
Is CVE-2026-27017 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (5th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-27017?: CVE-2026-27017 affects Refraction-networking Utls. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-27017?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-27017 have an EU (EUVD) identifier?: Yes. CVE-2026-27017 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-8031.
When was CVE-2026-27017 published?: CVE-2026-27017 was published on 2026-02-20 and last updated on 2026-06-17.

References

https://github.com/refraction-networking/utls/security/advisories/GHSA-7m29-f4hw-g2vx

Affected products (1)

cpe:2.3:a:refraction-networking:utls:*:*:*:*:*:go:*:*

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