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🛡️ CVE-2026-26994 — utls

🟡 CVSS 6.5 — Medium ✅ No Known Exploit CWE-693 NVD
6.5
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

Description

Before version 1.7.0, utls did not implement the TLS 1.3 downgrade protection mechanism specified in RFC 8446 Section 4.1.3 when using a utls ClientHello spec. This allowed an active network adversary to downgrade TLS 1.3 connections initiated by a utls client to a lower TLS version (e.g., TLS 1.2) by modifying the ClientHello message to exclude the SupportedVersions extension, causing the server to respond with a TLS 1.2 ServerHello (along with a downgrade canary in the ServerHello random field). Because utls did not check the downgrade canary in the ServerHello random field, clients would accept the downgraded connection without detecting the attack. This attack could also be used by an active network attacker to fingerprint utls connections.

Fix Commit or Pull Request

refraction-networking/utls#337, specifically refraction-networking/utls@f8892761e2a4d29054264651d3a86fda83bc83f9

References

  • https://github.com/refraction-networking/utls/issues/181

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality low, integrity low, availability none.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: None — nobody has to be tricked into anything.
  • Scope: Unchanged — the damage stays inside the vulnerable component.
  • Confidentiality impact: Low — limited, and the attacker does not choose what is affected.
  • Integrity impact: Low — limited, and the attacker does not choose what is affected.
  • Availability impact: None.

Weakness class

CVE-2026-26994 is classified as CWE-693: Protection Mechanism Failure. A protection exists but does not cover the case at hand, so it can be worked around.

Affected software

CVE-2026-26994 is recorded against 2 packages.

  • github.com/refraction-networking/utls
  • utls (fixed in 1.7.0)

Timeline and source

Published on 23 April 2025 and last revised on 22 February 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from NVD.

References

github.com (Web)
nvd.nist.gov (Advisory)
github.com (Web)
github.com (Web)
github.com (Web)
github.com (Package)

Other advisories for this package

github.com/refraction-networking/utls has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-693: Protection Mechanism Failure) in other software:

CVE-2026-26994 on other distributions

Each distribution ships its own build and its own fixed version. Pick the one you run:

Details

Severity Medium
CVSS Score 6.5
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
CWE CWE-693
Public Exploit ✅ No
Source NVD
Published 2025-04-23
Updated 2026-08-20
Modified 2026-02-22

Affected Packages

Software From version Fixed in
github.com/refraction-networking/utls — —
utls — 1.7.0

Similar Threats

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