TLS downgrade via unchecked ServerHello canary
Published Feb 20, 2026 · Updated Feb 20, 2026
Protection mechanism failure in Refraction Networking uTLS before 1.7.0 allows man-in-the-middle attackers to downgrade TLS connections. During UConn.clientHandshake, the client accepts a lower-version ServerHello without comparing its final eight random bytes with TLS downgrade canaries. The attack requires an active network position and a custom uTLS ClientHello; it silently forces TLS 1.2 or earlier and reveals a uTLS fingerprint.
Summary
What happened
Protection mechanism failure in Refraction Networking uTLS before 1.7.0 allows man-in-the-middle attackers to downgrade TLS connections. During UConn.clientHandshake, the client accepts a lower-version ServerHello without comparing its final eight random bytes with TLS downgrade canaries. The attack requires an active network position and a custom uTLS ClientHello; it silently forces TLS 1.2 or earlier and reveals a uTLS fingerprint.
The record
- CVE
- CVE-2026-26994
- Published
- Feb 20, 2026
- Updated
- Feb 20, 2026
- Vendor
- Refraction Networking
- Product
- uTLS
- Classifications
- CWE-693, T1557
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Feb 20, 2026CVE publishedPublication date reported by the CVE source.
- Feb 20, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=< 1.7.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Public exploit references
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Technologies
Your stack
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Your stack
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