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How the CVE Works:
The vulnerability occurs when uTLS (a TLS 1.3 library) fails to verify the downgrade canary in the ServerHello random field during TLS handshake negotiation. An attacker intercepting the connection can strip the SupportedVersions extension from the ClientHello, forcing the server to respond with a TLS 1.2 ServerHello. Since uTLS does not check the downgrade canary (a specific value in the random field indicating forced downgrade), the client accepts the lower TLS version, exposing the connection to weaker encryption and potential MITM attacks. This also allows fingerprinting of uTLS clients due to their non-standard behavior.
DailyCVE Form:
Platform: uTLS
Version: <1.7.0
Vulnerability: TLS downgrade bypass
Severity: Moderate
Date: 2025-04-23
What Undercode Say:
Exploitation:
- Intercept TLS handshake
- Modify ClientHello (remove SupportedVersions)
- Server responds with TLS 1.2
- Bypass canary check
Protection:
- Upgrade to uTLS ≥1.7.0
- Enforce TLS 1.3 strictly
- Monitor handshake anomalies
Detection Commands:
openssl s_client -connect target:443 -tls1_3 | grep "TLSv1.3" tcpdump -i eth0 'tcp port 443 and (tcp[((tcp[bash] >> 2) + 5)] = 0x16)' -vv
Code Patch Check:
// Verify downgrade canary in ServerHello
if isTLS13Downgrade(serverHello.Random) {
return errors.New("TLS downgrade detected")
}
Mitigation Script:
// Force TLS 1.3 in uTLS config
config := &utls.Config{
MinVersion: tls.VersionTLS13,
MaxVersion: tls.VersionTLS13,
}
Log Analysis:
grep "ServerHello" /var/log/tls.log | grep -v "TLSv1.3"
References:
- RFC 8446 Section 4.1.3
- GHSA-xxxx-xxxx-xxxx
- CVE-2025-XXXX
Sources:
Reported By: github.com
Extra Source Hub:
Undercode
