CVE-2023-23581
📋 TL;DR
A denial-of-service vulnerability in SoftEther VPN's vpnserver component allows attackers to crash the VPN service by sending specially crafted network packets. This affects SoftEther VPN servers running vulnerable versions, potentially disrupting VPN connectivity for all users.
💻 Affected Systems
- SoftEther VPN Server
📦 What is this software?
Vpn by Softether
Vpn by Softether
⚠️ Risk & Real-World Impact
Worst Case
Complete VPN service outage affecting all connected users and preventing new connections until service restart.
Likely Case
Temporary VPN service disruption requiring manual restart of the vpnserver process.
If Mitigated
Minimal impact if VPN service is behind proper network segmentation and has automated restart capabilities.
🎯 Exploit Status
Exploitation requires network access to the VPN server port (default 443/TCP, 992/TCP, or 5555/TCP).
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.02.5180 or later
Vendor Advisory: https://github.com/SoftEtherVPN/SoftEtherVPN/security/advisories/GHSA-4vjq-6hxq-9g3q
Restart Required: Yes
Instructions:
1. Download latest version from SoftEther VPN website. 2. Stop vpnserver service. 3. Install updated version. 4. Restart vpnserver service.
🔧 Temporary Workarounds
Network Access Restriction
linuxRestrict network access to VPN server ports to trusted IP addresses only.
iptables -A INPUT -p tcp --dport 443 -s trusted_ip -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP
Service Monitoring and Auto-restart
allConfigure monitoring to automatically restart vpnserver if it crashes.
systemctl enable vpnserver
Configure monitoring tool to restart service on failure
🧯 If You Can't Patch
- Implement strict network segmentation to limit access to VPN server ports
- Deploy VPN server behind a WAF or IPS with DoS protection capabilities
🔍 How to Verify
Check if Vulnerable:
Check SoftEther VPN Server version via admin console or configuration file.Check Version:
vpncmd localhost /SERVER /CMD AboutVerify Fix Applied:
Verify version is 5.02.5180 or later and test VPN connectivity.📡 Detection & Monitoring
Log Indicators:
- vpnserver process crashes
- Unexpected service restarts
- Connection spikes followed by service failure
Network Indicators:
- Multiple malformed packets to VPN server ports
- Sudden drop in VPN connections
SIEM Query:
source="vpnserver.log" AND ("crash" OR "segmentation fault" OR "abnormal termination")