CVE-2021-35027
📋 TL;DR
A directory traversal vulnerability in Zyxel VPN2S firewall firmware allows remote attackers to access sensitive files by manipulating file paths. This affects organizations using Zyxel VPN2S firewalls with vulnerable firmware versions, potentially exposing configuration files, credentials, or other sensitive data.
💻 Affected Systems
- Zyxel VPN2S Firewall
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Remote attacker gains administrative access to firewall, extracts all credentials and configurations, and pivots to internal network.
Likely Case
Remote attacker accesses sensitive configuration files containing credentials, VPN settings, or firewall rules.
If Mitigated
Attack is blocked at network perimeter, or sensitive files are inaccessible due to proper file permissions.
🎯 Exploit Status
Directory traversal vulnerabilities typically require minimal technical skill to exploit once details are known.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firmware version 1.20 or later
Vendor Advisory: https://www.zyxel.com/support/Zyxel_security_advisory_for_directory_traversal_and_command_injection_vulnerabilities_of_VPN2S_Firewall.shtml
Restart Required: Yes
Instructions:
1. Download firmware version 1.20 or later from Zyxel support portal. 2. Log into VPN2S web interface. 3. Navigate to Maintenance > Firmware Upgrade. 4. Upload and install the new firmware. 5. Reboot the device.
🔧 Temporary Workarounds
Restrict Web Interface Access
allLimit access to VPN2S web management interface to trusted IP addresses only.
Configure firewall rules to allow only specific source IPs to TCP ports 80/443 on VPN2S
Disable Web Interface
linuxTemporarily disable web management interface if not required.
Use CLI: system web-management disable
🧯 If You Can't Patch
- Implement strict network segmentation to isolate VPN2S from untrusted networks
- Monitor for unusual file access patterns in web server logs
🔍 How to Verify
Check if Vulnerable:
Check firmware version in web interface: System > Status > Firmware Version. If version is 1.12, device is vulnerable.Check Version:
ssh admin@vpn2s show version | grep FirmwareVerify Fix Applied:
After patching, verify firmware version shows 1.20 or later in System > Status > Firmware Version.📡 Detection & Monitoring
Log Indicators:
- Web server logs showing ../ or ..\ sequences in URL requests
- Multiple failed attempts to access sensitive file paths
Network Indicators:
- Unusual HTTP requests containing directory traversal sequences to VPN2S IP
SIEM Query:
source="vpn2s_web_logs" AND (url="*../*" OR url="*..\\*")