CVE-2020-29299
📋 TL;DR
This CVE allows authenticated administrators to execute arbitrary commands on affected Zyxel firewall and VPN products by injecting malicious input during password change operations. The vulnerability affects multiple Zyxel security appliance product lines including VPN On-premise, VPN Orchestrator, USG, USG FLEX, ATP, and NSG. Attackers with admin credentials can gain full system control.
💻 Affected Systems
- VPN On-premise
- VPN Orchestrator
- USG
- USG FLEX
- ATP
- NSG
📦 What is this software?
Zld by Zyxel
Zld by Zyxel
Zld by Zyxel
Zld by Zyxel
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise allowing attackers to execute arbitrary commands with root privileges, potentially leading to complete network takeover, data exfiltration, or deployment of persistent backdoors.
Likely Case
Privilege escalation from admin to root/system-level access, enabling attackers to modify configurations, intercept traffic, or pivot to other network segments.
If Mitigated
Limited impact if proper network segmentation, least privilege access, and monitoring are in place, though admin accounts remain at risk.
🎯 Exploit Status
Exploitation requires admin credentials but is straightforward once authenticated. No public exploit code is known, but the vulnerability is simple to weaponize.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: VPN On-premise ZLD V4.39 week38+, VPN Orchestrator SD-OS V10.03 week32+, USG ZLD V4.39 week38+, USG FLEX ZLD V4.55 week38+, ATP ZLD V4.55 week38+, NSG 1.33 patch 4+
Vendor Advisory: https://www.zyxel.com/support/Zyxel-security-advisory-for-command-injection-vulnerability-of-firewalls.shtml
Restart Required: Yes
Instructions:
1. Download the latest firmware from Zyxel support portal. 2. Backup current configuration. 3. Upload and install the firmware update via web interface or CLI. 4. Reboot the device. 5. Verify the update was successful.
🔧 Temporary Workarounds
Restrict Admin Access
allLimit administrative access to trusted IP addresses and networks only
Configure firewall rules to restrict management interface access to specific IP ranges
Implement Strong Authentication
allEnforce multi-factor authentication for all admin accounts
Configure RADIUS/TACACS+ with MFA for admin authentication
🧯 If You Can't Patch
- Implement strict network segmentation to isolate affected devices from critical systems
- Monitor all admin account activity and implement alerting for suspicious password change operations
🔍 How to Verify
Check if Vulnerable:
Check device firmware version via web interface (System > Maintenance > Firmware) or CLI using 'show version' command and compare against affected versionsCheck Version:
show versionVerify Fix Applied:
Verify firmware version is at or above the patched versions listed in the advisory📡 Detection & Monitoring
Log Indicators:
- Unusual admin password change activities
- Suspicious commands in system logs following password operations
- Multiple failed admin login attempts followed by successful login
Network Indicators:
- Unexpected outbound connections from firewall devices
- Anomalous traffic patterns from management interfaces
SIEM Query:
source="zyxel-firewall" AND (event_type="password_change" OR cmd="chg_exp_pwd") AND (user="admin" OR privileged_user=true)🔗 References
- https://www.zyxel.com/support/Zyxel-security-advisory-for-command-injection-vulnerability-of-firewalls.shtml
- https://www.zyxel.com/us/en/support/security_advisories.shtml
- https://www.zyxel.com/support/Zyxel-security-advisory-for-command-injection-vulnerability-of-firewalls.shtml
- https://www.zyxel.com/us/en/support/security_advisories.shtml
