CVE-2025-11730: Zyxel Firewall DDNS Command Injection

CVE-2025-11730 Overview

A post-authentication command injection vulnerability exists in the Dynamic DNS (DDNS) configuration CLI command across multiple Zyxel firewall product lines. This vulnerability affects Zyxel ATP series, USG FLEX series, USG FLEX 50(W) series, and USG20(W)-VPN series firmware versions from V5.35 through V5.41. An authenticated attacker with administrator privileges can execute arbitrary operating system (OS) commands on affected devices by supplying a specially crafted string as an argument to the vulnerable CLI command.

Critical Impact

Authenticated administrators can achieve full OS-level command execution on network security appliances, potentially compromising the entire network perimeter.

Affected Products

• Zyxel ATP series firmware versions V5.35 through V5.41
• Zyxel USG FLEX series firmware versions V5.35 through V5.41
• Zyxel USG FLEX 50(W) series firmware versions V5.35 through V5.41
• Zyxel USG20(W)-VPN series firmware versions V5.35 through V5.41

Discovery Timeline

• 2026-02-05 - CVE-2025-11730 published to NVD
• 2026-02-05 - Last updated in NVD database

Technical Details for CVE-2025-11730

Vulnerability Analysis

This vulnerability is classified as CWE-78 (Improper Neutralization of Special Elements used in an OS Command), commonly known as OS Command Injection. The flaw resides in the DDNS configuration CLI command processing logic within Zyxel ZLD firmware. When an authenticated administrator configures Dynamic DNS settings through the command-line interface, user-supplied input is passed to underlying operating system functions without proper sanitization or validation.

The attack requires network access to the device's management interface and valid administrator credentials. Once authenticated, an attacker can inject shell metacharacters or command separators within the DDNS configuration parameters. These injected commands are then executed with the privileges of the underlying operating system process, typically root or an equivalent privileged context on embedded network appliances.

The impact of successful exploitation is significant—an attacker gains the ability to execute arbitrary commands on the firewall appliance itself. This can lead to complete device compromise, including exfiltration of configuration data, credential harvesting, installation of persistent backdoors, lateral movement within the network, or disabling security functions entirely.

Root Cause

The root cause of CVE-2025-11730 is insufficient input validation and sanitization in the DDNS configuration CLI command handler. The firmware fails to properly escape or filter shell metacharacters (such as ;, |, &, $(), and backticks) before incorporating user-supplied DDNS parameters into OS command execution contexts. This allows specially crafted input to break out of the intended command structure and execute arbitrary commands on the underlying Linux-based operating system.

Attack Vector

The attack vector for this vulnerability is network-based, requiring the attacker to have:

1. Network connectivity to the device's management interface (typically accessible via SSH or web-based CLI)
2. Valid administrator credentials for authentication
3. Access to the DDNS configuration CLI commands

An attacker who has compromised administrator credentials through phishing, credential stuffing, or insider access can exploit this vulnerability by navigating to the DDNS configuration CLI command and providing a malicious payload as a configuration parameter. The injected commands are executed in the context of the firewall's operating system, potentially with root privileges.

The vulnerability mechanism involves crafting DDNS configuration arguments that contain shell command injection payloads. When the CLI processes these arguments, the unsanitized input is passed to system shell functions, resulting in command execution. For detailed technical information, refer to the Zyxel Security Advisory.

Detection Methods for CVE-2025-11730

Indicators of Compromise

• Unusual CLI session activity involving DDNS configuration commands with abnormal or encoded string parameters
• Unexpected outbound network connections originating from the firewall appliance to external hosts
• Anomalous processes spawned on the firewall device outside normal operational patterns
• Modification of system files, configuration, or the presence of unauthorized scripts on the appliance

Detection Strategies

• Monitor CLI session logs for DDNS configuration commands containing shell metacharacters (;, |, &, $(, backticks)
• Implement behavioral analysis on firewall appliances to detect unexpected process execution or network activity
• Review authentication logs for administrator access patterns, particularly from unusual source IPs or at unusual times
• Deploy network detection rules to identify potential command injection payloads in management traffic

Monitoring Recommendations

• Enable comprehensive logging for all CLI commands executed on Zyxel firewall appliances
• Configure SIEM alerts for suspicious patterns in DDNS configuration command arguments
• Implement session monitoring for privileged administrator accounts accessing firewall management interfaces
• Regularly audit administrator account usage and enforce least-privilege access principles

How to Mitigate CVE-2025-11730

Immediate Actions Required

• Update affected Zyxel devices to firmware versions V5.42 or later as specified in the vendor advisory
• Restrict management interface access to trusted IP addresses only using access control lists
• Review and audit all administrator accounts, removing unnecessary privileged access
• Enable multi-factor authentication for administrator accounts where supported

Patch Information

Zyxel has released firmware updates to address this command injection vulnerability. Affected organizations should download and apply the latest firmware versions from the official Zyxel support portal. Detailed patch information and download links are available in the Zyxel Security Advisory.

Workarounds

• Restrict CLI access to only essential personnel and audit all administrator accounts
• Implement network segmentation to limit management interface exposure to trusted management networks only
• Disable DDNS functionality if not required for operations until patches can be applied
• Monitor all CLI sessions in real-time and implement session recording for forensic purposes

# Configuration example - Restrict management access to trusted networks
# Access the Zyxel device CLI and configure management access restrictions

# Example: Limit management access to specific trusted subnet
configure terminal
management interface access-list
  permit 192.168.100.0/24
  deny any
exit

# Enable logging for CLI commands
logging cli-commands enable