CVE-2025-13943: Zyxel EX3301-T0 Auth Bypass Vulnerability

CVE-2025-13943 Overview

A post-authentication command injection vulnerability exists in the log file download function of multiple Zyxel CPE, DSL/Ethernet routers, fiber ONTs, and wireless extenders. This vulnerability allows an authenticated attacker to execute arbitrary operating system (OS) commands on affected devices, potentially leading to complete device compromise.

The vulnerability is classified as CWE-78 (Improper Neutralization of Special Elements used in an OS Command), indicating that user-supplied input is improperly sanitized before being passed to system command execution functions.

Critical Impact

Authenticated attackers can achieve full remote command execution on affected Zyxel network devices, enabling network pivoting, data exfiltration, and persistent access to enterprise and home networks.

Affected Products

• Zyxel EX3301-T0 firmware versions through 5.50(ABVY.7)C0
• Zyxel DX Series (DX3300-T0, DX3300-T1, DX3301-T0, DX4510-B0, DX4510-B1, DX5401-B1)
• Zyxel EX Series (EX2210-T0, EX3300-T0, EX3300-T1, EX3500-T0, EX3501-T0, EX3510-B0, EX3510-B1, EX3600-T0, EX5401-B1, EX5510-B0, EX5512-T0, EX5601-T0, EX5601-T1, EX7501-B0, EX7710-B0)
• Zyxel VMG Series (VMG3625-T50B, VMG4005-B50A, VMG4005-B60A, VMG4927-B50A, VMG8623-T50B)
• Zyxel PM Series (PM3100-T0, PM5100-T0, PM5100-T1, PM7300-T0, PM7500-00)
• Zyxel WX Series (WX3100-T0, WX3401-B1, WX5600-T0, WX5610-B0)
• Zyxel EMG Series (EMG3525-T50B, EMG5523-T50B, EMG6726-B10A)
• Zyxel EE Series (EE3301-00, EE5301-00, EE6510-10)
• Zyxel PE Series (PE3301-00, PE5301-01)
• Zyxel PX Series (PX3321-T1, PX5301-T0)
• Zyxel WE Series (WE3300-00, WE4600-00)
• Zyxel AX7501-B1, AM7510-00, DM4200-B0, GM4100-B0

Discovery Timeline

• 2026-02-24 - CVE-2025-13943 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2025-13943

Vulnerability Analysis

This command injection vulnerability resides in the log file download functionality of the affected Zyxel devices' web management interface. When an authenticated user requests a log file download, the device processes user-controllable parameters that are subsequently passed to underlying shell commands without proper sanitization or escaping.

The impact of successful exploitation is significant. An attacker who has obtained valid credentials (either through credential theft, brute force, or default credentials) can leverage this vulnerability to execute arbitrary commands with the privileges of the web server process, which typically runs as root on embedded Linux systems. This grants the attacker complete control over the network device.

Root Cause

The root cause is improper neutralization of special characters in user-supplied input before it is incorporated into OS commands (CWE-78). The log file download function fails to validate or sanitize parameters that are used in shell command construction, allowing metacharacters such as semicolons, pipes, or backticks to break out of the intended command context and execute attacker-controlled commands.

Attack Vector

The attack requires network access to the device's management interface and valid authentication credentials. Once authenticated, an attacker can craft malicious requests to the log file download endpoint, injecting OS commands through vulnerable parameters. The commands execute in the context of the web server, typically with root privileges on these embedded devices.

A typical exploitation scenario involves:

1. Attacker authenticates to the device's web interface using valid credentials
2. Attacker navigates to the log download functionality
3. Attacker crafts a request containing shell metacharacters and malicious commands in the vulnerable parameter
4. The device's backend processes the request, executing the injected commands
5. Attacker gains shell access or executes further malicious payloads

Detection Methods for CVE-2025-13943

Indicators of Compromise

• Unusual process spawning from web server processes (e.g., httpd, lighttpd, or similar)
• Unexpected outbound network connections from Zyxel devices to unknown IP addresses
• New or modified files in writable directories such as /tmp or /var
• Presence of reverse shell processes or netcat listeners on affected devices
• Anomalous log entries showing command injection patterns in HTTP request parameters

Detection Strategies

• Monitor HTTP traffic to Zyxel device management interfaces for requests containing shell metacharacters (;, |, $(), backticks) in log download parameters
• Implement network-based intrusion detection rules to identify command injection payloads targeting Zyxel endpoints
• Deploy endpoint detection on network segments to identify anomalous traffic patterns from network infrastructure devices
• Review authentication logs for suspicious login attempts followed by log download requests

Monitoring Recommendations

• Enable comprehensive logging on all Zyxel devices and forward logs to a centralized SIEM
• Configure alerts for multiple failed authentication attempts followed by successful logins
• Monitor for firmware changes or configuration modifications on affected devices
• Implement network segmentation monitoring to detect lateral movement from compromised devices

How to Mitigate CVE-2025-13943

Immediate Actions Required

• Update all affected Zyxel devices to the latest firmware version as specified in the Zyxel Security Advisory
• Change all default and weak credentials on affected devices immediately
• Restrict management interface access to trusted networks or IP addresses only
• Audit authentication logs for signs of compromise prior to patching

Patch Information

Zyxel has released security patches addressing this vulnerability. Refer to the Zyxel Security Advisory for specific firmware versions and download links for each affected product model. Organizations should prioritize patching based on device exposure and network criticality.

Workarounds

• Disable remote management access and only allow local/console administration where feasible
• Implement strict network access control lists (ACLs) to limit management interface access to authorized administrator workstations only
• Deploy a VPN requirement for remote administration to reduce attack surface
• Consider disabling the log download functionality if not operationally required until patches can be applied
• Enable multi-factor authentication if supported by the device firmware

# Example: Restrict management access via firewall rules
# Block external access to web management port (example for upstream firewall)
iptables -A INPUT -p tcp --dport 80 -s 0.0.0.0/0 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 0.0.0.0/0 -j DROP
# Allow only trusted admin network
iptables -A INPUT -p tcp --dport 80 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT