CVE-2024-40890: Zyxel VMG1312-B10A Firmware RCE Flaw

CVE-2024-40890 Overview

CVE-2024-40890 is a post-authentication command injection vulnerability affecting the CGI program in multiple Zyxel legacy DSL CPE (Customer Premises Equipment) devices. This vulnerability allows an authenticated attacker to execute arbitrary operating system commands on affected devices by sending specially crafted HTTP POST requests to the vulnerable CGI endpoint.

Important Note: This vulnerability was assigned while the affected products were already unsupported by the vendor. Organizations using these legacy devices face significant risk as no patches will be provided.

Critical Impact

This vulnerability is actively being exploited in the wild and has been added to CISA's Known Exploited Vulnerabilities (KEV) catalog. Authenticated attackers can achieve full command execution on vulnerable Zyxel DSL routers, potentially compromising network infrastructure and enabling lateral movement.

Affected Products

• Zyxel VMG1312-B10A Firmware
• Zyxel VMG1312-B10B Firmware
• Zyxel VMG1312-B10E Firmware
• Zyxel VMG3312-B10A Firmware
• Zyxel VMG3313-B10A Firmware
• Zyxel VMG3926-B10B Firmware
• Zyxel VMG4325-B10A Firmware (version 1.00(AAFR.4)C0_20170615 confirmed vulnerable)
• Zyxel VMG4380-B10A Firmware
• Zyxel VMG8324-B10A Firmware
• Zyxel VMG8924-B10A Firmware
• Zyxel SBG3300-N000 Firmware
• Zyxel SBG3300-NB00 Firmware
• Zyxel SBG3500-N000 Firmware
• Zyxel SBG3500-NB00 Firmware

Discovery Timeline

• 2025-02-04 - CVE-2024-40890 published to NVD
• 2025-10-27 - Last updated in NVD database

Technical Details for CVE-2024-40890

Vulnerability Analysis

This post-authentication command injection vulnerability (CWE-78: Improper Neutralization of Special Elements used in an OS Command) exists within the CGI program running on affected Zyxel DSL CPE devices. The vulnerability stems from insufficient input validation in HTTP POST request handling, allowing attackers who have obtained valid credentials to inject and execute arbitrary OS commands with the privileges of the web server process.

The attack requires network access to the device's management interface and valid authentication credentials. Given that many of these legacy devices may still use default or weak credentials, the authentication requirement may be trivially bypassed in many deployments. Once exploited, an attacker gains the ability to execute commands directly on the underlying Linux-based operating system, potentially leading to complete device compromise.

Root Cause

The root cause of CVE-2024-40890 is improper input sanitization in the CGI program's HTTP POST request handler. User-supplied input from POST parameters is passed directly to system shell commands without adequate filtering or escaping of special characters. This allows attackers to break out of the intended command context and inject additional OS commands using shell metacharacters such as semicolons (;), pipes (|), or command substitution operators.

Attack Vector

The attack is conducted over the network by sending a maliciously crafted HTTP POST request to the vulnerable CGI endpoint on the device's web management interface. The attack flow typically involves:

1. The attacker authenticates to the device's web interface using valid credentials (obtained through default credentials, credential stuffing, or other means)
2. A crafted HTTP POST request is sent containing command injection payloads in vulnerable parameters
3. The CGI program processes the request and passes the malicious input to system shell functions
4. The injected commands execute with the privileges of the web server process
5. The attacker achieves arbitrary command execution on the device

The vulnerability allows injection of arbitrary shell commands that can be used to download additional malware, establish reverse shells, modify device configuration, intercept network traffic, or pivot to other devices on the network.

Detection Methods for CVE-2024-40890

Indicators of Compromise

• Unusual HTTP POST requests to CGI endpoints on Zyxel device management interfaces containing shell metacharacters (;, |, $(), backticks)
• Unexpected outbound connections from Zyxel DSL CPE devices to external IP addresses
• Modified device configurations or unauthorized user accounts created on affected devices
• Evidence of command execution artifacts such as downloaded files in /tmp directories or unexpected processes running on the device

Detection Strategies

• Deploy network intrusion detection rules to identify HTTP POST requests containing command injection patterns targeting Zyxel CGI endpoints
• Monitor authentication logs on affected devices for brute-force attempts or successful logins from unexpected sources
• Implement egress filtering and monitor for unusual outbound traffic patterns from network edge devices
• Use SentinelOne Singularity to detect and respond to post-exploitation activities on networks where compromised devices may serve as pivot points

Monitoring Recommendations

• Enable comprehensive logging on network firewalls and web application firewalls protecting device management interfaces
• Regularly audit access logs on Zyxel devices for suspicious authentication patterns and unusual POST request activity
• Monitor for known indicators associated with botnet activity targeting IoT and network devices
• Review network segmentation to ensure management interfaces are not accessible from untrusted networks

How to Mitigate CVE-2024-40890

Immediate Actions Required

• Immediately identify all Zyxel VMG and SBG series devices in your environment that are affected by this vulnerability
• Restrict network access to device management interfaces using firewall rules, VPNs, or network segmentation to limit exposure
• Replace affected end-of-life devices with supported alternatives as no patches will be released for these legacy products
• Change default credentials on all devices and implement strong, unique passwords for device administration

Patch Information

No patches are available for CVE-2024-40890. Zyxel has confirmed that all affected products have reached end-of-life status and are no longer receiving security updates. According to the Zyxel Security Advisory, the recommended course of action is to replace these legacy DSL CPE devices with newer, supported models.

This vulnerability has been added to the CISA Known Exploited Vulnerabilities Catalog, requiring federal agencies to take immediate action.

Workarounds

• Disable remote management interfaces entirely if not required for operations
• Implement strict network access controls to limit management interface access to trusted internal IP addresses only
• Deploy a Web Application Firewall (WAF) in front of management interfaces to filter malicious POST requests
• Monitor and alert on authentication attempts to affected devices as a compensating control until replacement

# Example firewall rule to restrict management access (iptables)
# Replace 192.168.1.0/24 with your trusted management network
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP