Zyxel Command Injection Vulnerabilities in CPE and Extenders

On April 28, 2026, Zyxel issued a security advisory (AV26-399) detailing command injection vulnerabilities present in several of their customer premise equipment (CPE) and wireless extender product lines. The affected products include 4G LTE/5G NR CPE, DSL/Ethernet CPE, Fiber ONTs, and Wireless Extenders. The advisory urges users and administrators to promptly review the provided web links and apply the necessary updates to mitigate the risk of exploitation. Successful exploitation of these vulnerabilities could enable attackers to execute arbitrary commands on the affected devices, potentially leading to unauthorized access, device compromise, and network disruption. Due to the widespread use of these devices, particularly in home and small business environments, the potential impact is significant.

Attack Chain

1. Attacker identifies a vulnerable Zyxel device with an exposed management interface.
2. The attacker crafts a malicious HTTP request containing a command injection payload within a vulnerable parameter.
3. The request is sent to the Zyxel device through the web management interface.
4. The device processes the request and inadvertently executes the injected command due to insufficient input validation.
5. The attacker gains arbitrary command execution on the device’s operating system.
6. The attacker uses the compromised device to pivot further into the network.
7. The attacker may install malware or create a reverse shell for persistent access.
8. The attacker compromises other devices or exfiltrates sensitive data from the network.

Impact

Successful exploitation of these command injection vulnerabilities could allow attackers to gain complete control over the affected Zyxel devices. This could lead to unauthorized access to the network, modification of device configurations, and potential data breaches. Given the ubiquity of these Zyxel products, a large number of users and organizations are potentially at risk. The impact could range from disruption of internet services to full network compromise and data theft.

Recommendation

• Review the Zyxel security advisory (https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-command-injection-vulnerabilities-in-certain-4g-lte-5g-nr-cpe-dsl-ethernet-cpe-fiber-onts-and-wireless-extenders-04-28-2026) to identify affected devices and specific vulnerabilities.
• Apply the recommended firmware updates provided by Zyxel to patch the command injection vulnerabilities.
• Monitor web server logs for suspicious HTTP requests containing command injection attempts targeting Zyxel devices by deploying the “Detect Zyxel Command Injection Attempt” Sigma rule.
• Implement network segmentation to limit the impact of a potential device compromise.
• Regularly review and update device configurations to ensure strong passwords and disable unnecessary services.