Zyxel security advisory for XSS vulnerability and post-authentication command injection vulnerability in firewalls

CVE: CVE-2023-27990, CVE-2023-27991

Zyxel has released patches for some firewalls affected by a stored cross-site scripting (XSS) vulnerability and a post-authentication command injection vulnerability. Users are advised to install them for optimal protection.

What are the vulnerabilities?


The XSS vulnerability in some firewall versions could allow an authenticated attacker with administrator privileges to store malicious scripts in a vulnerable device. A successful XSS attack could then result in the stored malicious scripts being executed when the user visits the Logs page of the GUI on the device.


The post-authentication command injection vulnerability in the CLI command of some firewall versions could allow an authenticated attacker to execute some OS commands remotely. Note that WAN access is disabled by default on the firewall devices.

What versions are vulnerable—and what should you do?

After a thorough investigation, we’ve identified the vulnerable products that are within their vulnerability support period and released patches to address the vulnerability, as shown in the table below.

Affected series Affected version Patch availability
ATP ZLD V4.32~V5.35 ZLD V5.36
USG FLEX ZLD V4.50~V5.35 ZLD V5.36
USG FLEX 50(W) / USG20(W)-VPN ZLD V4.16~V5.35 ZLD V5.36
VPN ZLD V4.30~V5.35 ZLD V5.36
Got a question?

Please contact your local service rep or visit Zyxel’s Community for further information or assistance.


Thanks to Alessandro Sgreccia from Tecnical Service SRL for reporting the issues to us.

Revision history

2023-4-25: Initial release