A recently disclosed zero-day vulnerability in Zyxel network-attached storage (NAS) devices also impacts over twenty of the vendor’s firewalls.

The security flaw, which was issued CVE identifier CVE-2020-9054, can be exploited remotely, without authentication to execute arbitrary code on the affected devices.

Residing in the weblogin.cgi CGI program, the issue is that the username parameter is not properly sanitized. Thus, an attacker could exploit the flaw for command injection by including certain characters in the username.

While weblogin.cgi does not run as root, the vulnerable devices include a setuid utility that the attacker can abuse to run commands with root privileges.

The bug can be exploited by sending a specially-crafted HTTP POST or GET request to a vulnerable device. If the device is not directly exposed to the Internet, but protected by a firewall, exploitation is still possible if the user navigates to a malicious site.

Earlier this week, Zyxel published an advisory on the vulnerability, revealing that it impacted over a dozen NAS devices, including ten that were no longer supported.

On Wednesday, the networking devices vendor updated the advisory to add a total of 23 UTM, ATP, and VPN firewalls to the list of vulnerable products. The flaw, it says, impacts firmware versions ZLD V4.35 Patch 0 through ZLD V4.35 Patch 2.

The list of impacted devices now includes the following firewalls: ATP100, ATP200, ATP500, ATP800, USG20-VPN, USG20W-VPN, USG40, USG40W, USG60, USG60W, USG110, USG210, USG310, USG1100, USG1900, USG2200, VPN50, VPN100, VPN300, VPN1000, ZyWALL110, ZyWALL310, and ZyWALL1100.

An exploit for the vulnerability has been available on underground forums for a couple of weeks, and has already caught the attention of cybercrime groups, including the threat actor behind the Emotet Trojan.

Zyxel has released patches for all supported devices, which include the aforementioned firewall devices, as well as the NAS326, NAS520, NAS540, and NAS542 storage devices.

Users who cannot apply the available fixes are advised to isolate their devices from the Internet, to ensure they are not targeted in malicious attacks.

“Command injection within a login page is about as bad as it gets and the lack of any cross-site request forgery token makes this vulnerability particularly dangerous. As demonstrated by CERT, JavaScript running in the browser is enough to identify and exploit vulnerable devices on the network,” Craig Young, computer security researcher for Tripwire, told SecurityWeek in an emailed comment.

“Vulnerabilities like this are why I strongly recommend that vendors use HTTPS and network owners use segmentation. Generally speaking, it is not a good idea to perform general web browsing from the same network which has networked devices like printers, media boxes, file servers, or other systems which may serve vulnerable HTTP interfaces,” Young added.

Related: Zyxel Patches Zero-Day Vulnerability in Network Storage Products

Related: Zyxel Devices Can Be Hacked via DNS Requests, Hardcoded Credentials

Related: SOHOpelessly Broken 2.0: 125 Vulnerabilities Found in Routers, NAS Devices

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Ionut Arghire is an international correspondent for SecurityWeek.

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