Attacker Value
Very High
(2 users assessed)
(2 users assessed)
User Interaction
Privileges Required
Attack Vector


Disclosure Date: February 20, 2020
Exploited in the Wild
Add MITRE ATT&CK tactics and techniques that apply to this CVE.


Multiple ZyXEL network-attached storage (NAS) devices running firmware version 5.21 contain a pre-authentication command injection vulnerability, which may allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable device. ZyXEL NAS devices achieve authentication by using the weblogin.cgi CGI executable. This program fails to properly sanitize the username parameter that is passed to it. If the username parameter contains certain characters, it can allow command injection with the privileges of the web server that runs on the ZyXEL device. Although the web server does not run as the root user, ZyXEL devices include a setuid utility that can be leveraged to run any command with root privileges. As such, it should be assumed that exploitation of this vulnerability can lead to remote code execution with root privileges. By sending a specially-crafted HTTP POST or GET request to a vulnerable ZyXEL device, a remote, unauthenticated attacker may be able to execute arbitrary code on the device. This may happen by directly connecting to a device if it is directly exposed to an attacker. However, there are ways to trigger such crafted requests even if an attacker does not have direct connectivity to a vulnerable devices. For example, simply visiting a website can result in the compromise of any ZyXEL device that is reachable from the client system. Affected products include: NAS326 before firmware V5.21(AAZF.7)C0 NAS520 before firmware V5.21(AASZ.3)C0 NAS540 before firmware V5.21(AATB.4)C0 NAS542 before firmware V5.21(ABAG.4)C0 ZyXEL has made firmware updates available for NAS326, NAS520, NAS540, and NAS542 devices. Affected models that are end-of-support: NSA210, NSA220, NSA220+, NSA221, NSA310, NSA310S, NSA320, NSA320S, NSA325 and NSA325v2

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  • Attacker Value
    Very High
  • Exploitability
Technical Analysis

This affects products that are designed for Small – Medium enterprise more than home users so it is likely to have a higher value to an attacker.
As some of these devices like firewalls are designed to operate at the network perimeter It is fairly simple to identify vulnerable products.

The exploit is an unauthenticated remote code execution attack that leads to full root-level access on the affected device. This level of access can be used to pivot into the internal network or in the case of Firewall products it could be used to alter or intercept traffic from inside the organisation.

Exploit code has been seen for sale and not publicly released, however, the patches are now available to it is possible to reverse engineer the location of the exploit using information from the advisory and access to the firmware.

Additionally, ZyXel has devices that are no longer supported and will not receive a patch. Full details of affected products and patches can be found on their website.

Details on the reporting and identification can be found –

Technical Analysis

Wasn’t able to get a test device or emulate the firmware in QEMU, but I did dig into the patch a bit with Ghidra.

wvu@kharak:~/Downloads$ diff <(strings weblogin.cgi.unpatched) <(strings weblogin.cgi.patched)
> pcre_exec
> pcre_compile
> pcre_free_study
> pcre_study
> pcre_free
< free
> ^([a-zA-Z]|[a-zA-Z][a-zA-Z0-9._-]|[a-zA-Z][a-zA-Z0-9 ._-]{0,30}[a-zA-Z0-9._-])$
> ^[^!#-&(-]*$

Analysis of a patched function (not shown here) indicates Zyxel applied a regex to the username field.

But it appears they left what appears to be the command injection unfixed.



CVSS V3 Severity and Metrics
Base Score:
9.8 Critical
Impact Score:
Exploitability Score:
Attack Vector (AV):
Attack Complexity (AC):
Privileges Required (PR):
User Interaction (UI):
Scope (S):
Confidentiality (C):
Integrity (I):
Availability (A):

General Information


  • ZyXEL


  • NAS326,
  • NAS520,
  • NAS540,
  • NAS542,
  • NSA210,
  • NSA220,
  • NSA220+,
  • NSA221,
  • NSA310,
  • NSA320,
  • NSA320S,
  • NSA325,
  • NSA325v2
Technical Analysis