Zyxel NAS Firmware 5.21- Remote Code Execution

CVE Details

• Protection status: Protected by Atomicorp rules · About protection statuses
• CVE: CVE-2020-9054
• Name: Zyxel NAS Firmware 5.21- Remote Code Execution
• Severity: Critical
• CVSS: 9.8 (v3.1) (Critical)
• CVSS version: 3.1
• CVSS source: NVD
• CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  • Attack vector: Network
  • Attack complexity: Low
  • Privileges required: None
  • User interaction: None
  • Scope: Unchanged
  • Confidentiality impact: High
  • Integrity impact: High
  • Availability impact: High
• CWE: CWE-78 — Improper Neutralization of Special Elements used in an OS Command (‘OS Command Injection’)
• Related CAPEC attack patterns (via MITRE CWE relationships): CAPEC-108 (opens in a new tab) , CAPEC-15 (opens in a new tab) , CAPEC-43 (opens in a new tab) , CAPEC-6 (opens in a new tab) , CAPEC-88 (opens in a new tab)
• Vendor: zyxel
• Vulnerable products: nas326 firmware , nas520 firmware , nas540 firmware , nas542 firmware , atp100 firmware , atp200 firmware , atp500 firmware , atp800 firmware , usg20-vpn firmware , usg20w-vpn firmware , usg40 firmware , usg40w firmware , usg60 firmware , usg60w firmware , usg110 firmware , usg210 firmware , usg310 firmware , usg1100 firmware , usg1900 firmware , usg2200 firmware , vpn50 firmware , vpn100 firmware , vpn300 firmware , vpn1000 firmware , zywall110 firmware , zywall310 firmware , zywall1100 firmware
• CVE published: 2020-03-04
• CVE last modified: 2026-10-08
• Affected versions (NVD configurations):
  • nas326 firmware: < 5.21(aazf.10)c0
  • nas520 firmware: < 5.21(aasz.3)c0
  • nas540 firmware: < 5.21(aatb.7)c0
  • nas542 firmware: < 5.21(abag.7)c0
  • atp100 firmware: >= 4.35, < 4.35(abps.3)c0
  • atp200 firmware: >= 4.35, < 4.35(abfw.3)c0
  • atp500 firmware: >= 4.35, < 4.35(abfu.3)c0
  • atp800 firmware: >= 4.35, < 4.35(abiq.3)c0
  • usg20-vpn firmware: >= 4.35, < 4.35(abaq.3)c0
  • usg20w-vpn firmware: >= 4.35, < 4.35(abar.3)c0
  • usg40 firmware: >= 4.35, < 4.35(aala.3)c0
  • usg40w firmware: >= 4.35, < 4.35(aalb.3)c0
  • usg60 firmware: >= 4.35, < 4.35(aaky.3)c0
  • usg60w firmware: >= 4.35, < 4.35(aakz.3)c0
  • usg110 firmware: >= 4.35, < 4.35(aaph.3)c0
  • usg210 firmware: >= 4.35, < 4.35(aapi.3)c0
  • usg310 firmware: >= 4.35, < 4.35(aapj.3)c0
  • usg1100 firmware: >= 4.35, < 4.35(aapk.3)c0
  • usg1900 firmware: >= 4.35, < 4.35(aapl.3)c0
  • usg2200 firmware: >= 4.35, < 4.35(abae.3)c0
  • vpn50 firmware: >= 4.35, < 4.35(abhl.3)c0
  • vpn100 firmware: >= 4.35, < 4.35(abfv.3)c0
  • vpn300 firmware: >= 4.35, < 4.35(abfc.3)c0
  • vpn1000 firmware: >= 4.35, < 4.35(abip.3)c0
  • zywall110 firmware: >= 4.35, < 4.35(aaaa.3)c0
  • zywall310 firmware: >= 4.35, < 4.35(aaab.3)c0
  • zywall1100 firmware: >= 4.35, < 4.35(aaac.3)c0
• EPSS: 0.9999; 100.0th percentile as of 2026-10-26 (FIRST (opens in a new tab) )
• CISA KEV: Listed (added 2022-03-25)
  • Known ransomware campaign use: Unknown
• Latest Update: 2026-09-06
• Selected Rule IDs observed: 312659 , 340023 , 344360 , 344361 , 344363 , 344370 , 347009
• Matching rules: 7; rules with blocking actions: 7
• Would-block Rule IDs: 312659 , 340023 , 344360 , 344361 , 344363 , 344370 , 347009
• Request surfaces inspected: JSON request data, Request URI, Request argument names, Request arguments, Request cookies, SOAP request data, XML request data

Vulnerability Description

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

Recommended Actions

1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (312659 , 340023 , 344360 , 344361 , 344363 , 344370 , 347009 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

   • Affected versions: ZyXEL NAS326 versions through V5.21(AAZF.7)C0.
   • Affected versions: ZyXEL NAS520 versions through V5.21(AASZ.3)C0.
   • Affected versions: ZyXEL NAS540 versions through V5.21(AATB.4)C0.
   • Affected versions: ZyXEL NAS542 versions through V5.21(ABAG.4)C0.
   • Affected versions: ZyXEL NSA210 version all.
   • Affected versions: ZyXEL NSA220 version all.
   • Affected versions: ZyXEL NSA220+ version all.
   • Affected versions: ZyXEL NSA221 version all.
   • Affected versions: ZyXEL NSA310 versions through V4.75(AALH.2)C0.
   • Affected versions: ZyXEL NSA320 version all.
   • Affected versions: ZyXEL NSA320S versions through V4.75(AANV.2)C0.
   • Affected versions: ZyXEL NSA325 versions through V4.81(AAAJ.1)C0.
   • Affected versions: ZyXEL NSA325v2 versions through V4.81(AALS.1)C0.
   • CNA guidance source: certcc (opens in a new tab)
   • Vendor recommendation: ZyXEL has made firmware updates available for NAS326, NAS520, NAS540, NAS542, 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 devices.
   • Vendor workaround: Block access to the ZyXEL device web interface:

This issue can be mitigated by blocking (for example with a firewall) access to the web interface (80/tcp and 443/tcp) of any vulnerable ZyXEL device. Any machine that can access the ZyXEL web interface should not also be able to access the internet.

Restrict access to vulnerable ZyXEL devices:

Direct exploitation of this vulnerability can be mitigated by restricting access to vulnerable devices. In particular, do not expose such devices directly to the internet. Note however, that it is still possible for attackers to exploit devices that are not directly connected to the internet. For example, by way of viewing a web page.

• CISA KEV required action: Apply updates per vendor instructions.
• CISA KEV federal remediation due date: 2022-04-15

3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

• NVD (opens in a new tab)
• CVE Record (opens in a new tab)
• Exploit or technical research: https://cwe.mitre.org/data/definitions/78.html (opens in a new tab)
• Exploit or technical research: https://kb.cert.org/artifacts/cve-2020-9054.html (opens in a new tab)
• Exploit or technical research: https://kb.cert.org/vuls/id/498544/ (opens in a new tab)
• Exploit or technical research: https://krebsonsecurity.com/2020/02/zyxel-fixes-0day-in-network-storage-devices/ (opens in a new tab)
• Vendor advisory: https://www.zyxel.com/support/remote-code-execution-vulnerability-of-NAS-products.shtml (opens in a new tab)
• CVE authority reference: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-9054 (opens in a new tab)

Test or Research Source

• Source: http/cves/2020/CVE-2020-9054.yaml (opens in a new tab)
• Tags: cve, cve2020, injection, kev, rce, vkev, vuln, zyxel

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

• Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
• Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
• Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
• Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
• Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ (opens in a new tab) .