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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 15 Apr 2022.
Nas326 FirmwareOperating system · Zyxel

CVE-2020-9054

CRITICAL · 9.8 CVSS v3.1 Published 2020-03-04
Fix available
A fix is available. Upgrade to 4.35 / 5.21 or later.
See remediation →
100/100
Remediation priority · Urgent
In the wild High EPSS Public exploit Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Pre-authentication command injection vulnerability in ZyXEL NAS devices (NAS326, NAS520, NAS540, NAS542) running firmware before V5.21. The weblogin.cgi CGI program fails to sanitize the username parameter, allowing remote unauthenticated attackers to inject commands. Although the web server doesn't run as root, attackers leverage a setuid utility to achieve root-level code execution via specially-crafted HTTP POST/GET requests.

MitigationApply available firmware updates from ZyXEL for supported models (NAS326 V5.21(AAZF.7)C0 or later, NAS520 V5.21(AASZ.3)C0 or later, NAS540 V5.21(AATB.4)C0 or later, NAS542 V5.21(ABAG.4)C0 or later). For end-of-support models (NSA210, NSA220, NSA220+, NSA221, NSA310, NSA310S, NSA320, NSA320S, NSA325, NSA325v2), isolate from network or decommission.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Nas326 FirmwareOperating system
Affected:< 5.21\(aazf.7\)c0
Nas520 FirmwareOperating system
Affected:< 5.21\(aasz.3\)c0
Nas540 FirmwareOperating system
Affected:< 5.21\(aatb.4\)c0
Nas542 FirmwareOperating system
Affected:< 5.21\(abag.4\)c0
Atp100 FirmwareOperating system
Affected:>= 4.35, < 4.35\(abps.3\)c0
Atp200 FirmwareOperating system
Affected:>= 4.35, < 4.35\(abfw.3\)c0
Atp500 FirmwareOperating system
Affected:>= 4.35, < 4.35\(abfu.3\)c0
Atp800 FirmwareOperating system
Affected:>= 4.35, < 4.35\(abiq.3\)c0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify the ZyXEL device model
    Access the device web interface or check the device label/menu to determine the exact model number (NAS326, NAS520, NAS540, NAS542, ATP100, ATP200, ATP500, or ATP800)
    Affected if The device model is one of NAS326, NAS520, NAS540, NAS542, ATP100, ATP200, ATP500, or ATP800
  2. Determine the installed firmware version
    Log into the device web interface and navigate to the System > Firmware or Administration section to view the current firmware version, or use the device CLI if available
    Affected if The firmware version is lower than 5.21(aazf.7)c0 for NAS326, 5.21(aasz.3)c0 for NAS520, 5.21(aatb.4)c0 for NAS540, 5.21(abag.4)c0 for NAS542, or lower than 4.35(abps.3)c0/4.35(abfw.3)c0/4.35(abfu.3)c0/4.35(abiq.3)c0 for ATP models respectively
  3. Verify the web interface is enabled
    Check the device settings under Network Services or Remote Access to confirm the HTTP/HTTPS web management interface is turned on and accessible
    Affected if The web interface (weblogin.cgi) is enabled and reachable from the network being tested
  4. Confirm the device is exposed to network
    Attempt to access the device login page at /weblogin.cgi via HTTP or HTTPS from the network segment in question
    Affected if The device web login page is accessible over the network without authentication

The device is affected if it is a NAS326, NAS520, NAS540, NAS542, ATP100, ATP200, ATP500, or ATP800 running a firmware version below the specified thresholds AND the web interface is network-accessible.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 4.35 / 5.21 or later
Fixed in 4.355.21
Interim mitigation

Apply available firmware updates from ZyXEL for supported models (NAS326 V5.21(AAZF.7)C0 or later, NAS520 V5.21(AASZ.3)C0 or later, NAS540 V5.21(AATB.4)C0 or later, NAS542 V5.21(ABAG.4)C0 or later). For end-of-support models (NSA210, NSA220, NSA220+, NSA221, NSA310, NSA310S, NSA320, NSA320S, NSA325, NSA325v2), isolate from network or decommission.

Recommended fix High confidence

NAS326: V5.21(AAZF.7)C0 or later | NAS520: V5.21(AASZ.3)C0 or later | NAS540: V5.21(AATB.4)C0 or later | NAS542: V5.21(ABAG.4)C0 or later | ATP100/200/500/800: V4.35(ABPS.3/ABFW.3/ABFU.3/ABIQ.3)C0 or later

  1. 1. Identify the exact model of your ZyXEL NAS device (e.g., NAS326, NAS520, NAS540, NAS542, ATP100, ATP200, ATP500, or ATP800)
  2. 2. Determine the current firmware version by accessing the device's web management interface and checking the System Status or System Info section
  3. 3. Download the appropriate firmware update from the official ZyXEL support website (www.zyxel.com/support/download.shtml) for your specific model
  4. 4. In the web management interface, navigate to Administration > Firmware/OSS > Firmware Upgrade
  5. 5. Upload the downloaded firmware file and initiate the upgrade process
  6. 6. Wait for the firmware upgrade to complete - do not power off the device during this process
  7. 7. After the device reboots, verify the new firmware version matches or exceeds the fixed version for your model
  8. 8. Confirm the upgrade was successful by logging back into the web interface
Caveat Firmware upgrades on NAS devices may reset some configuration settings; backup configuration before upgrading and verify critical settings (like share permissions, user accounts, and network settings) after the upgrade

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Nas326 Firmware Exploited in the wild — priority engagement
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2020-9054 in production — separate from our analysis above.

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What belongs here
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  • Version or environment caveats, and links to real fixes
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