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Nas326 FirmwareOperating system · Zyxel

CVE-2020-13364

HIGH · 8.8 CVSS v3.1 Published 2020-08-06
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
94/100
Remediation priority · Urgent
Remotely reachable 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
A backdoor in certain Zyxel products allows remote TELNET access via a CGI script. This affects NAS520 V5.21(AASZ.4)C0, V5.21(AASZ.0)C0, V5.11(AASZ.3)C0, and V5.11(AASZ.0)C0; NAS542 V5.11(ABAG.0)C0, V5.20(ABAG.1)C0, and V5.21(ABAG.3)C0; NSA325 v2_V4.81(AALS.0)C0 and V4.81(AAAJ.1)C0; NSA310 4.22(AFK.0)C0 and 4.22(AFK.1)C0; NAS326 V5.21(AAZF.8)C0, V5.11(AAZF.4)C0, V5.11(AAZF.2)C0, and V5.11(AAZF.3)C0; NSA310S V4.75(AALH.2)C0; NSA320S V4.75(AANV.2)C0 and V4.75(AANV.1)C0; NSA221 V4.41(AFM.1)C0; and NAS540 V5.21(AATB.5)C0 and V5.21(AATB.3)C0.

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

A pre-existing backdoor in Zyxel NAS devices allows remote attackers to obtain TELNET access without authentication by exploiting a specific CGI script. The vulnerability affects multiple NAS models across various firmware versions, enabling attackers to gain command-line access to the device.

MitigationRestrict network access to affected devices, disable remote management interfaces if not required, and apply vendor firmware updates when available. If no update exists, consider network segmentation or device replacement.

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:< v5.21\(aazf.9\)c0
Nas520 FirmwareOperating system
Affected:< v5.21\(aasz.5\)c0
Nas540 FirmwareOperating system
Affected:< v5.21\(aatb.6\)c0
Nas542 FirmwareOperating system
Affected:< v5.21\(abag.6\)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
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/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 NAS device model
    Access the device web interface and check the System Status page, or run the command 'cat /proc/model' via SSH or serial console if available
    Affected if The model is one of Nas326, Nas520, Nas540, or Nas542
  2. Check the installed firmware version
    In the web interface, go to Maintenance > Firmware > Current Version, or run 'cat /etc/zyxel/os-release' via SSH/serial console
    Affected if The firmware version is below the fixed version for your model: Nas326 < v5.21(aazf.9)c0, Nas520 < v5.21(aasz.5)c0, Nas540 < v5.21(aatb.6)c0, Nas542 < v5.21(abag.6)c0
  3. Verify if the vulnerable CGI script is present
    Check for the existence of /usr/local/www/cgi-bin/backup/cgi-bin/ftok daemon on the device via SSH or by attempting to access the URL pattern 'http://<device-ip>/cgi-bin/backup/cgi-bin/ftok'
    Affected if The CGI script is found and responds to requests (the backdoor is present)
  4. Check if TELNET service is exposed
    Attempt to connect to the device on port 23 (TELNET) from an external host, or run 'netstat -tuln | grep 23' on the device
    Affected if TELNET port 23 is open and accessible from the network (enabling the backdoor to be exploited)

If you have a Nas326, Nas520, Nas540, or Nas542 with firmware below the fixed versions and the vulnerable CGI script is present with TELNET accessible, you are affected by this backdoor vulnerability.

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.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Restrict network access to affected devices, disable remote management interfaces if not required, and apply vendor firmware updates when available. If no update exists, consider network segmentation or device replacement.

Fix this in Nas326 Firmware Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA2.0 h
18.0 hours of engineering $3,200
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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-2020-13364 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
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