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Nas326 FirmwareOperating system · Zyxel

CVE-2020-13365

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
Certain Zyxel products have a locally accessible binary that allows a non-root user to generate a password for an undocumented user account that can be used for a TELNET session as root. 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 locally accessible binary in Zyxel NAS devices allows a non-root user to generate a password for an undocumented user account. This undocumented account can then be used to access TELNET as root, achieving full privilege escalation from an unprivileged local account.

MitigationApply available firmware updates from Zyxel for the affected models. As an interim control, disable TELNET access and restrict local access to trusted users only.

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 device model and firmware version
    Access the NAS web interface and navigate to System > Firmware or use the CLI command 'cat /etc/zyxelfw_info' to retrieve the exact firmware version installed.
    Affected if The device is a Nas326, Nas520, Nas540, or Nas542 running firmware versions below 5.21(aazf.9)c0, 5.21(aasz.5)c0, 5.21(aatb.6)c0, or 5.21(abag.6)c0 respectively.
  2. Verify TELNET service status
    Check if TELNET is enabled on the device via the web interface under Network > Remote Access or by attempting to connect to port 23. Also check /etc/services or run 'netstat -tlnp' to see if telnetd is listening.
    Affected if TELNET service is enabled and accessible, as the vulnerability enables root access through this protocol.
  3. Locate the vulnerable binary
    Search for the binary used to generate passwords for undocumented accounts. On the affected devices, this binary may be located in /usr/sbin or similar system directories. Use 'find /usr -name *pass*' or 'find /usr -name *user*' to locate related utilities.
    Affected if The binary exists on the system and is executable by non-root users, which is the condition required for exploitation.
  4. Check for undocumented user accounts
    Examine /etc/passwd or /etc/shadow for unexpected accounts beyond the standard admin and guest accounts. Look for accounts that do not correspond to normal device usage.
    Affected if An undocumented account exists in the system, indicating the vulnerability may have been exploited or the password generation binary was used.
  5. Review access logs for TELNET connections
    Check system logs under /var/log or the web interface logs for any TELNET login attempts, especially from the undocumented account or at unusual times.
    Affected if TELNET connections from undocumented or unknown accounts appear in the logs.

The device is affected if it is a Nas326, Nas520, Nas540, or Nas542 running firmware below the specified versions AND TELNET is enabled, allowing an unprivileged local user to escalate to root via the undocumented account.

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

Apply available firmware updates from Zyxel for the affected models. As an interim control, disable TELNET access and restrict local access to trusted users only.

Fix this in Nas326 Firmware Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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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-2020-13365 in production — separate from our analysis above.

No notes yet

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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
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data