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Atp200 FirmwareOperating system · Zyxel

CVE-2023-27991

HIGH · 8.8 CVSS v3.1 Published 2023-04-24
Fix available
A fix is available. Upgrade to 5.36 or later.
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
The post-authentication command injection vulnerability in the CLI command of Zyxel ATP series firmware versions 4.32 through 5.35, USG FLEX series firmware versions 4.50 through 5.35, USG FLEX 50(W) firmware versions 4.16 through 5.35, USG20(W)-VPN firmware versions 4.16 through 5.35, and VPN series firmware versions 4.30 through 5.35, which could allow an authenticated attacker to execute some OS commands remotely.

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 post-authentication command injection vulnerability exists in the CLI command interface of multiple Zyxel security appliance series (ATP, USG FLEX, USG20W-VPN, VPN). An authenticated attacker can inject and execute arbitrary OS commands through the CLI, potentially gaining full system control.

MitigationApply the latest firmware updates from Zyxel for all affected devices. Restrict CLI access to trusted administrative users and networks to reduce exposure.

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
Atp200 FirmwareOperating system
Affected:>= 4.32, < 5.36
Atp100 FirmwareOperating system
Affected:>= 4.32, < 5.36
Atp700 FirmwareOperating system
Affected:>= 4.32, < 5.36
Atp500 FirmwareOperating system
Affected:>= 4.32, < 5.36
Atp100w FirmwareOperating system
Affected:>= 4.32, < 5.36
Atp800 FirmwareOperating system
Affected:>= 4.32, < 5.36
Usg Flex 100 FirmwareOperating system
Affected:>= 4.50, < 5.36
Usg Flex 50 FirmwareOperating system
Affected:>= 4.50, < 5.36

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 your Zyxel device model
    Access the device web UI under Status > Device Information or run 'show system-info' in CLI to confirm the exact model name (e.g., ATP200, ATP100, ATP700, ATP500, ATP100w, ATP800, USG Flex 100, USG Flex 50)
    Affected if Device model is any of: ATP200, ATP100, ATP700, ATP500, ATP100w, ATP800, USG Flex 100, or USG Flex 50
  2. Determine the installed firmware version
    In the web UI go to Maintenance > Firmware Upgrade or run 'show system-info' in CLI to retrieve the current firmware version number
    Affected if Firmware version is >= 4.32 and < 5.36 for ATP series, OR >= 4.50 and < 5.36 for USG Flex series
  3. Verify if CLI interface is accessible
    Check if SSH, telnet, or console access to the device CLI is enabled under Network > CLI > Access Control or System > SSH/Telnet settings
    Affected if CLI access (SSH, telnet, or console) is enabled and reachable from your administrative network
  4. Confirm authentication method in use
    Review user accounts with CLI privileges under Object > User/Group or > Admin Access to identify which accounts can access the CLI
    Affected if Any administrative user account exists with CLI access privileges

You are affected if your device is an ATP or USG Flex model running a firmware version within the affected ranges AND CLI interface is accessible to authenticated users.

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 5.36 or later
Fixed in 5.36
Interim mitigation

Apply the latest firmware updates from Zyxel for all affected devices. Restrict CLI access to trusted administrative users and networks to reduce exposure.

Recommended fix High confidence

Firmware 5.36 or later

  1. 1. Identify the exact model number (ATP200, ATP100, ATP700, ATP500, ATP100w, ATP800, USG Flex 100, USG Flex 50, USG20W-VPN, USG20-VPN, or VPN series)
  2. 2. Visit the official Zyxel support website (www.zyxel.com) and navigate to the downloads/support area for your specific model
  3. 3. Download firmware version 5.36 or later (the first fixed release) for your model
  4. 4. Back up the current device configuration through the web GUI or CLI before proceeding
  5. 5. Access the device's web GUI or CLI and navigate to the firmware upgrade section
  6. 6. Upload the downloaded firmware file (5.36 or later) and initiate the upgrade process
  7. 7. Wait for the device to reboot and complete the firmware installation
  8. 8. After reboot, verify the firmware version has been updated to 5.36 or later in the system information
Caveat Firmware upgrades may reset some settings; backup configuration before upgrading and reapply after upgrade

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

Fix this in Atp200 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
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,120.

Scan for this in your stack

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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-2023-27991 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