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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 21 Jun 2023.
Atp100 FirmwareOperating system · Zyxel

CVE-2023-28771

CRITICAL · 9.8 CVSS v3.1 Published 2023-04-25
Fix available
A fix is available. Upgrade to 5.35 / 5.36 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
Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware versions 4.60 through 5.35, USG FLEX series firmware versions 4.60 through 5.35, and ATP series firmware versions 4.60 through 5.35, which could allow an unauthenticated attacker to execute some OS commands remotely by sending crafted packets to an affected device.

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

This is a command injection vulnerability in Zyxel security gateway devices (ZyWALL/USG, VPN, USG FLEX, and ATP series) caused by improper error message handling. An unauthenticated remote attacker can send specially crafted packets to the affected device to execute arbitrary OS commands, achieving full compromise without any credentials.

MitigationApply vendor firmware updates: upgrade ZyWALL/USG to version 4.74 or later, and upgrade VPN/USG FLEX/ATP series to version 5.36 or later. If immediate patching is not possible, restrict network access to the management interfaces via firewall rules or VPN access controls.

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
Atp100 FirmwareOperating system
Affected:>= 4.60, < 5.36
Atp100w FirmwareOperating system
Affected:>= 4.60, < 5.35
Atp200 FirmwareOperating system
Affected:>= 4.60, < 5.36
Atp500 FirmwareOperating system
Affected:>= 4.60, < 5.36
Atp700 FirmwareOperating system
Affected:>= 4.60, < 5.36
Atp800 FirmwareOperating system
Affected:>= 4.60, < 5.36
Usg Flex 100 FirmwareOperating system
Affected:>= 4.60, < 5.36
Usg Flex 100w FirmwareOperating system
Affected:>= 4.60, < 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
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 your Zyxel device model
    Access the web management interface and navigate to System > Status > Device Information, or run the CLI command 'show system-info' to confirm the exact model name (e.g., ATP100, USG Flex 100, ATP500, etc.)
    Affected if The device model is one of: ATP100, ATP100w, ATP200, ATP500, ATP700, ATP800, USG Flex 100, or USG Flex 100w
  2. Check the installed firmware version
    In the web interface, go to System > Status > Device Information to view the Firmware Version. Alternatively, use CLI command 'show system-info' or 'sysrouthash' to retrieve the version number
    Affected if The firmware version is >= 4.60 but < 5.35 (for ATP100w) or < 5.36 (for all other affected models)
  3. Confirm remote management is accessible
    Check if the device management interface (HTTP/HTTPS on ports 80/443 or the dedicated management port) is reachable from untrusted networks. Review firewall access rules under Configuration > Security Policy > Control Preview to see which source IPs or zones can reach the management interface
    Affected if The management interface is exposed to the internet or untrusted networks without restriction
  4. Verify the device has not been patched to a fixed version
    Compare your current firmware version against the fixed releases: ZyWALL/USG should be >= 4.74, and VPN/USG FLEX/ATP series should be >= 5.36
    Affected if The firmware version remains below these fixed versions

You are affected if your device is an ATP or USG Flex model running firmware between 4.60 and the fixed version, and the management interface is accessible to untrusted networks.

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 5.35 / 5.36 or later
Fixed in 5.355.36
Interim mitigation

Apply vendor firmware updates: upgrade ZyWALL/USG to version 4.74 or later, and upgrade VPN/USG FLEX/ATP series to version 5.36 or later. If immediate patching is not possible, restrict network access to the management interfaces via firewall rules or VPN access controls.

Recommended fix High confidence

Firmware version 5.36 or later (5.35 for Atp100w)

  1. 1. Identify the exact model of the Zyxel device (Atp100, Atp100w, Atp200, Atp500, Atp700, Atp800, USG Flex 100, or USG Flex 100w)
  2. 2. Access the device's web management interface or CLI
  3. 3. Navigate to the firmware upgrade section (typically under Maintenance > Firmware Upgrade or System > File Manager)
  4. 4. Download the appropriate firmware version 5.36 or later from the official Zyxel support website (support.zyxel.com)
  5. 5. Upload the firmware file to the device and initiate the upgrade process
  6. 6. Wait for the upgrade to complete and the device to reboot
  7. 7. Verify the firmware version has been updated successfully
  8. 8. Confirm the device is functioning normally post-upgrade
Caveat Review Zyxel release notes for any configuration or feature changes between your current version and 5.36; backup configuration before upgrading

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

Fix this in Atp100 Firmware Exploited in the wild — priority engagement
  • Consultation6.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,600
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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-2023-28771 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
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data