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Actively exploited

CVE-2023-28771 Zyxel Multiple Firewalls OS Command Injection Vulnerability

Zyxel ATP, USG FLEX, VPN, and ZyWALL/USG firewalls allow for improper error message handling which could allow an unauthenticated attacker to execute OS commands remotely by sending crafted packets to an affected device.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 31 May 2023.
  • Rated Critical severity (CVSS 9.8, scored by Vendor).
  • It has been in the catalogue for 1,255 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

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.

Affected Versions

ProductVersions
Zyxel ZyWALL/USG series firmware4.60 through 4.73
Zyxel VPN series firmware4.60 through 5.35
Zyxel USG FLEX series firmware4.60 through 5.35
Zyxel ATP series firmware4.60 through 5.35

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Zyxel Multiple Firewalls. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated critical, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 21 Jun 2023 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited Zyxel Flaws

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CVE-2026-7273Zyxel GS1900 Series Switches Stack-Based Buffer Overflow VulnerabilityAdded 21 Sep 2026CVE-2024-40891Zyxel DSL CPE OS Command Injection VulnerabilityAdded 11 Feb 2025CVE-2024-40890Zyxel DSL CPE OS Command Injection VulnerabilityAdded 11 Feb 2025CVE-2024-11667 · RansomwareZyxel Multiple Firewalls Path Traversal VulnerabilityAdded 3 Dec 2024CVE-2017-6884 · RansomwareZyxel EMG2926 Routers Command Injection VulnerabilityAdded 18 Sep 2023CVE-2017-18368Zyxel P660HN-T1A Routers Command Injection VulnerabilityAdded 7 Aug 2023

All 13 exploited Zyxel vulnerabilities

Not Sure If You Are Exposed?

Our attack surface and vulnerability teams can check your internet-facing estate for CVE-2023-28771 and every other exploited flaw on the list.

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2023-28771
  3. CVE record: CVE-2023-28771
  4. FIRST EPSS
  5. zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-remote-command-injection
  6. packetstormsecurity.com/files/172820/Zyxel-IKE-Packet-Decoder-Unauthenticated-Remote-Code-Execution.