dbcve.org
ZldOperating system · Zyxel

CVE-2023-4398

HIGH · 7.5 CVSS v3.1 Published 2023-11-28
Fix available
A fix is available. Upgrade to after 5.37 or later.
See remediation →
84/100
Remediation priority · High
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
An integer overflow vulnerability in the source code of the QuickSec IPSec toolkit used in the VPN feature of the Zyxel ATP series firmware versions 4.32 through 5.37, USG FLEX series firmware versions 4.50 through 5.37, USG FLEX 50(W) series firmware versions 4.16 through 5.37, USG20(W)-VPN series firmware versions 4.16 through 5.37, and VPN series firmware versions 4.30 through 5.37, could allow an unauthenticated attacker to cause denial-of-service (DoS) conditions on an affected device by sending a crafted IKE packet.

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

An integer overflow vulnerability exists in the QuickSec IPSec toolkit's IKE packet processing within Zyxel VPN devices (ATP, USG FLEX, USG20W-VPN, and VPN series). An unauthenticated attacker can send a specially crafted IKE packet to trigger the overflow, causing the affected device to crash or become unresponsive, resulting in denial of service.

MitigationAdministrators should update affected Zyxel devices to the latest firmware versions beyond 5.37, which contain the patched QuickSec IPSec toolkit. Until patches are applied, consider blocking or rate-limiting IKE traffic at perimeter firewalls or disabling unnecessary VPN access.

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
ZldOperating system
Affected:>= 4.32, <= 5.37>= 4.50, <= 5.37>= 4.16, <= 5.37>= 4.30, <= 5.37

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
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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 Zyxel web UI or CLI and navigate to Status > Device Information to view the model name and firmware version. Alternatively, use CLI command 'show system info' or 'syswrapper.sh show', then compare the firmware version against the affected ranges: 4.32 to 5.37, 4.50 to 5.37, 4.16 to 5.37, or 4.30 to 5.37 depending on the device series.
    Affected if The firmware version falls within any of the 4.16-5.37, 4.30-5.37, 4.32-5.37, or 4.50-5.37 ranges.
  2. Confirm IKE/IPSec VPN is configured or enabled
    In the Zyxel web UI, go to CONFIGURATION > VPN > IPsec to check if any IKE/IPSec VPN connections are configured or enabled. In CLI, use 'show ipsec config' or 'show ike config' to list active IKE policies.
    Affected if Any IKE/IPSec VPN tunnel, policy, or gateway is configured or the VPN feature is active.
  3. Check if device WAN interface accepts IKE traffic
    In the web UI, go to CONFIGURATION > Network > Interface and verify if the WAN interface allows IKE (UDP port 500/4500) traffic. Alternatively, check the firewall rules under CONFIGURATION > Firewall to see if inbound IKE traffic from untrusted zones is permitted.
    Affected if The WAN or external-facing interface allows IKE traffic (UDP 500/4500) from the internet or untrusted networks.

The device is affected if it is a Zyxel ATP, USG FLEX, USG20W-VPN, or VPN series model running firmware version 5.37 or lower with IKE/IPSec VPN enabled and exposed to untrusted IKE traffic.

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
Upgrade available Upgrade to a release after 5.37
Interim mitigation

Administrators should update affected Zyxel devices to the latest firmware versions beyond 5.37, which contain the patched QuickSec IPSec toolkit. Until patches are applied, consider blocking or rate-limiting IKE traffic at perimeter firewalls or disabling unnecessary VPN access.

Fix this in Zld Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,500
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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-2023-4398 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