CVE-2023-33010 Zyxel Multiple Firewalls Buffer Overflow Vulnerability
Zyxel ATP, USG FLEX, USG FLEX 50(W), USG20(W)-VPN, VPN, and ZyWALL/USG firewalls contain a buffer overflow vulnerability in the ID processing function that could allow an unauthenticated attacker to cause denial-of-service (DoS) conditions and remote code execution on an affected device.
From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.
Should You Worry?
- CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 5 Jun 2023.
- Rated Critical severity (CVSS 9.8, scored by Vendor).
- It has been in the catalogue for 1,250 days. Systems still unpatched now have almost certainly been scanned for it.
What It Is
A buffer overflow vulnerability in the ID processing function in Zyxel ATP series firmware versions 4.32 through 5.36 Patch 1, USG FLEX series firmware versions 4.50 through 5.36 Patch 1, USG FLEX 50(W) firmware versions 4.25 through 5.36 Patch 1, USG20(W)-VPN firmware versions 4.25 through 5.36 Patch 1, VPN series firmware versions 4.30 through 5.36 Patch 1, ZyWALL/USG series firmware versions 4.25 through 4.73 Patch 1, could allow an unauthenticated attacker to cause denial-of-service (DoS) conditions and even a remote code execution on an affected device.
Affected Versions
| Product | Versions |
|---|---|
| Zyxel ATP series firmware | 4.32 through 5.36 Patch 1 |
| Zyxel USG FLEX series firmware | 4.50 through 5.36 Patch 1 |
| Zyxel USG FLEX 50(W) firmware | 4.25 through 5.36 Patch 1 |
| Zyxel USG20(W)-VPN firmware | 4.25 through 5.36 Patch 1 |
| Zyxel VPN series firmware | 4.30 through 5.36 Patch 1 |
| Zyxel ZyWALL/USG series firmware | 4.25 through 4.73 Patch 1 |
From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.
What To Do Now
- 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
- 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
Prove it is fixed
Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.
Penetration Testing
The Compliance Clock
General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.
Not Sure If You Are Exposed?
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