Exploited. On CISA's list since 2023-06-05.
Zyxel Multiple Firewalls Buffer Overflow Vulnerability
CVE-2023-33009 · Zyxel Multiple Firewalls · Published 2023-05-24
How bad would it be?
9.8 out of 10
CVSS rates it critical: about as damaging as flaws get.
How likely is it to be used?
28%
Estimated chance of exploitation in the next 30 days, higher than 98% of all scored flaws (EPSS, 2026-10-29).
Is it already being used?
Yes
CISA has seen it exploited in real attacks. For US government agencies the fix is deadline passed 1222 days ago (2023-06-26).
What CISA says to do
Apply updates per vendor instructions.
Zyxel ATP, USG FLEX, USG FLEX 50(W), USG20(W)-VPN, VPN, and ZyWALL/USG firewalls contain a buffer overflow vulnerability in the notification function that could allow an unauthenticated attacker to cause denial-of-service (DoS) conditions and remote code execution on an affected device.
What the flaw is
A buffer overflow vulnerability in the notification function in Zyxel ATP series firmware versions 4.60 through 5.36 Patch 1, USG FLEX series firmware versions 4.60 through 5.36 Patch 1, USG FLEX 50(W) firmware versions 4.60 through 5.36 Patch 1, USG20(W)-VPN firmware versions 4.60 through 5.36 Patch 1, VPN series firmware versions 4.60 through 5.36 Patch 1, ZyWALL/USG series firmware versions 4.60 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.
From the NVD.
Fixes and advisories
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Understand the kind of weakness behind this flaw. These resources teach the weakness, rather than reproduce this specific CVE. Follow the vendor's advice above to fix the affected product.
CWE-120 · CISA KEV; NVD record: nvd@nist.gov; NVD record: security@zyxel.com.tw. We have not selected a close learning match for this label.
CWE is a catalogue of weakness types. Labels come from the sources shown and can differ or change. Resource matches reviewed 16 October 2026.
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Exploited flaws from CISA's KEV catalogue. Chance of exploitation from EPSS by FIRST. Descriptions and scores from the NVD. This product uses the NVD API but is not endorsed or certified by the NVD.