CVE-2023-4398: Zyxel Zld

High severity, CVSS 7.5. EPSS: 0.9% chance of exploitation in the next 30 days.

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.

Affected products

• Zyxel Zld: from 4.32, up to and including 5.37; from 4.50, up to and including 5.37; from 4.16, up to and including 5.37; from 4.30, up to and including 5.37

Published 2023-11-28. Last modified 2026-06-17.