What’s Vulnerable › Vulnerabilities

CVE-2023-34140

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2023-07-17 · updated 2026-06-17
• Weakness: CWE-120

Description

A buffer overflow vulnerability in the Zyxel ATP series firmware versions 4.32 through 5.36 Patch 2, USG FLEX series firmware versions 4.50 through 5.36 Patch 2, USG FLEX 50(W) series firmware versions 4.16 through 5.36 Patch 2, USG20(W)-VPN series firmware versions 4.16 through 5.36 Patch 2, VPN series firmware versions 4.30 through 5.36 Patch 2, NXC2500 firmware versions 6.10(AAIG.0) through 6.10(AAIG.3), and NXC5500 firmware versions 6.10(AAOS.0) through 6.10(AAOS.4), could allow an unauthenticated, LAN-based attacker to cause denial of service (DoS) conditions by sending a crafted request to the CAPWAP daemon.

Affected products

• Zyxel NXC2500 firmware: from 6.10\(aaig.0\) through 6.10\(aaig.3\)
• Zyxel NXC5500 firmware: from 6.10\(aaos.0\) through 6.10\(aaos.4\)
• Zyxel USG 20(W)-VPN firmware: from 4.16 before 5.37 — fixed in 5.37
• Zyxel Usg 2200-Vpn Firmware: from 4.30 before 5.37 — fixed in 5.37
• Zyxel firewall (ZLD): from 4.50 before 5.37 — fixed in 5.37
• Zyxel USG FLEX 100(W) firmware: from 4.50 before 5.37 — fixed in 5.37
• Zyxel USG FLEX 200 firmware: from 4.50 before 5.37 — fixed in 5.37
• Zyxel USG FLEX 500 firmware: from 4.50 before 5.37 — fixed in 5.37
• Zyxel Usg Flex 50 Firmware: from 4.50 before 5.37 — fixed in 5.37
• Zyxel USG FLEX 50(W) firmware: from 4.50 before 5.37 — fixed in 5.37
• Zyxel USG FLEX 700 firmware: from 4.50 before 5.37 — fixed in 5.37
• Zyxel Zywall Atp100 Firmware: from 4.32 before 5.37 — fixed in 5.37
• Zyxel Zywall Atp100W Firmware: from 4.32 before 5.37 — fixed in 5.37
• Zyxel Zywall Atp200 Firmware: from 4.32 before 5.37 — fixed in 5.37
• Zyxel Zywall Atp500 Firmware: from 4.32 before 5.37 — fixed in 5.37
• Zyxel Zywall Atp700 Firmware: from 4.32 before 5.37 — fixed in 5.37
• Zyxel Zywall Atp800 Firmware: from 4.32 before 5.37 — fixed in 5.37
• Zyxel Zywall Vpn100 Firmware: from 4.30 before 5.37 — fixed in 5.37
• Zyxel ZyWALL VPN2S Firmware: from 4.30 before 5.37 — fixed in 5.37
• Zyxel Zywall Vpn300 Firmware: from 4.30 before 5.37 — fixed in 5.37
• Zyxel Zywall Vpn50 Firmware: from 4.30 before 5.37 — fixed in 5.37
• Zyxel Zywall Vpn 100 Firmware: from 4.30 before 5.37 — fixed in 5.37
• Zyxel Zywall Vpn 300 Firmware: from 4.30 before 5.37 — fixed in 5.37
• Zyxel Zywall Vpn 50 Firmware: from 4.30 before 5.37 — fixed in 5.37

How to fix

Upgrade to a fixed version: Zyxel USG 20(W)-VPN firmware 5.37; Zyxel Usg 2200-Vpn Firmware 5.37; Zyxel firewall (ZLD) 5.37; Zyxel USG FLEX 100(W) firmware 5.37; Zyxel USG FLEX 200 firmware 5.37; Zyxel USG FLEX 500 firmware 5.37; Zyxel Usg Flex 50 Firmware 5.37; Zyxel USG FLEX 50(W) firmware 5.37; Zyxel USG FLEX 700 firmware 5.37; Zyxel Zywall Atp100 Firmware 5.37; Zyxel Zywall Atp100W Firmware 5.37; Zyxel Zywall Atp200 Firmware 5.37. Check the vendor advisory for mitigations.

References

• https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-multiple-vulnerabilities-in-firewalls-and-wlan-controllers
• https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-multiple-vulnerabilities-in-firewalls-and-wlan-controllers

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