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…

published 2023-07-17 3 y ago modified 2026-06-17 4 mo ago also EUVD-2023-38242

cvss: 6.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 22%
public exploits: 0; none known

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.

Patch / fix

fixed in 20

zyxel usg_20w-vpn_firmware 5.37
zyxel usg_2200-vpn_firmware 5.37
zyxel usg_flex_100_firmware 5.37
zyxel usg_flex_100w_firmware 5.37
zyxel usg_flex_200_firmware 5.37
zyxel usg_flex_50_firmware 5.37
zyxel usg_flex_500_firmware 5.37
zyxel usg_flex_50w_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
zyxel zywall_atp500_firmware 5.37
zyxel zywall_atp700_firmware 5.37
zyxel zywall_atp800_firmware 5.37
zyxel zywall_vpn100_firmware 5.37
zyxel zywall_vpn2s_firmware 5.37
zyxel zywall_vpn300_firmware 5.37
zyxel zywall_vpn50_firmware 5.37
zyxel zywall_vpn_100_firmware 5.37

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: partial
Assessed: 2 y ago (2024-10-21)

Exploit prediction

0.30% chance of exploitation in the next 30 days, higher than 22% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	6.5	CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

Zyxel ATP series firmware
Zyxel USG FLEX series firmware
Zyxel USG FLEX 50(W) series firmware
Zyxel USG20(W)-VPN series firmware
Zyxel VPN series firmware
Zyxel NXC2500 firmware
Zyxel NXC5500 firmware

Classification

Weakness

CWE-120 Buffer Copy without Checking Size of Input

References

zyxel.com /global/en/support/security-advisories/zyxel-security-advisory-for-multiple-vulnerabilities-in-firewalls-and-w
vendor-advisory

Sources

CVE List CVE-2023-34140
NVD CVE-2023-34140

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary