The cross-site scripting (XSS) vulnerability in Zyxel ATP series firmware versions 4.32 through 5.35, USG FLEX series firmware versions 4.5…

published 2023-04-24 3 y ago modified 2026-06-17 4 mo ago also EUVD-2023-31715

cvss: 4.8; 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 27%
public exploits: 0; none known

The cross-site scripting (XSS) vulnerability in Zyxel ATP series firmware versions 4.32 through 5.35, USG FLEX series firmware versions 4.50 through 5.35, USG FLEX 50(W) firmware versions 4.16 through 5.35, USG20(W)-VPN firmware versions 4.16 through 5.35, and VPN series firmware versions 4.30 through 5.35, which could allow an authenticated attacker with administrator privileges to store malicious scripts in a vulnerable device. A successful XSS attack could then result in the stored malicious scripts being executed when the user visits the Logs page of the GUI on the device.

Patch / fix

fixed in 19

zyxel atp200_firmware 5.36
zyxel atp100_firmware 5.36
zyxel atp700_firmware 5.36
zyxel atp500_firmware 5.36
zyxel atp100w_firmware 5.36
zyxel atp800_firmware 5.36
zyxel usg_flex_100_firmware 5.36
zyxel usg_flex_50_firmware 5.36
zyxel usg_flex_200_firmware 5.36
zyxel usg_flex_500_firmware 5.36
zyxel usg_flex_700_firmware 5.36
zyxel usg_flex_100w_firmware 5.36
zyxel usg_20w-vpn_firmware 5.36
zyxel usg_flex_50w_firmware 5.36
zyxel usg20-vpn_firmware 5.36
zyxel vpn100_firmware 5.36
zyxel vpn1000_firmware 5.36
zyxel vpn300_firmware 5.36
zyxel vpn50_firmware 5.36

Exploit prediction

0.34% chance of exploitation in the next 30 days, higher than 27% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	4.8	CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N
NVD	3.1	4.8	CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N

Affected products

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

Classification

Weakness

CWE-79 Improper Neutralization of Input During Web Page Generation

References

zyxel.com /global/en/support/security-advisories/zyxel-security-advisory-for-xss-vulnerability-and-post-authentication-c
vendor-advisory

Sources

CVE List CVE-2023-27990
NVD CVE-2023-27990

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