What’s Vulnerable › Vulnerabilities

CVE-2023-27991

• Severity: high (CVSS 8.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2023-04-24 · updated 2026-06-17
• Weakness: CWE-78

Description

The post-authentication command injection vulnerability in the CLI command of 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 to execute some OS commands remotely.

Affected products

• Zyxel Atp100 Firmware: from 4.32 before 5.36 — fixed in 5.36
• Zyxel Atp100W Firmware: from 4.32 before 5.36 — fixed in 5.36
• Zyxel Atp200 Firmware: from 4.32 before 5.36 — fixed in 5.36
• Zyxel Atp500 Firmware: from 4.32 before 5.36 — fixed in 5.36
• Zyxel Atp700 Firmware: from 4.32 before 5.36 — fixed in 5.36
• Zyxel Atp800 Firmware: from 4.32 before 5.36 — fixed in 5.36
• Zyxel Usg20-Vpn Firmware: from 4.30 before 5.36 — fixed in 5.36
• Zyxel USG 20(W)-VPN firmware: from 4.16 before 5.36 — fixed in 5.36
• Zyxel firewall (ZLD): from 4.50 before 5.36 — fixed in 5.36
• Zyxel USG FLEX 100(W) firmware: from 4.50 before 5.36 — fixed in 5.36
• Zyxel USG FLEX 200 firmware: from 4.50 before 5.36 — fixed in 5.36
• Zyxel USG FLEX 500 firmware: from 4.50 before 5.36 — fixed in 5.36
• Zyxel Usg Flex 50 Firmware: from 4.50 before 5.36 — fixed in 5.36
• Zyxel USG FLEX 50(W) firmware: from 4.16 before 5.36 — fixed in 5.36
• Zyxel USG FLEX 700 firmware: from 4.50 before 5.36 — fixed in 5.36
• Zyxel Vpn1000 Firmware: from 4.30 before 5.36 — fixed in 5.36
• Zyxel Vpn100 Firmware: from 4.30 before 5.36 — fixed in 5.36
• Zyxel Vpn300 Firmware: from 4.30 before 5.36 — fixed in 5.36
• Zyxel Vpn50 Firmware: from 4.30 before 5.36 — fixed in 5.36

How to fix

Upgrade to a fixed version: Zyxel Atp100 Firmware 5.36; Zyxel Atp100W Firmware 5.36; Zyxel Atp200 Firmware 5.36; Zyxel Atp500 Firmware 5.36; Zyxel Atp700 Firmware 5.36; Zyxel Atp800 Firmware 5.36; Zyxel Usg20-Vpn Firmware 5.36; Zyxel USG 20(W)-VPN firmware 5.36; Zyxel firewall (ZLD) 5.36; Zyxel USG FLEX 100(W) firmware 5.36; Zyxel USG FLEX 200 firmware 5.36; Zyxel USG FLEX 500 firmware 5.36. Check the vendor advisory for mitigations.

References

• https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-xss-vulnerability-and-post-authentication-command-injection-vulnerability-in-firewalls
• https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-xss-vulnerability-and-post-authentication-command-injection-vulnerability-in-firewalls

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