How severe is CVE-2023-27991?

CVE-2023-27991 has a CVSS 3.x base score of 8.8, rated high severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2023-27991 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (71st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2023-27991?

CVE-2023-27991 primarily affects Zyxel Atp200 Firmware. In total, 19 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2023-27991?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2023-27991 published?

CVE-2023-27991 was published on 2023-04-24 and last updated on 2026-06-17.

CVE-2023-27991

CVE-2023-27991 is a high-severity vulnerability in Zyxel Atp200 Firmware with a CVSS 3.x base score of 8.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-78.

Key facts

Severity: High (CVSS 3.x base score 8.8)
EPSS exploit prediction: 2% (71st percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-78
Affected product: Zyxel Atp200 Firmware
Published: 2023-04-24
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2023-27991?: 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.
How severe is CVE-2023-27991?: CVE-2023-27991 has a CVSS 3.x base score of 8.8, rated high severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2023-27991 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (71st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2023-27991?: CVE-2023-27991 primarily affects Zyxel Atp200 Firmware. In total, 19 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2023-27991?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2023-27991 published?: CVE-2023-27991 was published on 2023-04-24 and last updated on 2026-06-17.

References

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

Affected products (19)

cpe:2.3:o:zyxel:atp200_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:atp100_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:atp700_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:atp500_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:atp100w_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:atp800_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_100_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_50_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_200_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_500_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_700_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_100w_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_20w-vpn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_50w_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg20-vpn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:vpn100_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:vpn1000_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:vpn300_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:vpn50_firmware:*:*:*:*:*:*:*:*

More vulnerabilities in Zyxel Atp200 Firmware

CVE-2023-33010 — Critical (CVSS 9.8): A buffer overflow vulnerability in the ID processing function in Zyxel ATP series firmware versions 4.32 through 5.36…
CVE-2023-33009 — Critical (CVSS 9.8): A buffer overflow vulnerability in the notification function in Zyxel ATP series firmware versions 4.60 through 5.36…
CVE-2023-28771 — Critical (CVSS 9.8): Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware…
CVE-2022-30525 — Critical (CVSS 9.8): A OS command injection vulnerability in the CGI program of Zyxel USG FLEX 100(W) firmware versions 5.00 through 5.21…
CVE-2022-0342 — Critical (CVSS 9.8): An authentication bypass vulnerability in the CGI program of Zyxel USG/ZyWALL series firmware versions 4.20 through…
CVE-2020-29583 — Critical (CVSS 9.8): Firmware version 4.60 of Zyxel USG devices contains an undocumented account (zyfwp) with an unchangeable password. The…

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