CVE-2023-28771: Zyxel Multiple Firewalls OS Command Injection Vulnerability

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 31 May 2023, 36 days after the CVE record was published.

How likely is exploitation in the next 30 days?

99.3%

EPSS score, percentile 99.9 of all scored records

How severe?

9.8 Critical

CVSS v3.1 base score, given by the CNA

What should I do?

CISA deadline 21 Jun 2023. Apply updates per vendor instructions.

The CVE record links no vendor advisory, patch or mitigation.

Which versions are affected?

Affected

• 4.60 through 4.73
• 4.60 through 5.35
• 4.60 through 5.35
• 4.60 through 5.35

Versions by product

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

CISA due date for this action: 21 June 2023.

Advisories and patches

The CVE record links no vendor advisory, patch or mitigation.

Linked from CISA's catalog entry

• https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls
• https://nvd.nist.gov/vuln/detail/CVE-2023-28771

Affected products and versions

As listed in the CVE record.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

Zyxel ATP, USG FLEX, VPN, and ZyWALL/USG firewalls allow for improper error message handling which could allow an unauthenticated attacker to execute OS commands remotely by sending crafted packets to an affected device.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

• CISA added CVE-2023-28771 to its Known Exploited Vulnerabilities (KEV) catalog on 31 May 2023.
• CISA's ransomware flag for this entry is "Unknown": no evidence of ransomware use in the catalog, which is not proof of none.
• The due date CISA sets for the required action is 21 Jun 2023.
• The CVE record was published on 25 Apr 2023, 36 days before it was added to KEV.
• The CVE record carries a CVSS v3.1 base score of 9.8 (Critical), given by its CNA.
• FIRST's EPSS model puts the probability of exploitation in the next 30 days at 99.3% (percentile 99.9), as of 28 Oct 2026.
• CISA's SSVC assessment in the CVE record: exploitation active, automatable yes, technical impact total.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

Description

Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware versions 4.60 through 5.35, USG FLEX series firmware versions 4.60 through 5.35, and ATP series firmware versions 4.60 through 5.35, which could allow an unauthenticated attacker to execute some OS commands remotely by sending crafted packets to an affected device.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Weakness

• CWE-78: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

References

• zyxel.com
• packetstormsecurity.com

More exploited Zyxel vulnerabilities

• CVE-2026-7273: Zyxel GS1900 Series Switches Stack-Based Buffer Overflow Vulnerability (added 21 Sep 2026)
• CVE-2024-40890: Zyxel DSL CPE OS Command Injection Vulnerability (added 11 Feb 2025)
• CVE-2024-40891: Zyxel DSL CPE OS Command Injection Vulnerability (added 11 Feb 2025)
• CVE-2024-11667: Zyxel Multiple Firewalls Path Traversal Vulnerability (added 3 Dec 2024)
• CVE-2017-6884: Zyxel EMG2926 Routers Command Injection Vulnerability (added 18 Sep 2023)
• CVE-2017-18368: Zyxel P660HN-T1A Routers Command Injection Vulnerability (added 7 Aug 2023)