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Exploited. On CISA's list since 2023-05-31.
CVE-2023-28771 · Zyxel Multiple Firewalls · Published 2023-04-25
9.8 out of 10
CVSS rates it critical: about as damaging as flaws get.
99%
Estimated chance of exploitation in the next 30 days, among the top 1% of all scored flaws (EPSS, 2026-10-29).
Yes
CISA has seen it exploited in real attacks. For US government agencies the fix is deadline passed 1227 days ago (2023-06-21).
Apply updates per vendor instructions.
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.
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.
From the NVD.
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Input changes a command that the server runs through its operating system.
CWE-78 · CISA KEV; NVD record: nvd@nist.gov; NVD record: security@zyxel.com.tw
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CWE is a catalogue of weakness types. Labels come from the sources shown and can differ or change. Resource matches reviewed 16 October 2026.
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Exploited flaws from CISA's KEV catalogue. Chance of exploitation from EPSS by FIRST. Descriptions and scores from the NVD. This product uses the NVD API but is not endorsed or certified by the NVD.