CVE-2023-27991Network attack vector

Authenticated OS command execution via CLI injection

Published Apr 24, 2023 · Updated Feb 4, 2025

Command injection in Zyxel firewall firmware through 5.35 allows remote authenticated users to execute OS commands through the CLI. The flaw is in CLI command processing, but Zyxel has not disclosed the affected command, parameter, or how input reaches an OS command interpreter. A valid account and management CLI access are required; WAN access is disabled by default, and exploitation executes commands with the CLI process's privileges.

CVSS severity8.8
High
EPSS probability1.45%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Command injection in Zyxel firewall firmware through 5.35 allows remote authenticated users to execute OS commands through the CLI. The flaw is in CLI command processing, but Zyxel has not disclosed the affected command, parameter, or how input reaches an OS command interpreter. A valid account and management CLI access are required; WAN access is disabled by default, and exploitation executes commands with the CLI process's privileges.

The record

CVE
CVE-2023-27991
Published
Apr 24, 2023
Updated
Feb 4, 2025
Vendor
Zyxel Networks
Product
Zyxel ATP Series Firmware
Classifications
CWE-78, T1059
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Apr 24, 2023CVE publishedPublication date reported by the CVE source.
  2. Feb 4, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=4.32 through 5.35

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Zyxel Networks · Zyxel ATP Series Firmwareversion=4.32 through 5.35
Zyxel Networks · Zyxel USG FLEX 50(W) Firmwareversion=4.16 through 5.35
Zyxel Networks · Zyxel USG20(W)-VPN Firmwareversion=4.16 through 5.35
Zyxel Networks · Zyxel VPN Series Firmwareversion=4.30 through 5.35
Zyxel Networks · Zyxel USG FLEX Series Firmwareversion=4.50 through 5.35

Published CVSS scores

8.8Zyxel CorporationCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
T1059Command and Scripting Interpreter

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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