CVE-2021-35029Known exploitationNetwork attack vector

Unauthenticated command execution via unsanitized web input

Published Jul 2, 2021 · Updated Aug 4, 2024

Authentication bypass in affected Zyxel firewall firmware allows remote attackers to execute arbitrary commands through its web interface. The web management interface fails to sanitize input strings before authentication processing, allowing crafted input to grant access. Exposure requires on-premises ZLD firmware with remote management or SSL VPN enabled; Nebula cloud management mode is not affected.

CVSS severity9.8
Critical
EPSS probability2.25%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Authentication bypass in affected Zyxel firewall firmware allows remote attackers to execute arbitrary commands through its web interface. The web management interface fails to sanitize input strings before authentication processing, allowing crafted input to grant access. Exposure requires on-premises ZLD firmware with remote management or SSL VPN enabled; Nebula cloud management mode is not affected.

The record

CVE
CVE-2021-35029
Published
Jul 2, 2021
Updated
Aug 4, 2024
Vendor
Zyxel Networks
Product
Zyxel ATP Series Firmware
Classifications
CWE-287, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 2, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record 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=4.35 through 5.01

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Zyxel Networks · Zyxel ATP Series Firmwareversion=4.35 through 5.01
Zyxel Networks · Zyxel ZyWALL/USG Series Firmwareversion=4.35 through 4.64
Zyxel Networks · Zyxel VPN Series Firmwareversion=4.35 through 5.01
Zyxel Networks · Zyxel USG FLEX Series Firmwareversion=4.35 through 5.01

Published CVSS scores

9.8Zyxel CorporationCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/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
Privilegesunauthenticated
Known exploitationReported
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-287Improper Authentication
T1190Exploit Public-Facing Application

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.

Reported exploitation

Technologies

Your stack

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Your stack

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