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Usg1900 FirmwareOperating system · Zyxel

CVE-2021-35029

CRITICAL · 9.8 CVSS v3.1 Published 2021-07-02
Fix available
A fix is available. Upgrade to after 4.64 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
An authentication bypasss vulnerability in the web-based management interface of Zyxel USG/Zywall series firmware versions 4.35 through 4.64 and USG Flex, ATP, and VPN series firmware versions 4.35 through 5.01, which could allow a remote attacker to execute arbitrary commands on an affected device.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Authentication bypass in Zyxel USG/Zywall, USG Flex, ATP, and VPN series web management interfaces allows remote attackers to execute arbitrary commands without valid credentials. The vulnerability affects firmware versions 4.35-4.64 (USG/Zywall) and 4.35-5.01 (USG Flex/ATP/VPN).

MitigationUpdate firmware to versions beyond 4.64 (USG/Zywall) and 5.01 (USG Flex/ATP/VPN). If patching is immediately infeasible, restrict web management interface access to trusted IPs via firewall rules or disable remote management on external interfaces.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Usg1900 FirmwareOperating system
Affected:>= 4.35, <= 4.64
Usg1100 FirmwareOperating system
Affected:>= 4.35, <= 4.64
Usg310 FirmwareOperating system
Affected:>= 4.35, <= 4.64
Usg210 FirmwareOperating system
Affected:>= 4.35, <= 4.64
Usg110 FirmwareOperating system
Affected:>= 4.35, <= 4.64
Usg40 FirmwareOperating system
Affected:>= 4.35, <= 4.64
Usg40w FirmwareOperating system
Affected:>= 4.35, <= 4.64
Usg60 FirmwareOperating system
Affected:>= 4.35, <= 4.64

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify the Zyxel device model
    Access the device console or web interface and locate the model name (USG, Zywall, ATP, USG Flex, or VPN series). Check the product label or system information page.
    Affected if The device is one of: USG1900, USG1100, USG310, USG210, USG110, USG40, USG40w, USG60, or other USG Flex/ATP/VPN series models.
  2. Check the installed firmware version
    Log into the web management interface and navigate to Status > Device Information, or use the CLI command 'show system-info' or 'sysrepoctl -l' to retrieve the firmware version.
    Affected if The firmware version falls within 4.35 to 4.64 (for USG/Zywall models) or 4.35 to 5.01 (for USG Flex/ATP/VPN models).
  3. Verify the web management interface is enabled
    In the web interface, go to Configuration > Network > HTTP/HTTPS server settings, or check via CLI with 'show web-server' to confirm the web management service is active.
    Affected if The web management interface (HTTP or HTTPS) is enabled on any interface.
  4. Determine if remote management is exposed to untrusted networks
    Check the interface bindings under Configuration > Network > Interface settings, or run 'show ip http server' and 'show ip https server' to see which interfaces the web service listens on. Review firewall rules under Configuration > Firewall to see if the WAN or external interfaces allow inbound HTTP/HTTPS traffic.
    Affected if The web management interface is bound to or accessible from WAN/external interfaces, or firewall rules permit inbound HTTP (port 80) or HTTPS (port 443) from untrusted IP addresses.

A user is affected if they run a Zyxel USG/Zywall/ATP/Flex/VPN device on firmware versions 4.35-4.64 (or 4.35-5.01 for Flex/ATP/VPN) with the web management interface enabled and accessible from untrusted networks.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to a release after 4.64
Interim mitigation

Update firmware to versions beyond 4.64 (USG/Zywall) and 5.01 (USG Flex/ATP/VPN). If patching is immediately infeasible, restrict web management interface access to trusted IPs via firewall rules or disable remote management on external interfaces.

Recommended fix Moderate confidence

Firmware version > 4.64 (USG/Zywall series) or >= 5.01 (USG Flex/ATP/VPN series) - obtain latest stable release from Zyxel

  1. Identify the specific USG/Zywall device model from the affected list (Usg1900, Usg1100, Usg310, Usg210, Usg110, Usg40, Usg40w, or Usg60)
  2. Access the Zyxel support website at www.zyxel.com and navigate to the firmware download section for your device model
  3. Download the latest available firmware version (version 5.01 or later for USG Flex/ATP/VPN series, or the most recent version beyond 4.64 for USG/Zywall series)
  4. Review the firmware release notes to confirm the security patch for CVE-2021-35029 is included
  5. Back up the current device configuration before upgrading
  6. Upload and install the new firmware through the device's web-based management interface or CLI
  7. Verify the upgrade completed successfully and the device is functioning normally
  8. Confirm the new firmware version is applied and the authentication bypass vulnerability is patched
Caveat Review Zyxel release notes for any configuration or feature changes between your current version and the upgrade version; some legacy settings may require adjustment after upgrade

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Usg1900 Firmware Scoped from the published advisory
  • Consultation6.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
16.0 hours of engineering $2,880
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-35029 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data