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Nbg6604 FirmwareOperating system · Zyxel

CVE-2021-35034

CRITICAL · 9.1 CVSS v3.1 Published 2021-12-29
Fix available
A fix is available. Upgrade to 1.00 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 insufficient session expiration vulnerability in the CGI program of the Zyxel NBG6604 firmware could allow a remote attacker to access the device if the correct token can be intercepted.

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 · moderate confidence

The Zyxel NBG6604 firmware contains an insufficient session expiration vulnerability in its CGI program. Sessions do not properly timeout, allowing a remote attacker who intercepts a valid token to maintain persistent access to the device without authentication expiration.

MitigationApply the latest Zyxel firmware patch for NBG6604. If no patch is available, implement compensating controls such as network segmentation, aggressive ACLs restricting access to management interfaces, and monitoring for unauthorized token usage.

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
Nbg6604 FirmwareOperating system
Affected:< 1.00\(abir.9\)c0

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
None

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

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 installed firmware version
    Access the device's web management interface and navigate to the Status or System Info page to view the firmware version. Alternatively, log into the device via SSH or console and use the 'firmware version' or 'show version' command.
    Affected if The firmware version is lower than 1.00(abir.9)c0, meaning the version string reads something like 1.00(abcd.8)c0 or earlier.
  2. Locate session timeout configuration
    In the web interface, examine the Administration, Management, or Security settings sections for any session timeout, session expiration, or idle timeout options.
    Affected if There is no session timeout option available, or the setting is missing from the CGI/web configuration interface.
  3. Test session persistence behavior
    Log into the device, note the session token, wait for an extended period (e.g., 30 minutes or longer without activity), then attempt to use the original token to access the device without re-authenticating.
    Affected if The original session token remains valid and grants access after a prolonged period of inactivity, indicating sessions do not expire properly.
  4. Review CGI session handling
    If you have access to the device's file system (via firmware extraction or debug mode), locate the CGI binary or program files and examine configuration files related to session management for timeout parameters.
    Affected if No session timeout parameter is defined in the CGI or session configuration, or the timeout value is set to zero or an excessively long duration.

The device is affected if it runs firmware version lower than 1.00(abir.9)c0 and sessions remain valid indefinitely without requiring re-authentication after inactivity.

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 1.00 or later
Fixed in 1.00
Interim mitigation

Apply the latest Zyxel firmware patch for NBG6604. If no patch is available, implement compensating controls such as network segmentation, aggressive ACLs restricting access to management interfaces, and monitoring for unauthorized token usage.

Recommended fix Moderate confidence

NBG6604 Firmware version 1.00(abir.9)c0 or later

  1. 1. Access the Zyxel NBG6604 router web management interface.
  2. 2. Navigate to the Administration or System Settings section.
  3. 3. Locate the firmware upgrade option.
  4. 4. Download the firmware version 1.00(abir.9)c0 or later from the official Zyxel support website (www.zyxel.com).
  5. 5. Upload the firmware file and allow the device to complete the update process.
  6. 6. After reboot, verify the new firmware version is installed.
  7. 7. After upgrading, log out of the session completely and verify that sessions properly terminate.
Caveat Standard firmware upgrade risks apply - ensure stable power during update; configuration reset may be required

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

Fix this in Nbg6604 Firmware Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,800
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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-2021-35034 in production — separate from our analysis above.

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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