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Zenwifi Ax \(xt8\) FirmwareOperating system · Asus

CVE-2021-3128

HIGH · 7.5 CVSS v3.1 Published 2021-04-12
Fix available
A fix is available. Upgrade to 3.0.0.4.386.42095 / 9.0.0.4.386.41994 or later.
See remediation →
84/100
Remediation priority · High
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
In ASUS RT-AX3000, ZenWiFi AX (XT8), RT-AX88U, and other ASUS routers with firmware < 3.0.0.4.386.42095 or < 9.0.0.4.386.41994, when IPv6 is used, a routing loop can occur that generates excessive network traffic between an affected device and its upstream ISP's router. This occurs when a link prefix route points to a point-to-point link, a destination IPv6 address belongs to the prefix and is not a local IPv6 address, and a router advertisement is received with at least one global unique IPv6 prefix for which the on-link flag is set.

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

This is an IPv6 routing loop vulnerability in ASUS routers where improper handling of router advertisements with global unique prefixes and on-link flags causes traffic to loop between the affected device and the upstream ISP router. The loop generates excessive network traffic, effectively creating a denial-of-service condition.

MitigationUpdate router firmware to version 3.0.0.4.386.42095 or higher (for branch 3.0.0.4) or 9.0.0.4.386.41994 or higher (for branch 9.0.0.4). If immediate updates are not possible, consider disabling IPv6 as a temporary workaround.

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
Zenwifi Ax \(xt8\) FirmwareOperating system
Affected:< 3.0.0.4.386.42095< 9.0.0.4.386.41994
Rt Ax3000 FirmwareOperating system
Affected:< 3.0.0.4.386.42095< 9.0.0.4.386.41994
Rt Ax55 FirmwareOperating system
Affected:< 3.0.0.4.386.42095< 9.0.0.4.386.41994
Rt Ax56u FirmwareOperating system
Affected:< 3.0.0.4.386.42095< 9.0.0.4.386.41994
Rt Ax58u FirmwareOperating system
Affected:< 3.0.0.4.386.42095< 9.0.0.4.386.41994
Rt Ax68u FirmwareOperating system
Affected:< 3.0.0.4.386.42095< 9.0.0.4.386.41994
Rt Ax82u FirmwareOperating system
Affected:< 3.0.0.4.386.42095< 9.0.0.4.386.41994
Rt Ax86u FirmwareOperating system
Affected:< 3.0.0.4.386.42095< 9.0.0.4.386.41994

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
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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 router model
    Locate the model number on the device label or log into the web admin interface (typically at 192.168.1.1) and check the Device Info or Status page
    Affected if Model is not one of: Asus Zenwifi Ax (xt8), Asus Rt Ax3000, Asus Rt Ax55, Asus Rt Ax56u, Asus Rt Ax58u, Asus Rt Ax68u, Asus Rt Ax82u, Asus Rt Ax86u
  2. Check firmware version
    In the router admin interface, go to Administration > Firmware Upgrade or System > Firmware Version and note the exact version number
    Affected if Firmware version is below 3.0.0.4.386.42095 (for branch 3.0.0.4) or below 9.0.0.4.386.41994 (for branch 9.0.0.4)
  3. Verify IPv6 is enabled
    In the router admin interface, navigate to IPv6 or Wireless > IPv6 settings and confirm the IPv6 functionality is turned on
    Affected if IPv6 is enabled on the router

The router is affected if it is one of the listed models, runs firmware version below the thresholds, and has IPv6 enabled, causing router advertisement handling to create a routing loop with upstream ISP equipment.

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.

From vendor data
Upgrade available Upgrade to 3.0.0.4.386.42095 / 9.0.0.4.386.41994 or later
Fixed in 3.0.0.4.386.420959.0.0.4.386.41994
Interim mitigation

Update router firmware to version 3.0.0.4.386.42095 or higher (for branch 3.0.0.4) or 9.0.0.4.386.41994 or higher (for branch 9.0.0.4). If immediate updates are not possible, consider disabling IPv6 as a temporary workaround.

Fix this in Zenwifi Ax \(xt8\) Firmware Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing3.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,210
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,936.

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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-2021-3128 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