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Classic Buffer OverflowWeakness · CWE-120

CVE-2026-3871

MEDIUM · 6.5 CVSS v3.1 Published 2026-06-02
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
70/100
Remediation priority · Elevated
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
A buffer overflow vulnerability in the UPnP DeletePortMapping() command in Zyxel VMG4005-B50B firmware versions through 5.13(ABRL.5.4)C0 could allow an adjacent attacker to trigger a temporary denial-of-service (DoS) condition affecting the UPnP function of the 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 · moderate confidence

A buffer overflow vulnerability exists in the UPnP DeletePortMapping() function of Zyxel VMG4005-B50B routers running firmware versions up to 5.13(ABRL.5.4)C0. An adjacent (network-local) attacker can send malformed UPnP requests to trigger the overflow, causing a temporary denial of service that disrupts the UPnP service on the affected device.

MitigationApply the vendor firmware patch when available; in the interim, disable the UPnP function if not required, and restrict network access to the device via VLAN segmentation or firewall rules to limit adjacent attackers.

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

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
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/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 device model
    Access router web interface or check device label to confirm model is Zyxel VMG4005-B50B
    Affected if Device is not a Zyxel VMG4005-B50B - not affected by this specific CVE
  2. Check firmware version
    Log into router web interface, navigate to Status or System Info page to view firmware version. Alternatively, check via CLI if available.
    Affected if Firmware version is 5.13(ABRL.5.4)C0 or any version prior to it (all versions through this release are affected)
  3. Verify UPnP is enabled
    Access router web interface, navigate to Network Settings or Advanced Settings, look for UPnP setting. Confirm whether it is toggled ON or enabled.
    Affected if UPnP is disabled - the DeletePortMapping() function is not accessible, reducing exploitability to non-existent

Device is affected if it is a Zyxel VMG4005-B50B running firmware version 5.13(ABRL.5.4)C0 or earlier AND has UPnP functionality enabled.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the vendor firmware patch when available; in the interim, disable the UPnP function if not required, and restrict network access to the device via VLAN segmentation or firewall rules to limit adjacent attackers.

Have this fixed Scoped from the published advisory
  • Consultation4.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,180
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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-2026-3871 in production — separate from our analysis above.

No notes yet

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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