CVE-2026-3870
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 · uneditedA buffer overflow vulnerability in the UPnP AddPortMapping() 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 · high confidenceA buffer overflow vulnerability exists in the UPnP AddPortMapping() function of Zyxel VMG4005-B50B broadband gateway firmware versions up to 5.13(ABRL.5.4)C0. An adjacent network attacker with local network access can exploit this buffer overflow to crash the UPnP service, causing a temporary denial-of-service condition.
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 checksWork through these to decide whether this CVE applies to you.
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Confirm device modelLog into the router admin interface or check the device label to verify the model is VMG4005-B50BAffected if The device is not a Zyxel VMG4005-B50B, then it is not affected by this specific CVE
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Check firmware versionAccess the router web interface (typically at 192.168.1.1) and navigate to the Status or Administration section to view the firmware version. Compare it to 5.13(ABRL.5.4)C0Affected if The installed firmware version is 5.13(ABRL.5.4)C0 or any earlier version, indicating the device falls within the affected range
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Verify UPnP is enabledIn the router web interface, go to Network Settings or Advanced settings and check the UPnP (Universal Plug and Play) status. UPnP is typically found under NAT, Firewall, or Network settingsAffected if UPnP is currently enabled on the device, making the vulnerable AddPortMapping() function accessible to an attacker
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Assess network exposureDetermine if untrusted devices can access the router's LAN segment. This vulnerability requires an adjacent network attacker on the same network segment to reach the UPnP serviceAffected if The router's LAN network is accessible to untrusted or guest devices, allowing adjacent network attackers to reach the UPnP service
A user is affected if they have a Zyxel VMG4005-B50B router running firmware version 5.13(ABRL.5.4)C0 or earlier, with UPnP enabled, and the device is on a network where untrusted adjacent attackers can reach the LAN segment.
Generated from the published advisory. Verify against your own configuration.
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 dataDisable the UPnP function on the affected device if not required, and apply any vendor firmware updates when available. Consider network segmentation to restrict adjacent attackers from reaching the device's UPnP service.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-3870 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-3870 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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