CVE-2021-27239
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 · uneditedThis vulnerability allows network-adjacent attackers to execute arbitrary code on affected installations of NETGEAR R6400 and R6700 firmware version 1.0.4.98 routers. Authentication is not required to exploit this vulnerability. The specific flaw exists within the upnpd service, which listens on UDP port 1900 by default. A crafted MX header field in an SSDP message can trigger an overflow of a fixed-length stack-based buffer. An attacker can leverage this vulnerability to execute code in the context of root. Was ZDI-CAN-11851.
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 confidenceStack-based buffer overflow in the upnpd service (UDP port 1900) allows unauthenticated, network-adjacent attackers to execute arbitrary code as root on NETGEAR R6400 and R6700 routers (firmware 1.0.4.98) via a crafted MX header field in SSDP messages.
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< 1.0.0.68< 1.0.0.102< 1.0.0.66< 1.0.3.60< 1.0.0.54< 1.0.1.94< 1.0.0.72< 1.0.4.48CVSS 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
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:A/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 checksWork through these to decide whether this CVE applies to you.
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Identify router modelDetermine the exact Netgear model number of the device (e.g., D6220, D6400, D7000, D8500, Dc112a, Ex7000, Ex7500, R6250, R6400, or R6700)Affected if The model is any of these: D6220, D6400, D7000, D8500, Dc112a, Ex7000, Ex7500, R6250, R6400, or R6700
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Check firmware versionAccess the router admin interface or use the manufacturer-provided method to retrieve the current firmware version, then compare against the affected version rangesAffected if Firmware version is below: D6220 < 1.0.0.68, D6400 < 1.0.0.102, D7000 < 1.0.0.66, D8500 < 1.0.3.60, Dc112a < 1.0.0.54, Ex7000 < 1.0.1.94, Ex7500 < 1.0.0.72, R6250 < 1.0.4.48 (and R6400/R6700 < 1.0.4.98)
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Verify UPnP service statusCheck if the UPnP feature is enabled in the router settings under the UPnP configuration sectionAffected if UPnP is enabled on the device (the vulnerability only affects devices with UPnP active)
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Test UDP port 1900 accessibilityPerform a network scan to determine if UDP port 1900 (the SSDP port) is accessible from the networkAffected if UDP port 1900 is exposed and reachable from an untrusted network segment
The device is affected if it is a listed Netgear model running a firmware version below the specified threshold AND has UPnP enabled with UDP port 1900 accessible.
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.
dbcve · scoped1.0.0.541.0.0.661.0.0.68
Restrict access to UDP port 1900 at the network perimeter or disable UPnP; replace affected routers as they are likely end-of-life and may not receive official firmware patches.
Upgrade to firmware version 1.0.0.68 or later for D6220, 1.0.0.102 or later for D6400, 1.0.0.66 or later for D7000, 1.0.3.60 or later for D8500, 1.0.0.54 or later for Dc112a, 1.0.1.94 or later for Ex7000, 1.0.0.72 or later for Ex7500, 1.0.4.48 or later for R6250
- 1. Identify the NETGEAR router model from the affected list (D6220, D6400, D7000, D8500, Dc112a, Ex7000, Ex7500, R6250)
- 2. Access the router's web interface by navigating to http://routerlogin.net or http://192.168.1.1
- 3. Go to Settings > Administration > Firmware Update or similar menu path
- 4. Check the current firmware version under the Router Info or Maintenance section
- 5. If the current version is below the fixed release for your model, download the corresponding firmware update from the NETGEAR support website (support.netgear.com)
- 6. In the router admin panel, select the option to manually upload the firmware file
- 7. Browse and select the downloaded firmware file, then initiate the update
- 8. Wait for the firmware upload and installation to complete - do not power off the router during this process
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Testing2.0 h
- Review / QA1.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2021-27239 — 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-2021-27239 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
- No weaponised exploit code, or anything meant to cause harm
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