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Classic Buffer OverflowWeakness · CWE-120

CVE-2021-47854

CRITICAL · 9.8 CVSS v3.1 Published 2026-01-21
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
100/100
Remediation priority · Urgent
Public exploit 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
DD-WRT version 45723 contains a buffer overflow vulnerability in the UPNP network discovery service that allows remote attackers to potentially execute arbitrary code. Attackers can send crafted M-SEARCH packets with oversized UUID payloads to trigger buffer overflow conditions on the target 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 confidence

DD-WRT version 45723 contains a buffer overflow vulnerability in the UPNP network discovery service. Remote attackers can send crafted M-SEARCH packets with oversized UUID payloads to overflow a buffer, potentially enabling arbitrary code execution on the targeted device.

MitigationUpgrade to a patched DD-WRT version that addresses this vulnerability, or disable the UPNP service if not required by the network.

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

CVSS:3.1/AV:N/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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify DD-WRT version
    Log into the router web interface (typically http://192.168.1.1) and check the status page for the firmware version, or run 'nvram get os_version' via telnet/SSH
    Affected if The installed version is 45723 or lower (any version prior to the patch for this CVE)
  2. Verify UPNP service is enabled
    In the DD-WRT web interface, go to Services > LAN > UPNP and confirm the 'UPNP' setting is set to 'Enable', or run 'nvram get upnp_enable' via telnet/SSH
    Affected if UPNP is enabled and accessible on the network
  3. Check UPNP listening interface
    In the UPNP settings, verify which network interface UPNP is bound to (LAN only vs WAN+LAN), or check iptables rules for UPNP traffic: 'iptables -L -n | grep -i upnp'
    Affected if UPNP is exposed to the WAN interface or an untrusted network
  4. Confirm UPNP service is running
    Run 'ps | grep upnp' or check the router's process list via telnet/SSH to verify the upnp daemon is actively running
    Affected if The upnpd process is running on the device

A user is affected if they are running DD-WRT version 45723 or earlier AND have the UPNP service enabled and exposed to the network where an attacker can send M-SEARCH packets.

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

Upgrade to a patched DD-WRT version that addresses this vulnerability, or disable the UPNP service if not required by the network.

Recommended fix Moderate confidence

Latest stable DD-WRT release for the specific router model (version higher than 45723)

  1. 1. Identify the current DD-WRT firmware version on the affected device via the web interface (Status -> Router) or via command line (nvram get os_version)
  2. 2. Navigate to the official DD-WRT download page at https://download1.dd-wrt.com/ to check for newer firmware versions
  3. 3. Download the latest stable DD-WRT firmware build for your specific router model
  4. 4. Access the DD-WRT web interface, go to Administration -> Firmware Upgrade
  5. 5. Upload and flash the newer firmware version
  6. 6. After the device reboots, verify the new firmware version is installed
  7. 7. As a temporary mitigation before upgrading, disable UPNP service in the web interface under Services -> UPnP -> Enable UPnP (uncheck)
Caveat firmware upgrades on embedded devices can sometimes reset configuration; ensure you have a backup of your DD-WRT configuration before upgrading

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

Have this fixed Scoped from the published advisory
  • Consultation3.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,440
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An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,304.

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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-2021-47854 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
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  • Version or environment caveats, and links to real fixes
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