---
title: "CVE-2021-27137: DD-WRT Stack-Based Buffer Overflow Vulnerability"
url: https://erroneo.us/security/cve/CVE-2021-27137/
description: "CVE-2021-27137 (DD-WRT DD-WRT): Exploited in the wild (CISA KEV). An issue was discovered in router/upnp/src/ssdp.c in DD-WRT before 45724. An unsafe strcpy…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-07-16
date_modified: 2026-07-22
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2021-27137"
kev: true
cvss: 8.1
epss: 0.03995
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2021-27137: DD-WRT Stack-Based Buffer Overflow Vulnerability

**Signal:** Exploited: patch now if you run it. CISA has confirmed exploitation in the wild. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Yes, if you run DD-WRT DD-WRT. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-07-21, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-07-24 ; treat that as the outer limit, not the target.
- EPSS 4.0% , percentile 90: FIRST’s model estimates a 4.0% chance of exploitation activity in the next 30 days, higher than 90% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Editorial verdict: Check exposure. DD-WRT CVE-2021-27137: UPnP Overflow Used by C0XMO Botnet — Only routers running an old DD-WRT build with UPnP turned on are at risk; find any you own, update or retire them, and switch UPnP off where nobody needs it.

## What it is

An issue was discovered in router/upnp/src/ssdp.c in DD-WRT before 45724. An unsafe strcpy in the UPnP handling functionality allows an unauthenticated remote attacker to send a request that would overflow an internal fixed buffer. Exploitation requires the DD-WRT user to enable UPnP (which is off by default, and only listens on internal interfaces by default). This occurs in ssdpmsearch (reachable by an M-SEARCH request).

## Affected products (CVE record)

- DD-WRT DD-WRT: < 45724

## Severity

- mitre CVSS 3.1: 8.1 `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H`

## Exploitation

- CISA KEV: added 2026-07-21; federal due date 2026-07-24. Known ransomware use: Unknown.
- CISA required action: Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
- CISA SSVC assessment: exploitation active , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 4.00% probability of exploitation activity in the next 30 days; percentile 90.

## Sources

- [Listed in CISA KEV notes (svn.dd-wrt.com)](https://svn.dd-wrt.com/changeset/45724)
- [CVE.org record CVE-2021-27137](https://www.cve.org/CVERecord?id=CVE-2021-27137)
- [NVD entry CVE-2021-27137](https://nvd.nist.gov/vuln/detail/CVE-2021-27137)
- [CISA Known Exploited Vulnerabilities catalog entry](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27137)
- [Listed in CISA KEV notes (cisa.gov)](https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk)
- [Listed in CISA KEV notes (cisa.gov)](https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk)
- [ssd-disclosure.com](https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/)
- [securityaffairs.com](https://securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.html)
- [bleepingcomputer.com](https://www.bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/)
- [fortinet.com](https://www.fortinet.com/blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmo)

---
Source page: https://erroneo.us/security/cve/CVE-2021-27137/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt