# CVE-2021-27137: DD-WRT stack buffer overflow in UPnP SSDP parser

Source: https://junglewise.ai/threats/cve-2021-27137-dd-wrt-stack-buffer-overflow-in-upnp-ssdp-parser  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-16. Updated: 2026-07-21  
License: CC BY 4.0 with a link back

CVE-2021-27137 · Severity: critical · CVSS 8.1 · Exploited in the wild · Published 2026-07-16

DD-WRT, a popular open-source firmware for wireless routers, contains a security flaw in its UPnP (Universal Plug and Play) feature. If UPnP is enabled, an attacker on the local network can send a specially crafted request to take full control of the router. This could lead to the theft of internet traffic, unauthorized access to other devices on the network, or the router being recruited into a botnet.

## Quick answers

**Is CVE-2021-27137 exploited in the wild?** Yes. CVE-2021-27137 is known to be exploited in the wild, first recorded on 21 July 2026. Treat it as urgent.

**How severe is CVE-2021-27137?** Critical severity, with a CVSS score of 8.1, published 16 July 2026.

**Which products and versions are affected?** DD-WRT before 45724.

**Is there a fix for CVE-2021-27137?** A patch was published on 2026-07-16: Fix confirmed in build 45724 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2021-27137 attacked?** It is attacked over the network, in an operating system.

**Am I affected by CVE-2021-27137?** Check whether you run DD-WRT at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A stack-based buffer overflow exists in router/upnp/src/ssdp.c within the ssdp_msearch function of DD-WRT. The vulnerability is caused by an unsafe strcpy call when handling M-SEARCH requests, allowing an unauthenticated attacker to overflow a fixed-size internal buffer. While UPnP is disabled by default and typically only listens on internal interfaces, enabling it exposes the device to remote code execution. This vulnerability has been observed being exploited in the wild by the 'c0xmo' botnet. The issue is resolved in DD-WRT build 45724.

## Affected products

- DD-WRT DD-WRT before 45724

## Timeline

- 2021-02-10: disclosed: CVE reserved date
- 2026-07-16: advisory: NVD publication date
- 2026-07-16: exploited: Reports of botnet exploitation (c0xmo) linked to this vulnerability
- 2026-07-16: patched: Fix confirmed in build 45724