DD-WRT UPnP SSDP Buffer Overflow Vulnerability (CVE-2021-27137) Analysis and Mitigation Guide

1. Firmware Upgrade (Recommended)

Apply the official patch by upgrading your DD-WRT router to build 45724 or later, which replaces unsafe string operations with bounded alternatives [references].

2. Temporary Mitigation (If patching is not immediately possible)

Since UPnP is disabled by default in DD-WRT, ensure it is turned off if it was previously enabled.

Via DD-WRT Web GUI:

• Log in to the DD-WRT Web Administration interface.
• Navigate to NAT / QoS -> UPnP.
• Set UPnP Service to Disable.
• Click Save and then Apply Settings.

Via Command Line Interface (SSH/Telnet):

# Disable UPnP service and commit changes
nvram set upnp_enable=0
nvram commit

# Terminate the running UPnP daemon
killall upnpd

This vulnerability (CVE-2021-27137) is a stack-based buffer overflow (CWE-121) residing in the UPnP (Universal Plug and Play) handling component (router/upnp/src/ssdp.c) of the DD-WRT open-source router firmware.

Root Cause & Exploit Mechanics

• Unsafe String Copy: The vulnerability exists within the ssdp_msearch function, which processes incoming Simple Service Discovery Protocol (SSDP) discovery requests. The function utilizes the unsafe strcpy function to copy user-controlled input into a fixed-size internal stack buffer without validating the input length.
• Attack Vector: An attacker can transmit a specially crafted SSDP M-SEARCH request over UDP port 1900. By supplying an overly long header value, the attacker overflows the stack buffer, allowing them to overwrite the return address and hijack the control flow.
• In-the-Wild Exploitation: This vulnerability is actively exploited and listed on the CISA KEV catalog. Threat actors, including Gafgyt variants like the c0xmo botnet, leverage this flaw to compromise IoT devices, propagate malware, and terminate competing botnet processes on infected routers [references].