DD-WRT UPnP SSDP Buffer Overflow via Unsafe strcpy
CVE-2021-27137 Published on July 16, 2026
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 ssdp_msearch (reachable by an M-SEARCH request).
Known Exploited Vulnerability
This DD-WRT Stack-Based Buffer Overflow Vulnerability is part of CISA's list of Known Exploited Vulnerabilities. DD-WRT contains a stack-based buffer overflow vulnerability that could allow an unauthenticated attacker to overflow an internal buffer used by UPnP and trigger a code execution vulnerability.
The following remediation steps are recommended / required by July 24, 2026: 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 applicab
Vulnerability Analysis
CVE-2021-27137 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is consided to have a high level of attack complexity. This vulnerability is known to be actively exploited by threat actors. The potential impact of an exploit of this vulnerability is considered to be very high.
Weakness Type
What is a Stack Overflow Vulnerability?
A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function).
CVE-2021-27137 has been classified to as a Stack Overflow vulnerability or weakness.
Products Associated with CVE-2021-27137
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Affected Versions
DD-WRT:- Before 45724 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.