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

High severity, CVSS 8.1. Actively exploited: in CISA KEV since 2026-07-21. EPSS: 4% chance of exploitation in the next 30 days.

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).

Affected products

• DD-WRT DD-WRT: before 45724 (fixed in 45724)

Published 2026-07-16. Last modified 2026-07-22.