Actively exploited in the wild
DD-WRT Stack-Based Buffer Overflow Vulnerability
DD-WRT - DD-WRT · Listed in the CISA KEV since 2026-07-21. This indicates confirmed attacks in production environments.
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.
CVE-2021-27137
HighCVSS 8.1KEVExploitation Probability (EPSS)
Very high risk95th percentile - higher than 95% of all known CVEs
Summary
An issue was discovered in DD-WRT before version 45724 in router/upnp/src/ssdp.c. An unsafe strcpy in UPnP handling allows an unauthenticated remote attacker to overflow an internal fixed buffer via an M-SEARCH request. Exploitation requires UPnP to be enabled (disabled by default, listening only on internal interfaces).
Risk Assessment
The risk for the organization is potential remote code execution or router crash, leading to network disruption and loss of internet access. The attack is limited to scenarios where UPnP is enabled and exposed externally.
Recommendation
Immediately update DD-WRT to version 45724 or later. If updating is not possible, disable UPnP in the router settings and ensure it is not accessible from external interfaces.
Original NVD description (English source)
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).