CVE-2021-27137
DD-WRT Stack-Based Buffer Overflow Vulnerability
DD-WRT DD-WRT
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).
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 21 July 2026. US federal agencies were required to remediate it by 24 July 2026.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 16.5%, which is higher than 96.7% of all scored vulnerabilities. EPSS is a forecast of activity, not a measure of severity, and it is rescored daily.
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Frequently asked questions
Is CVE-2021-27137 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2021-27137.
What does CVE-2021-27137 affect?
CVE-2021-27137 affects DD-WRT DD-WRT. 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).
How severe is CVE-2021-27137?
CVE-2021-27137 is rated HIGH with a CVSS base score of 8.1. EPSS puts the probability of exploitation in the next 30 days at 16.5%, higher than 96.7% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 24 July 2026.
How should organisations respond to CVE-2021-27137?
Apply the vendor patch for DD-WRT DD-WRT as the first priority, and treat any internet-facing instance as the most urgent. Because this vulnerability is associated with ransomware activity, also review whether the affected systems were reachable before patching, rather than assuming that patching alone closes the incident.