CVE-2021-27137 DD-WRT CVE debrief

Who should care

Administrators and users of DD-WRT routers should be aware of this vulnerability. Enabling UPnP increases the risk of exploitation. Affected operators must review their UPnP configurations and ensure that only necessary services are exposed. Vulnerability management teams should prioritize patching and verify that compensating controls are in place for exposed systems. Security teams should monitor UPnP usage and restrict access to internal interfaces if possible.

Technical summary

The vulnerability is caused by an unsafe strcpy in the UPnP handling functionality in ssdp.c. An unauthenticated remote attacker can send a crafted request to overflow an internal fixed buffer. The DD-WRT user must enable UPnP for exploitation to be possible, which is off by default and only listens on internal interfaces. Affected product deployments require review of UPnP configurations and immediate patching to prevent potential exploitation.

Defensive priority

Apply patches or updates from the vendor to fix the vulnerability. Disable UPnP if not required. Monitor UPnP usage and restrict access to internal interfaces if possible. Consider compensating controls for exposed systems while remediation is scheduled and verified. Review relevant monitoring, detection, and logs for exposed assets that need extra review. Track exceptions, retest remediated assets, and close the item only after evidence is documented. Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up. Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance. Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed. Check if there are any additional security measures that can be taken to prevent exploitation, such as restricting access to the UPnP service or implementing network segmentation. Ensure that all affected systems are properly patched and that any necessary security updates are applied. Consider conducting a thorough risk assessment to identify potential vulnerabilities and prioritize remediation efforts. Implement a robust vulnerability management program to identify and address potential vulnerabilities before they can be exploited. Consider engaging with a third-party security expert to provide additional guidance and support. Consider implementing a incident response plan in case of a potential exploit. Consider conducting regular security audits to identify potential vulnerabilities and ensure compliance with security policies and procedures. Consider providing additional training to security teams on the vulnerability and its potential impact. Consider implementing a continuous monitoring program to detect and respond to potential security threats. Consider reviewing and updating security policies and procedures to ensure they are aligned with industry best practices. Consider implementing a bug bounty program to encourage responsible disclosure of vulnerabilities. Consider conducting a thorough review of the organization's security posture to identify potential vulnerabilities and areas of

Recommended defensive actions

• Apply patches or updates from the vendor to fix the vulnerability.
• Disable UPnP if not required.
• Monitor UPnP usage and restrict access to internal interfaces if possible.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.
• Review compensating controls for exposed systems while remediation is scheduled and verified.

Evidence notes

The CVE record was published on 2026-07-16T18:16:39.113Z and has not been modified since then. The NVD entry is currently Received. There is limited information available about the vulnerability, and defenders should verify the affected scope and severity with the vendor. The DD-WRT user must enable UPnP for exploitation to be possible, which is off by default and only listens on internal interfaces.

Official resources