A security vulnerability has been identified in DD-WRT, a widely used router firmware that many small business and individual users rely on to manage their network connections. The flaw is a stack-based buffer overflow, a type of coding error that lets unrequested extra data overflow a small internal memory buffer used by UPnP, a standard feature that lets devices on your network automatically find and communicate with each other without manual setup. This issue does not require an attacker to have any login credentials for your DD-WRT device to exploit it. If successfully triggered, the flaw can allow the attacker to run unauthorized, harmful code on the device running the DD-WRT firmware.
Source: CISA Known Exploited Vulnerabilities Catalog — https://nvd.nist.gov/vuln/detail/CVE-2021-27137