Netcore is a popular brand for networking equipment in China which is also sold under the Netis brand name outside of China. The routers have an open UDP port listening at port 53413. This port is accessible from the WAN side of the router. This means that if the router in question has an externally accessible IP address (i.e., almost all residential and SMB users), an attacker from anywhere on the Internet can access this backdoor.
This backdoor is “protected” by a single, hardcoded password located in the router’s firmware. Netcore/Netis routers appear to all have the same password.