The Home Network Administration Protocol (HNAP) is an HTTP-Simple Object Access Protocol (SOAP)-based protocol that can be implemented inside of network devices to allow advanced programmatic configuration and management by remote entities.
Network equipment manufacturers can better identify their devices and provide improved, more cost effective support. Using HNAP, manufacturers can display custom, device-specific information and tasks in applications that use HNAP in device discovery and management, such as Cisco’s Network Magic.
Read more here – http://www.cisco.com/web/partners/downloads/guest/hnap_protocol_whitepaper.pdf
HNAP is useful to anyone in particular to ISPs who manage a fleet of devices deployed as part of their internet subscriptions, but exploits against HNAP are not new.
Read more about vulnerabilities in HNAP implementations here – https://isc.sans.edu/forums/diary//17648