Emby (formerly Media Browser) is a media server designed to organize, play, and stream audio and video to a variety of devices. Emby is open-source, and uses a client-server model. Two comparable media servers are Plex and Windows Media Center.
The issue can be triggered by an unauthenticated actor within the home network (LAN) only. The attacker doesn't need to specify a valid username to reset the password. He or she can enter a random string, and using the file disclosure issue it's possible to read the PIN needed for resetting. This in turn will disclose all the valid usernames in the emby server and reset all the passwords for all the users with a blank password. Attackers can exploit this to gain unauthenticated and unauthorized access to the emby media server management interface.