Emby Server is a user-installable home media server. Versions below 4.9.1.81 allow an attacker to gain full administrative access to an Emby Server (for Emby Server administration, not at the OS level). Other than network access, no specific preconditions need to be fulfilled for a server to be vulnerable. This issue is fixed in version 4.9.1.81.
Emby Server is a personal media server. Prior to version 4.8.1.0 and prior to Beta version 4.9.0.0-beta, a malicious user can send an authentication request with a manipulated X-Emby-Client value, which gets added to the devices section of the admin dashboard without sanitization. This issue has been patched in version 4.8.1.0 and Beta version 4.9.0.0-beta.