Ampache is a web based audio/video streaming application and file manager. Versions prior to 4.4.1 allow unauthenticated access to Ampache using the subsonic API. To successfully make the attack you must use a username that is not part of the site to bypass the auth checks. For more details and workaround guidance see the referenced GitHub security advisory.
CWE-287 - Improper Authentication
Improper (or broken) authentication attacks are widespread, and have accounted for many of the worst data breaches in recent years. Improper authentication attacks are a class of vulnerabilities where an attacker impersonates a legitimate user by exploiting weaknesses in either session management or credential management to gain access to the user’s account. This can result in disclosure of sensitive information, and can lead to system compromise, theft, identity theft, and fraud.