Subsonic API: any authenticated user can delete or read any other user's playlist (IDOR)

gonic is a music streaming server / free-software subsonic server API implementation. Prior to version 0.21.0, the Subsonic API endpoints /rest/deletePlaylist.view and /rest/getPlaylist.view perform no per-resource authorization. Once authenticated as any user (admin or not), an attacker can delete any playlist owned by any other user (including admin) by passing its id and read the full contents (name, comment, song list) of any other user's private (non-public) playlist by passing its id. The Subsonic playlist id is base64url("/.m3u"). Because filenames are user-supplied or time-derived and the userID is a small integer, IDs are guessable and frequently exposed (e.g. a previously-public playlist that was later made private still has the same ID). This breaks the multi-user trust boundary of gonic: a low-privileged user can wipe an administrator's curated playlists, and a user can exfiltrate any private playlist they obtain an ID for. The issue was fixed in commit 6dd71e6a3c966867ef8c900d359a7df75789f410, which is part of version 0.21.0.