Streamable HTTP mode exposes LINE Desktop read/send tools without MCP authentication

Summary

line-desktop-mcp supports a --http-mode Streamable HTTP transport for use with clients such as n8n. In this mode the server binds to 0.0.0.0 and exposes the MCP /mcp endpoint without an MCP-layer authentication check. Any network client that can reach the port can initialize a session, list tools, and call tools that read LINE Desktop chat history or send LINE messages through the already logged-in desktop application.

This is High for deployments where the HTTP port is reachable beyond the local host, because the server acts with the user authority of the logged-in LINE Desktop session. It is lower if the listener is strictly firewalled to trusted local clients.

Affected version

Repository: dtwang/line-desktop-mcp

Current source checked: fbed0d2d3048e63f48a356a1267ed8ec5e78f3ae on main, committed 2026-05-14.

Published npm package checked: [email protected].

Source evidence

README.md documents Streamable HTTP mode:

npx line-desktop-mcp@latest --http-mode --port 3000

The same README documents MCP endpoints at /mcp and explains that this mode is intended for clients such as n8n.

src/server.js registers LINE Desktop tools including:

• get_line_chatroom_history_default
• get_line_chatroom_history_long
• get_line_chatroom_history_short
• send_message_manual
• send_message_auto

Those tool handlers call into the desktop automation layer: getChatHistory(...) and sendChatMessage(...).

In HTTP mode, src/server.js creates an Express app and Streamable HTTP transport, accepts POSTs to /mcp, creates sessions, connects the transport to the MCP server, and calls transport.handleRequest(...). I did not find an authentication or bearer-token check before session creation or tool invocation.

The listener is explicitly network-bound:

app.listen(port, 0.0.0.0, () => {
  console.error(`LINE Desktop MCP Server running on Streamable HTTP mode`);
  console.error(`  Local:   http://127.0.0.1:${port}${endpoint}`);
  console.error(`  Network: http://0.0.0.0:${port}${endpoint}`);
});

Vulnerability chain

1. A user starts the server with --http-mode --port 3000.
2. The server binds on 0.0.0.0:3000, not only loopback.
3. A network client reaches /mcp and sends the normal MCP initialize request.
4. The server creates a Streamable HTTP session without authenticating the caller.
5. The caller can list and invoke LINE Desktop tools.
6. Tool calls execute through the logged-in LINE Desktop application on the user workstation.

Impact

An unauthenticated network client can read LINE chat history through the MCP history tools and can send LINE messages through the send-message tools, including send_message_auto when the tool call requests immediate sending. The attacker does not need LINE credentials or a LINE API token; they only need network reachability to the MCP HTTP port.

The practical impact is disclosure of private LINE conversations and unauthorized messages sent as the logged-in desktop user.

Suggested fix

Require authentication before accepting Streamable HTTP MCP sessions or tool calls. For example:

• require a bearer token or local secret when --http-mode is used;
• bind HTTP mode to 127.0.0.1 by default unless the operator explicitly opts into network exposure;
• refuse to start 0.0.0.0 HTTP mode without authentication;
• document that host.docker.internal / n8n setups must still authenticate to the MCP server.

A defense-in-depth improvement would also keep send_message_auto disabled unless explicitly enabled by a server-side flag, because it converts MCP tool access into immediate message sending as the desktop user.