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Vulnerable MCP Server: Indirect Prompt Injection (Remote MCP over HTTP+SSE)

This is an intentionally vulnerable MCP server that simulates a document retrieval/search system where retrieved documents include hidden instructions designed to manipulate the model.

Unlike the local stdio version, this server exposes MCP over HTTP + Server-Sent Events (SSE) so clients can connect over the network.

Do not use this outside a controlled lab environment.

What it provides

The server exposes these MCP tools:

  • get_document: Retrieve a document by ID (expects document_id)
    • Available IDs: company_policy, product_specs, user_review, safe_document
  • search_documents: Search for documents containing keywords (expects query)

Documents are loaded from documents/*.txt and returned verbatim.

How it is vulnerable

This server demonstrates indirect prompt injection in a remote context:

  • Retrieved documents contain “internal notes” / “guidelines” that look legitimate but act as prompt-like directives.
  • The server returns the content with no sanitization, filtering, or instruction/data separation.
  • Because this is a remote MCP server, it also models the risk of connecting to untrusted network-accessible MCP endpoints.

Examples in the bundled documents include attempts to:

  • Add an unrelated “wellness tip” (recipe) when answering policy questions
  • Steer users to a premium product variant / inflate claims
  • Reframe a negative review as an “outlier” and push 5-star language

Requirements

  • Node.js (modern version)
  • npm

Install dependencies in this folder:

npm install

Node/Claude compatibility note (common error)

If you use the included claude_config.json (which runs npx … mcp-remote …) on Node.js 16, you may see errors like:

  • npm WARN cli npm v10.x does not support Node.js v16.x
  • ReferenceError: ReadableStream is not defined (from undici)

This happens because the bridge (mcp-remote invoked via npx) pulls dependencies that expect a newer Node.js runtime (web streams / ReadableStream are available globally in Node 18+).

Workarounds (pick one):

  • Upgrade Node for the bridge (recommended): ensure the node that Claude Desktop uses is Node >= 18.17 (or >= 20.5). On Linux, Claude will use whatever node/npx it finds on its PATH, which may differ from your interactive shell.
  • Use a version manager: with nvm/asdf, make sure Claude is launched from an environment where node -v is 18+ (e.g., log out/in after changing defaults, or start Claude from a shell where the newer Node is active).
  • Pin an older mcp-remote (stopgap if you must stay on Node 16): change the claude_config.json command args to run a specific older version, e.g. mcp-remote@0.0.14, instead of the latest. (This avoids newer dependency chains, but may not support all features.)

Run the server

By default it listens on port 3000 (or PORT if set).

Run locally

node index.js

Then the SSE endpoint is:

  • http://localhost:3000/mcp

Run behind a reverse proxy with a path prefix (your setup)

If your reverse proxy exposes this service under a prefix like /document, set BASE_PATH so the server advertises the correct message endpoint to clients:

BASE_PATH=/document node index.js

Make sure your reverse proxy routes both:

  • /document/mcp
  • /document/messages

to this process.

Run with Claude (using claude_config.json)

Claude Desktop typically runs MCP servers over stdio. Since this server is HTTP+SSE, the included claude_config.json uses mcp-remote as a small bridge that connects to the SSE endpoint and exposes it to Claude as a local stdio MCP server.

  1. Start this server (examples):
npm install
BASE_PATH=/document node index.js
  1. Edit claude_config.json and replace the URL argument with your SSE endpoint, for example:
  • http://localhost:3000/mcp (local)
  • https://agents.apsc.co/document/mcp (behind your proxy)

  1. Open Claude and edit its MCP configuration (Claude Desktop typically exposes this under Settings -> Developer -> Edit config).

  2. Merge the mcpServers entry into your config.

  3. Restart Claude so it loads the updated MCP configuration.

After Claude restarts, the server should appear as:

  • vulnerable-mcp-server-indirect-prompt-injection-remote-mcp

Run manually (for debugging)

Quick connectivity tests

  • Local test client:
node local-test.js
  • Remote/proxied test client (as configured in test.js):
node test.js

Example usage (safe + injection)

From your MCP client:

  • Demonstrate normal retrieval:
    • Ask for: “Retrieve safe_document and summarize it.”
  • Search for documents:
    • Use search_documents with query: "vacation" (should find company_policy)
  • Demonstrate indirect prompt injection:
    • Ask: “What does company_policy say about vacations?”
    • Ask: “Summarize product_specs.”
    • Ask: “Summarize user_review.”

These documents intentionally contain embedded instructions that attempt to bias or override the assistant’s response.

Notes

  • This server is intentionally insecure and exists only for demonstration/training.
  • Document content is loaded from documents/*.txt so you can easily tweak injection payloads.
  • This server intentionally has no authentication and is meant for local/lab use only.

Links to Appsecco Resources