vulnerable-mcp-server-indirect-prompt-injection

Vulnerable MCP Server: Indirect Prompt Injection

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.

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)

get_document returns the full document as plain text, including any embedded hidden instructions.

How it is vulnerable

This server demonstrates indirect prompt injection:

• Retrieved documents contain prompt-like directives embedded in normal-looking content (examples include bracketed “SYSTEM INSTRUCTION” text, HTML comments, and delimited injection blocks).
• The server returns this content verbatim with no sanitization, no filtering, and no separation between “data” and “instructions”.

In a real RAG / document assistant system, this can lead the model to follow attacker-controlled instructions that override user intent or policy.

Requirements

• Node.js (modern version)
• npm

Install dependencies in this folder:

npm install

Run with Claude (using claude_config.json)

This folder includes claude_config.json, which is a ready-to-merge snippet for Claude's MCP server config.

1. Install dependencies:

npm install

2. Edit claude_config.json and replace the placeholder:

• /full/path/to/vulnerable-mcp-server-indirect-prompt-injection/index.js

Use an absolute path.

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

4. Merge the mcpServers entry into your config. If you already have mcpServers, add just the server entry:

{
  "mcpServers": {
    "vulnerable-mcp-server-indirect-prompt-injection": {
      "command": "node",
      "args": [
        "/absolute/path/to/vulnerable-mcp-server-indirect-prompt-injection/index.js"
      ]
    }
  }
}

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

After Claude restarts, the server should appear as:

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

Run manually (for debugging)

This server speaks MCP over stdio. Most users should run it via an MCP-capable client (like Claude) rather than manually.

If you just want to confirm it starts:

npm install
node index.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: “What does user_review say about the product?”

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.
• The “hidden instructions” are embedded in different formats to illustrate how injections can be smuggled in:
  • Bracketed pseudo-system directives
  • HTML comments
  • Delimited injection blocks

Links to Appsecco Resources

• Appsecco LinkedIn
• Appsecco YouTube
• Appsecco Website