MCP guide · Security Proxy

What is MCP Security Proxy? Prompt injection and PII shield — @guardbee/mcp-security-proxy

@guardbee/mcp-security-proxy is a transparent security layer in front of any MCP server. It catches prompt-injection patterns, masks PII in responses, and audits every hop — no code changes in the target server.

What does it do?

Use it before exposing filesystem, DB, or third-party MCP tools to Claude/Cursor. Requests pass the proxy first; hostile instructions are blocked or warned.

Who is it for?

Teams connecting multiple MCP servers to Claude or Cursor
Security owners reducing prompt-injection risk
Product teams preventing PII leakage in tool responses

What it is not

Acting as a business-logic MCP by itself (it wraps another server)
Replacing a network WAF or API gateway

Features

Feature	Description
Prompt injection defense	16+ patterns including jailbreaks and hidden commands
PII masking	IDs, IBANs, emails, JWTs, API keys in responses
Block / warn	Per-interceptor mode
Audit log	Console or file
Zero code change	Launch the target after —

MCP tools

Tool	Description
`proxy_status`	Active interceptors and blocked-request stats
`Target tools`	Downstream MCP tools pass through transparently

Install with npm

Global install (handy for Claude Desktop / Cursor):

bash

npm install -g @guardbee/mcp-security-proxy

or via npx (recommended with -y in config):

bash

npx -y @guardbee/mcp-security-proxy

npm: @guardbee/mcp-security-proxy

The proxy runs the command after -- as the target MCP — e.g. wrap the filesystem server.

Claude Desktop configuration

Config file paths:

macOS~/Library/Application Support/Claude/claude_desktop_config.json
Windows%APPDATA%\Claude\claude_desktop_config.json
Linux~/.config/Claude/claude_desktop_config.json

Merge this block, then restart Claude Desktop:

claude_desktop_config.json

{
  "mcpServers": {
    "secure-filesystem": {
      "command": "npx",
      "args": [
        "-y",
        "@guardbee/mcp-security-proxy",
        "--",
        "npx",
        "-y",
        "@modelcontextprotocol/server-filesystem",
        "/tmp"
      ]
    }
  }
}

Cursor and other MCP clients

In Cursor, add the same command / args / env under Settings → MCP. stdio MCP servers use the same contract as Claude Desktop.

Example prompts in Claude

“Show recent blocked requests via proxy_status”
“List /tmp through this secured MCP”
“Confirm emails are masked in responses”

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Frequently asked questions

Which MCP servers work?

Most stdio-launched MCP servers; you supply the command after –.

Block or warn?

Configure per interceptor — prefer block in sensitive environments.

Performance?

Pattern checks are cheap; watch audit file I/O at high volume.

What is MCP Security Proxy? Prompt injection and PII shield