mcp-bastion-python 1.0.1

Security middleware for MCP servers protecting LLM agents from prompt injection, resource exhaustion, and PII leakage

MCP-Bastion

Enterprise-Grade Security Middleware for the Model Context Protocol

Author: Viquar Khan

Releases are published automatically to npm and PyPI via GitHub Actions when tags are pushed.

The Model Context Protocol (MCP) has rapidly become the universally accepted standard for connecting AI agents to enterprise databases and APIs. However, this connectivity introduces a massive new attack surface: unpredictable, non-deterministic agentic behavior.

MCP-Bastion is a lightweight, drop-in security middleware designed to wrap around any existing Python or TypeScript MCP server. Instead of relying on passive logging, human-in-the-loop approvals, or third-party APIs, MCP-Bastion provides an active, 100% local defense layer. It intercepts standard JSON-RPC traffic to stop threats before they cross the enterprise boundary.

Under 5ms proxy overhead. MCP-Bastion provides:

• Prompt Injection Defense: Meta PromptGuard runs locally to block adversarial payloads and jailbreaks.
• PII Redaction: Uses Microsoft Presidio to detect and mask PII before it reaches the LLM context.
• Infinite Loop Protection: Token buckets and cycle detection stop runaway agents from burning API budget.

Secure your MCP server without changing business logic.

---

Core Features

Zero-Click Prompt Injection Prevention

Integrates Meta's PromptGuard model locally to detect and block malicious payloads, jailbreaks, and adversarial tokenization before they reach your external tools.

PII Redaction

Microsoft Presidio scans outbound tool results and masks PII (redaction, substitution, generalization).

Infinite Loop and Denial of Wallet Protection

Implements stateful cycle detection and configurable FinOps token-bucket algorithms to automatically terminate runaway agents and prevent massive API bill overruns.

100% Local Execution (Data Privacy)

All security classification and data redaction happen entirely within the local memory space of your server. Sensitive data never leaves your enterprise network for third-party safety evaluations.

Low Latency

Drop-in middleware, under 5ms overhead.

Framework Integration

Hooks into MCP SDKs (TypeScript, Python) and FastMCP via standard middleware. No business logic changes.

---

Why MCP-Bastion (Competitive Comparison)

Early security packages (mcp-guardian, mcp-shield) focus on logging or static scanning. MCP-Bastion adds an active defense layer.

1. Active Defense vs. Passive Logging

The Competition	MCP-Bastion
Tools like mcp-guardian focus on tracing, logging, human-in-the-loop approvals.	Automated interception. MCP-Bastion scrubs PII before it leaves the server.

2. Local Inference vs. Third-Party APIs

The Competition	MCP-Bastion
Many guardrail proxies send prompts to external APIs (e.g. OpenAI moderation) to check for malice.	PromptGuard-86M and Presidio run locally. Data stays on your network.

3. Stateful Denial of Wallet Protection

The Competition	MCP-Bastion
Most tools focus on static vulns or basic rate limits.	Tracks tool call history per session. Stops runaway loops before they burn API budget.

4. Drop-in Middleware vs. Standalone Gateway

The Competition	MCP-Bastion
Some solutions need standalone proxy servers.	Library hooks into server.setRequestHandler (TS) or middleware (Python). No extra infra.

---

Structure

Path	Description
src/mcp_bastion/	Python package: PromptGuard, Presidio, rate limiting
packages/core/	TypeScript package: rate limiting; ML via Python sidecar
examples/	Python examples: basic middleware, full demo (examples/README.md)
scripts/validate_checklist.py	Enterprise validation runner
VALIDATION_CHECKLIST.md	Validation guide and MCP Inspector steps
SETUP_GUIDE.md	Setup, config, and validation

Installation

Python

uv add mcp-bastion-python
# or
pip install mcp-bastion-python

TypeScript

npm install @mcp-bastion/core

Developer Guide

Integration examples for Python and TypeScript.

---

Quick Start (Python)

Add MCP-Bastion to an existing MCP server in three steps:

from mcp_bastion import MCPBastionMiddleware, compose_middleware

# 1. Create the security middleware
bastion = MCPBastionMiddleware(
    enable_prompt_guard=True,
    enable_pii_redaction=True,
    enable_rate_limit=True,
)

# 2. Compose with your middleware chain (Bastion runs first)
middleware = compose_middleware(bastion)

# 3. Pass the composed middleware to your MCP server
# (integration depends on your server framework)

Examples:

Example	Description
examples/python_server_example.py	Basic middleware chain
examples/full_demo.py	All features: add, PII, rate limit, prompt injection

# Windows: $env:PYTHONPATH="src"; python examples/full_demo.py
# Linux/Mac: PYTHONPATH=src python examples/full_demo.py

Enterprise validation:

PYTHONPATH=src python scripts/validate_checklist.py

See VALIDATION_CHECKLIST.md and SETUP_GUIDE.md.

---

Python Tutorial: FastMCP Server

FastMCP server with MCP-Bastion.

Step 1: Install dependencies

pip install mcp mcp-bastion-python

Step 2: Create your server file (server.py)

from mcp.server.fastmcp import FastMCP
from mcp_bastion import MCPBastionMiddleware, compose_middleware

# Create the MCP server
mcp = FastMCP("My Secure Server")

# Create MCP-Bastion middleware
# It intercepts tool calls and resource reads before they execute
bastion = MCPBastionMiddleware(
    enable_prompt_guard=True,   # Block malicious prompts via PromptGuard
    enable_pii_redaction=True,  # Mask PII in outgoing content
    enable_rate_limit=True,     # Cap at 15 iterations, 60s timeout
)

# Compose middleware chain (pass to your server's middleware config if supported)
middleware = compose_middleware(bastion)

# Register a tool (protected when middleware is wired into your server)
@mcp.tool()
def get_weather(city: str) -> str:
    """Get weather for a city."""
    return f"Weather in {city}: 22C, sunny"

# Resource (PII redacted)
@mcp.resource("user://profile/{user_id}")
def get_profile(user_id: str) -> str:
    """Get user profile. PII redacted."""
    return f"User {user_id}: John Doe, SSN 123-45-6789, [e-mail]"

if __name__ == "__main__":
    mcp.run(transport="streamable-http")

Step 3: Run the server

python server.py

MCP-Bastion:

• Scans tool args for prompt injection
• Redacts PII from resource responses
• Blocks sessions over 15 calls or 60s

---

Python: Custom Rate Limits

Custom config example:

from mcp_bastion import MCPBastionMiddleware
from mcp_bastion.pillars.rate_limit import TokenBucketRateLimiter
from mcp_bastion.pillars.prompt_guard import PromptGuardEngine

# Stricter limits
rate_limiter = TokenBucketRateLimiter(
    max_iterations=10,
    timeout_seconds=30,
    token_budget=25_000,
)

# Higher threshold = fewer blocks, more risk
prompt_guard = PromptGuardEngine(threshold=0.92)

bastion = MCPBastionMiddleware(
    prompt_guard=prompt_guard,
    rate_limiter=rate_limiter,
    enable_prompt_guard=True,
    enable_pii_redaction=True,
    enable_rate_limit=True,
)

# Disable PII redaction if your data has no PII
bastion_no_pii = MCPBastionMiddleware(enable_pii_redaction=False)

---

Python: Custom Middleware

Extend Middleware to add logging, metrics, or custom logic:

from mcp_bastion.base import Middleware, MiddlewareContext, compose_middleware

class LoggingMiddleware(Middleware):
    async def on_message(self, context, call_next):
        result = await call_next(context)
        # log method, elapsed, etc.
        return result

middleware = compose_middleware(bastion, LoggingMiddleware())

See examples/full_demo.py for a complete example.

---

TypeScript: Wrap an MCP Server

Step 1: Install dependencies

npm install @modelcontextprotocol/sdk @mcp-bastion/core

Step 2: Create your server (server.ts)

import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import {
  wrapWithMcpBastion,
  wrapCallToolHandler,
} from "@mcp-bastion/core";

const server = new Server({ name: "my-mcp-server", version: "1.0.0" });

// Wrap the server with MCP-Bastion (rate limiting only by default)
// For prompt injection and PII, run the Python sidecar and set sidecarUrl
wrapWithMcpBastion(server, {
  enableRateLimit: true,
  maxIterations: 15,
  timeoutMs: 60_000,
  // Optional: enable ML features via Python sidecar
  sidecarUrl: process.env.MCP_BASTION_SIDECAR || "",
  enablePromptGuard: !!process.env.MCP_BASTION_SIDECAR,
  enablePiiRedaction: !!process.env.MCP_BASTION_SIDECAR,
});

// Register tools (handlers are automatically wrapped)
server.setRequestHandler("tools/call" as any, async (request) => {
  if (request.params?.name === "get_weather") {
    return {
      content: [{ type: "text", text: "Sunny, 22C" }],
      isError: false,
    };
  }
  throw new Error("Unknown tool");
});

async function main() {
  const transport = new StdioServerTransport();
  await server.connect(transport);
}

main();

Step 3: Run with rate limiting only

npx tsx server.ts

Step 4: Run with full ML features (Python sidecar)

For prompt injection and PII redaction, run a Python HTTP service that exposes /prompt-guard and /pii-redact endpoints (see the Python package for sidecar implementation). Then:

# Start the Python sidecar, then the TypeScript server
MCP_BASTION_SIDECAR=http://localhost:8000 npx tsx server.ts

---

TypeScript: Wrap Individual Handlers

Wrap specific handlers only:

import {
  wrapCallToolHandler,
  wrapReadResourceHandler,
} from "@mcp-bastion/core";
import {
  CallToolRequestSchema,
  ReadResourceRequestSchema,
} from "@modelcontextprotocol/sdk/types.js";

// Wrap only the tool handler
const safeToolHandler = wrapCallToolHandler(
  async (request) => {
    // Your tool logic
    return { content: [{ type: "text", text: "OK" }], isError: false };
  },
  { enableRateLimit: true, maxIterations: 10 }
);

// Wrap only the resource handler (for PII redaction)
const safeResourceHandler = wrapReadResourceHandler(
  async (request) => {
    const contents = await fetchResource(request.params.uri);
    return { contents };
  },
  { sidecarUrl: "http://localhost:8000", enablePiiRedaction: true }
);

server.setRequestHandler(CallToolRequestSchema, safeToolHandler);
server.setRequestHandler(ReadResourceRequestSchema, safeResourceHandler);

---

Configuration Reference

Option	Python	TypeScript	Default	Description
enable_prompt_guard	Yes	Yes	True (Python) / False (TS)	Block malicious prompts via PromptGuard
enable_pii_redaction	Yes	Yes	True (Python) / False (TS)	Mask PII in outgoing content
enable_rate_limit	Yes	Yes	True	Enforce iteration and timeout caps
max_iterations	Via TokenBucketRateLimiter	Yes	15	Max tool calls per session
timeout_seconds / timeoutMs	Via TokenBucketRateLimiter	Yes	60	Session timeout
token_budget	Via TokenBucketRateLimiter	-	50,000	FinOps token cap per request
sidecarUrl	-	Yes	""	Python sidecar URL for ML features
threshold	Via PromptGuardEngine	-	0.85	Malicious probability cutoff
setLogLevel	-	Yes	"info"	TypeScript: "debug" | "info" | "warn" | "error"

---

Error Handling

When MCP-Bastion blocks a request, it returns standard MCP/JSON-RPC errors:

Code	Exception	When
-32001	PromptInjectionError	Tool args contain jailbreak/injection
-32002	RateLimitExceededError	Session exceeds iteration or timeout limit
-32003	TokenBudgetExceededError	Session exceeds token budget

# Python: exceptions
from mcp_bastion.errors import (
    PromptInjectionError,
    RateLimitExceededError,
    TokenBudgetExceededError,
)
import logging
logger = logging.getLogger(__name__)

try:
    result = await middleware(context, call_next)
except PromptInjectionError as e:
    logger.warning("blocked: %s", e.to_mcp_error())
except RateLimitExceededError as e:
    logger.warning("blocked: %s", e.to_mcp_error())
except TokenBudgetExceededError as e:
    logger.warning("blocked: %s", e.to_mcp_error())

// TypeScript: handlers return isError: true
import { logger, setLogLevel } from "@mcp-bastion/core";
setLogLevel("debug");  // optional: "debug" | "info" | "warn" | "error"
const result = await guardedHandler(request);
if (result.isError) {
  logger.error("blocked", result.content);
}

---

Testing

MCP Inspector:

# Start your guarded server
python server.py   # or: npx tsx server.ts

# In another terminal, launch the Inspector
npx -y @modelcontextprotocol/inspector

Connect via HTTP (http://localhost:8000/mcp) or stdio, then:

1. List tools and call one with benign arguments (should succeed)
2. Call a tool with "Ignore previous instructions" (should be blocked)
3. Trigger 16+ tool calls in one session (should hit rate limit)

---

Testing

# Python (PYTHONPATH=src on Windows: $env:PYTHONPATH="src")
pytest tests/ -v

# TypeScript
npm run test --workspace=@mcp-bastion/core

# Full validation checklist (build, pillars, latency)
PYTHONPATH=src python scripts/validate_checklist.py

# MCP Inspector (manual)
npx -y @modelcontextprotocol/inspector

Third-Party Components

See NOTICE for licenses. MCP-Bastion uses Meta Llama Prompt Guard 2 (Llama 4 Community License) and Microsoft Presidio.

License

MIT