Eunomia MCP Authorization Middleware

Add policy-based authorization to your MCP servers built on FastMCP with one line of code.

[!NOTE] Eunomia is the official authorization middleware of FastMCP!

Overview

Features

• 🔒 Policy-Based Authorization: Control which agents can access which MCP tools, resources, and prompts
• 📊 Audit Logging: Track all authorization decisions and violations
• 🔄 Centralized Policy Enforcement: Optionally use a remote Eunomia server for centralized policy enforcement
• ⚡ FastMCP Integration: One-line middleware integration with FastMCP servers
• 🔧 Flexible Configuration: JSON-based policies for complex dynamic rules with CLI tooling

Architecture

The Eunomia middleware intercepts all MCP requests to your server and automatically maps MCP methods to authorization checks.

Listing Operations

The middleware behaves as a filter for listing operations (tools/list, resources/list, prompts/list), hiding to the client components that are not authorized by the defined policies.

sequenceDiagram
    participant MCPClient as MCP Client
    participant EunomiaMiddleware as Eunomia Middleware
    participant MCPServer as FastMCP Server
    participant EunomiaServer as Eunomia Server

    MCPClient->>EunomiaMiddleware: MCP Listing Request (e.g., tools/list)
    EunomiaMiddleware->>MCPServer: MCP Listing Request
    MCPServer-->>EunomiaMiddleware: MCP Listing Response
    EunomiaMiddleware->>EunomiaServer: Authorization Checks
    EunomiaServer->>EunomiaMiddleware: Authorization Decisions
    EunomiaMiddleware-->>MCPClient: Filtered MCP Listing Response

Execution Operations

The middleware behaves as a firewall for execution operations (tools/call, resources/read, prompts/get), blocking operations that are not authorized by the defined policies.

sequenceDiagram
    participant MCPClient as MCP Client
    participant EunomiaMiddleware as Eunomia Middleware
    participant MCPServer as FastMCP Server
    participant EunomiaServer as Eunomia Server

    MCPClient->>EunomiaMiddleware: MCP Execution Request (e.g., tools/call)
    EunomiaMiddleware->>EunomiaServer: Authorization Check
    EunomiaServer->>EunomiaMiddleware: Authorization Decision
    EunomiaMiddleware-->>MCPClient: MCP Unauthorized Error (if denied)
    EunomiaMiddleware->>MCPServer: MCP Execution Request (if allowed)
    MCPServer-->>EunomiaMiddleware: MCP Execution Response (if allowed)
    EunomiaMiddleware-->>MCPClient: MCP Execution Response (if allowed)

Installation

pip install eunomia-mcp

Quickstart

Create a MCP Server with Middleware

Basic Integration with embedded Eunomia server

from fastmcp import FastMCP
from eunomia_mcp import create_eunomia_middleware

# Create your FastMCP server
mcp = FastMCP("Secure MCP Server 🔒")

@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two numbers"""
    return a + b

# Add middleware to your server
middleware = create_eunomia_middleware(policy_file="mcp_policies.json")
mcp.add_middleware(middleware)

if __name__ == "__main__":
    mcp.run()

Advanced Integration with remote Eunomia server

Configure the middleware with custom options for production deployments:

from fastmcp import FastMCP
from eunomia_mcp import create_eunomia_middleware

mcp = FastMCP("Secure MCP Server 🔒")

# Configure middleware with custom options
middleware = create_eunomia_middleware(
    use_remote_eunomia=True,
    eunomia_endpoint="https://your-eunomia-server.com",
    enable_audit_logging=True,
)

mcp.add_middleware(middleware)

[!IMPORTANT]

You can use Eunomia as a centralized policy decision point for multiple MCP servers, which is especially relevant in enterprise scenarios.

You can run the Eunomia server in the background with Docker:

docker run -d -p 8421:8421 --name eunomia ttommitt/eunomia-server:latest

Or refer to Eunomia documentation for additional running options.

Policy Configuration

Use the eunomia-mcp CLI in your terminal to manage your MCP authorization policies:

Creating Your First Policy

# Create a default policy configuration file
eunomia-mcp init

# Create a custom policy configuration file from your FastMCP server instance
eunomia-mcp init --custom-mcp "app.server:mcp"

# Generate both policy configuration file and a sample FastMCP server with Eunomia authorization
eunomia-mcp init --sample

You can edit the created mcp_policies.json policy configuration file to your liking. Refer to the templates for example policies and rules.

Validating Policies

# Validate your policy file
eunomia-mcp validate mcp_policies.json

Deploying Policies

[!NOTE]

This operation is needed only if you are using a remote Eunomia server, otherwise you can skip this step.

# Push your policy to Eunomia server
eunomia-mcp push mcp_policies.json

# Push your policy and overwrite existing ones
eunomia-mcp push mcp_policies.json --overwrite

Further Reading

MCP Method Mappings

The middleware extracts contextual attributes from the MCP request and passes them to the decision engine; these attributes can therefore be referenced inside policies to define dynamic rules.

Agent Authentication

Default Identification

Agents are identified through HTTP headers:

X-Agent-ID: claude
X-User-ID: user123
User-Agent: Claude
Authorization: Bearer api-key-here

Custom Principal Extraction

You can customize principal extraction by subclassing the middleware:

from eunomia_core import schemas
from eunomia_mcp.middleware import EunomiaMcpMiddleware

class CustomAuthMiddleware(EunomiaMcpMiddleware):
    def _extract_principal(self) -> schemas.PrincipalCheck:
        # Custom logic to extract principal from JWT, etc.
        return schemas.PrincipalCheck(
            uri="user:john.doe",
            attributes={"role": "admin", "department": "engineering"}
        )

Logging

Enable comprehensive audit logging:

import logging

# Configure logging
logging.basicConfig(level=logging.INFO)
logger = logging.getLogger("eunomia_mcp")

# Authorization success
# INFO: Authorized request | MCP method: tools/call | MCP uri: mcp:tools:file_read | User-Agent: Claude

# Authorization violation
# WARNING: Authorization violation: Access denied for tools/call | MCP method: tools/call | MCP uri: mcp:tools:file_read | User-Agent: Claude

Examples

(Sample) Planetary Weather MCP

WhatsApp MCP to Authorized Contacts

Documentation

Explore the detailed documentation for more advanced configuration and scenarios.