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AgentCore Policy-Based Access Control

Demonstrates Cedar policy-based access control for MCP tools through AgentCore Gateway.

Overview

chat-agent (JWT) → Gateway → Policy Engine → MCP Runtime
                      ↓
              Cedar policy checks
              user's username tag

Key Finding: Use unless for Denying Tools

AgentCore rejects standalone forbid policies as "Overly Restrictive".

Solution: Use permit ... unless to deny specific tools:

permit(
  principal is AgentCore::OAuthUser,
  action,
  resource == AgentCore::Gateway::"..."
) when {
  principal.hasTag("username") &&
  principal.getTag("username") == "alice"
} unless {
  action == AgentCore::Action::"travel-mcp___searchFlights"
};

Setup Steps

cd /Users/shakirin/Projects/agentcore/samples/policy/scripts/policy
source .venv/bin/activate

# 1. Create policy engine (one-time)
python 01-create-policy-engine.py

# 2. Create policy (update ENGINE_ID in script first)
python 02-create-policy.py

# 3. Attach to gateway (update IDs in script first)
python 03-attach-policy-engine.py

Test Results

Tool Policy Result
searchHotels Permitted ✅ Returns hotel data
searchFlights Denied via unless ❌ Tool not available

JWT Token Requirements

The policy uses username tag from Cognito user tokens:

{
  "username": "alice",
  "client_id": "...",
  "token_use": "access"
}

Get user token:

TOKEN=$(aws cognito-idp initiate-auth \
  --client-id $CLIENT_ID \
  --auth-flow USER_PASSWORD_AUTH \
  --auth-parameters "USERNAME=alice,PASSWORD=$PASSWORD,SECRET_HASH=$SECRET_HASH" \
  --region us-east-1 \
  --query 'AuthenticationResult.AccessToken' --output text)

Files

  • policy.cedar - Working Cedar policy with unless clause
  • policy_commands.py - Helper functions for policy management
  • 01-create-policy-engine.py - Create policy engine
  • 02-create-policy.py - Create/update policy
  • 03-attach-policy-engine.py - Attach engine to gateway