When building REST APIs on AWS API Gateway, security is paramount. Protecting your endpoints ensures that only authorized users can access your resources. A custom Lambda authorizer lets you implement your authentication and authorization logic for API Gateway. This blog will guide you through creating a custom authorizer using static credentials (for testing) and Auth0 (for production).
A custom authorizer is a Lambda function that you attach to an API Gateway route. It validates incoming requests based on authentication tokens or headers and then generates a policy document to grant or deny access.
Custom authorizers are perfect when:
We’ll create a custom authorizer that:
1. Create and Write the Lambda Authorizer Function
Here’s the complete Lambda function:
import json
import os
import logging
import base64
import requests
import jwt
logger = logging.getLogger()
logger.setLevel(logging.INFO)
def lambda_handler(event, context):
try:
print('Event:', event)
# Extract Authorization header
auth = None
try:
auth = event['headers']['Authorization']
except KeyError:
raise Exception("Unauthorized: Authorization header missing")
# Extract username and password from the header
username, password = authorization_header(auth)
# Authenticate user (static for testing, Auth0 for production)
authenticated_data = authenticate_user(username, password)
if authenticated_data:
# Return a successful authorization response
return generate_authorizer_response(True, username)
else:
# Authentication failed
return generate_authorizer_response(False, username, "Invalid credentials")
except Exception as e:
logger.error(f"Error: {str(e)}")
return generate_authorizer_response(False, "Internal Server Error")
def authenticate_user(email, password):
"""
Temporarily authenticate using static credentials.
Replace with Auth0 logic for production.
"""
STATIC_EMAIL = "test@example.com"
STATIC_PASSWORD = "TestPassword123"
if email == STATIC_EMAIL and password == STATIC_PASSWORD:
return {
"nickname": STATIC_EMAIL.split('@')[0],
"email": STATIC_EMAIL
}
else:
print("Authentication failed: Invalid email or password")
return None
def authorization_header(auth_token):
"""
Extract and decode username/password from the Basic Authorization header.
"""
auth = auth_token.split(" ")
if not auth or auth[0].lower() != 'basic':
raise Exception("Unauthorized: Invalid header format")
try:
auth_decoded = base64.b64decode(auth[1]).decode('utf-8')
username, password = auth_decoded.split(':')
except Exception as e:
raise Exception("Unauthorized: Invalid credentials encoding")
return username, password
def generate_authorizer_response(is_authorized, username, additional_info=None):
"""
Generate IAM policy for the API Gateway.
"""
return {
'principalId': username,
'policyDocument': {
'Version': '2012-10-17',
'Statement': [
{
'Action': 'execute-api:Invoke',
'Effect': 'Allow' if is_authorized else 'Deny',
'Resource': '*' # Replace '*' with specific ARNs
}
]
},
'context': {
'additionalInfo': additional_info
}
}Create the Lambda Function:
Permissions:
Attach the AWSLambdaBasicExecutionRole policy to the Lambda execution role.
To connect the Lambda authorizer with your API:
Add a Resource:
Add a Method:
Go to the API Gateway Console:
Update the API Gateway Methods:
Restrict Policy Resources:
Secure Secrets:
Logging:
With a custom Lambda authorizer, you can implement flexible and secure authentication for your AWS API Gateway endpoints. Starting with static credentials and transitioning to Auth0, you can rapidly prototype and scale your solution without compromising security.
Next Steps: Integrate additional identity providers like Cognito, or explore multi-tenancy support in your APIs!
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