How can custom Config rules built with Lambda functions evaluate complex, organization specific compliance requirements that AWS managed rules do not cover?

A custom Config rule invokes a Lambda function you write whenever a relevant resource changes, and that function contains your own logic to evaluate whatever specific business or security requirement you need to check, such as ensuring an EC2 instance's tags follow a specific naming convention or that a database's backup retention period meets an internally mandated minimum, giving you unlimited flexibility beyond what any predefined managed rule could offer.

def lambda_handler(event, context):
    configuration_item = event['configurationItem']
    is_compliant = configuration_item['tags'].get('CostCenter') is not None
    return {'compliance_type': 'COMPLIANT' if is_compliant else 'NON_COMPLIANT'}