AWS AppSync forwards application request headers to AWS Lambda custom authorizer functions

AWS AppSync is a fully managed service that enables developers to build digital experiences based on multiple data sources. With AppSync, you create GraphQL APIs that your applications interact with over the internet (public APIs) or inside your VPC (private APIs). A method of authorization is always required to access your AppSync API. Developers can chose from several authorization modes to authorize their requests based on their business requirements, including calling an AWS Lambda function to implement custom authorization. 

Source:: Amazon AWS