How to access http headers in custom authorizer AWS lambda function

This is now possible by using an Authoriser of type 'Request' instead of Token

Full details are here: https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-use-lambda-authorizer.html

Fundamentally, all headers are passed in the event object for a Request authorisation

ie headers object on event

    "headers": {
        "X-wibble": "111",
        "X-wobble": "222",
        "x-amzn-ssl-client-hello": "*Deleted*",
        "Via": "1.1 .cloudfront.net (CloudFront)",
        "CloudFront-Is-Desktop-Viewer": "true",
        "CloudFront-Is-SmartTV-Viewer": "false",
        "CloudFront-Forwarded-Proto": "https",
        "X-Forwarded-For": "*Deleted*",
        "CloudFront-Viewer-Country": "GB",
        "Accept": "*/*",
        "User-Agent": "curl/7.55.1",
        "X-Amzn-Trace-Id": "Root=*Deleted*",
        "Host": "*Deleted*.execute-api.eu-west-1.amazonaws.com",
        "X-Forwarded-Proto": "https",
        "X-Amz-Cf-Id": "*Deleted*",
        "CloudFront-Is-Tablet-Viewer": "false",
        "X-Forwarded-Port": "443",
        "CloudFront-Is-Mobile-Viewer": "false"
    }

Here is a SAM template:

ApiGatewayApi:
    Type: AWS::Serverless::Api
    Properties:
      StageName: Prod
      Auth:
        Authorizers:
          MyAuthorizer:
            FunctionPayloadType: REQUEST
            FunctionArn: !GetAtt AuthLambda.Arn
            Identity:
              Headers:
                - X-API-KEY
                - X-API-ID