Custom User Authentication through SOAP Call from Cognito

You can use a Pre authentication Lambda trigger. Now within your Lambda function you can make a call to your SOAP service (provided it has connectivity and has access) and receive the response (authenticate). For Pre authentication response parameters, Amazon Cognito does not expect any additional return information in the response. Your function can return an error to reject the sign-in attempt, or use API operations to query and modify your resources. Once authenticate Cognito can return response to your UI.