External Validation of Customer User/Password

Can anyone tell me is there is an API Endpoint to validate a customer account with their email/pass combination.

After a customer purchases a Subscription and they have to enter a email/pass for the eCommerce parts I want to then call in to validate from an external location.  In this case a Firebase Function.

eg I have a login in my App that accepts a user/pass combination and I validate that in a number of ways.

Secondary to that can I then get details of the Subscription purchased somehow?