Yes, the current MyBB 1.x series doesn't have any public API to any o the core functionality at the minute. There have been a few attempts by people to create plugins to provide such functionality in the past, but none have been completed or kept up to date as far as I know. It's certainly something we plan to provide in the future.
Probably the best approach would be to write a plugin that provides logins (doing all of the internal processing in PHP) and returns any required information back to your client application. There is no plugin to do this at the minute as far as I'm aware, so you'd either have to write one yourself or hire somebody to write one unfortunately.
Probably the best approach would be to write a plugin that provides logins (doing all of the internal processing in PHP) and returns any required information back to your client application. There is no plugin to do this at the minute as far as I'm aware, so you'd either have to write one yourself or hire somebody to write one unfortunately.