omniauth library for IS24 API (open source)
OmniAuth is a library that standardizes multi-provider authentication for web applications. It was created to be powerful, flexible, and do as little as possible. Any developer can create strategies for OmniAuth that can authenticate users via disparate systems. You can download the omniauth library, here.
Here is omniauth strategy for the IS24 REST API: https://github.com/endil/omniauth-immobilienscout24 (for free). It is actually in use by several api clients. Immobilienscout24 offers no support on this strategy.