Crowd 2.1 and later.
Note that these might be out of date. For the current versions of Crowd see https://docs.atlassian.com/atlassian-crowd/latest/REST/
The Crowd REST APIs allow you to address the Crowd data entities as 'resources'. This means that the data entities are identified by URIs and operated on by HTTP requests (GET, POST, PUT and DELETE). Whenever you GET one of these resources, you receive a representation encoded in XML or JSON. Below are details of the resources made available by the APIs.