graylog2-server vulnerable to instantiation of arbitrary classes triggered by API request

Graylog is a free and open log management platform. Starting in version 2.0.0 and prior to versions 5.1.11 and 5.2.4, arbitrary classes can be loaded and instantiated using a HTTP PUT request to the `/api/system/cluster_config/` endpoint. Graylog's cluster config system uses fully qualified class names as config keys. To validate the existence of the requested class before using them, Graylog loads the class using the class loader. If a user with the appropriate permissions performs the request, arbitrary classes with 1-arg String constructors can be instantiated. This will execute arbitrary code that is run during class instantiation. In the specific use case of `java.io.File`, the behavior of the internal web-server stack will lead to information exposure by including the entire file content in the response to the REST request. Versions 5.1.11 and 5.2.4 contain a fix for this issue.