This vulnerability in Apache Solr 1.2 to 6.6.2 and 7.0.0 to 7.2.1 relates to an XML external entity expansion (XXE) in the `&dataConfig=` parameter of Solr's DataImportHandler. It can be used as XXE using file/ftp/http protocols in order to read arbitrary local files from the Solr server or the internal network.
