When using the Index Replication feature, Apache Solr nodes can pull index files from a master/leader node using an HTTP API which accepts a file name. However, Solr before 5.5.4 and 6.x before 6.4.1 did not validate the file name, hence it was possible to craft a special request involving path traversal, leaving any file readable to the Solr server process exposed. Solr servers protected and restricted by firewall rules and/or authentication would not be at risk since only trusted clients and users would gain direct HTTP access.
https://www.debian.org/security/2018/dsa-4124
https://access.redhat.com/errata/RHSA-2018:1451
https://access.redhat.com/errata/RHSA-2018:1450
https://access.redhat.com/errata/RHSA-2018:1449