Description
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.
Remediation
References
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
https://access.redhat.com/errata/RHSA-2018:1448
https://access.redhat.com/errata/RHSA-2018:1447
https://lists.apache.org/thread.html/a6a33a186f293f9f9aecf3bd39c76252bfc49a79de4321dd2a53b488%40%3Csolr-user.lucene.apache.org%3E