Because of the recent vulnerabilities found in Solr a new version of the 5.x branch has been released – version 5.5.5. If you are using Solr 5.x and for some reason, you can’t update to 6.x or 7.x branch this is the version you should upgrade to.
Apache Lucene library 5.5.5 can be downloaded from http://www.apache.org/dyn/closer.cgi/lucene/java. If you want to download Solr search server 5.5.5 go to http://www.apache.org/dyn/closer.cgi/lucene/solr.