In Apache Solr versions 5.0.0 to 5.5.5 and 6.0.0 to 6.6.5, the Config API allows to configure the JMX server via an HTTP POST request. By pointing it to a malicious RMI server, an attacker could take advantage of Solr's unsafe deserialization to trigger remote code execution on the Solr side.
https://www.oracle.com/technetwork/security-advisory/cpujul2019-5072835.html
https://www.oracle.com/security-alerts/cpuoct2020.html
https://security.netapp.com/advisory/ntap-20190327-0003/
https://access.redhat.com/errata/RHSA-2019:2413