CVE-2019-17558

Apache Solr 5.0.0 to Apache Solr 8.3.1 are vulnerable to a Remote Code
Execution through the VelocityResponseWriter. A Velocity template can be
provided through Velocity templates in a configset velocity/ directory or
as a parameter. A user defined configset could contain renderable,
potentially malicious, templates. Parameter provided templates are disabled
by default, but can be enabled by setting params.resource.loader.enabled
by defining a response writer with that setting set to true. Defining a
response writer requires configuration API access. Solr 8.4 removed the
params resource loader entirely, and only enables the configset-provided
template rendering when the configset is trusted (has been uploaded by an
authenticated user).