Pi-hole's Web interface (based on AdminLTE) provides a central location to manage one's Pi-hole and review the statistics generated by FTLDNS. Prior to version 5.8, cross-site scripting is possible when adding a client via the groups-clients management page. This issue was patched in version 5.8.
Pi-hole is a Linux network-level advertisement and Internet tracker blocking application. The Stored XSS exists in the Pi-hole Admin portal, which can be exploited by the malicious actor with the network access to DNS server. See the referenced GitHub security advisory for patch details.