Pi-hole's Web interface provides a central location to manage a Pi-hole instance and review performance statistics. Prior to Pi-hole Web interface version 5.5.1, the function to add domains to blocklists or allowlists is vulnerable to a stored cross-site-scripting vulnerability. User input added as a wildcard domain to a blocklist or allowlist is unfiltered in the web interface. Since the payload is stored permanently as a wildcard domain, this is a persistent XSS vulnerability. A remote attacker can therefore attack administrative user accounts through client-side attacks. Pi-hole Web Interface version 5.5.1 contains a patch for this vulnerability.