Alf.io is a free and open source event attendance management system. An administrator on the alf.io application is able to upload HTML files that trigger JavaScript payloads. As such, an attacker gaining administrative access to the alf.io application may be able to persist access by planting an XSS payload. This issue has been addressed in version 2.0-M4-2402. Users are advised to upgrade. There are no known workarounds for this vulnerability.
https://github.com/alfio-event/alf.io/security/advisories/GHSA-gpmg-8f92-37cf