Select2 is an auto-complete widget. In multiple places where CiviCRM uses Select2, it was vulnerable to stored cross-site scripting (XSS) attack.
(We believe that exploiting this requires that both the attacker and the victim have a high-level of access to the same CiviCRM deployment.)
CiviCRM version 5.64.3 and earlier
CiviCRM version 5.64.4, 5.65.0 and 5.63.4 (ESR)
Upgrade to the fixed version of CiviCRM
Jean-Marie Heitz of Uepal.
Coleman Watts of CiviCRM.
Seamus Lee of JMA Consulting/CiviCRM.