ArcGIS Server contains an improper authentication vulnerability in an undocumented administrative endpoint. An unauthenticated attacker could exploit this issue by sending a crafted request to the endpoint. Successful exploitation may result in disruption of the web-based browsing interface. This issue affects ArcGIS Server 12.0 and earlier.
https://www.esri.com/arcgis-blog/products/trust-arcgis/administration/april2026_security_bulletin