Network Policy Server (NPS) in Microsoft Windows Server 2008 SP2 and R2 SP1 and Server 2012 Gold and R2 misparses username queries, which allows remote attackers to cause a denial of service (RADIUS authentication outage) via crafted requests, aka "Network Policy Server RADIUS Implementation Denial of Service Vulnerability."
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2016/ms16-021