My guess is oauth. He basically used fake Microsoft account to access services during the hack and them used his actual microsoft account to access something else. He used VPN for the hack but he connected from his actual IP to the second service. The only thing linking those was the GID. So they figured out his actual IP address and checked other logins from this IP around the same time and found out his other accounts.
My guess is oauth. He basically used fake Microsoft account to access services during the hack and them used his actual microsoft account to access something else. He used VPN for the hack but he connected from his actual IP to the second service. The only thing linking those was the GID. So they figured out his actual IP address and checked other logins from this IP around the same time and found out his other accounts.