Researchers say the “PhishPoint” tactic has already impacted 10 percent of Office 365 users globally.

A severe vulnerability in the way Microsoft Office 365 handles federated identities via SAML put an attacker in position to have access to any account and data, including email messages and files stored in the cloud-based service. Microsoft pushed through a mitigation to the service on Jan. 5, seven hours after being notified by researchers...