Enumerating Users and Mailboxes in Microsoft Outlook 365 Web
During our research into Microsoft 365 security, we discovered a flaw in Outlook on the web (OWA) that exposed information about users and their mailboxes. By manipulating certain request headers against the “/owa/service.svc” endpoint, an attacker could not only confirm whether a user account existed, but also determine if that account had a mailbox associated with it.