Back in mid-January 2021, representatives of Mimecast warned that an unknown hacker had one of its digital certificates. The attacker then abused it to gain access to some Microsoft 365 customer accounts.

The compromised certificate was used by several of the company’s products (Mimecast Sync and Recover, Continuity Monitor, and IEP) to connect to Microsoft’s infrastructure.

At the same time, it was reported that only 10% of customers used the above products [...]