Earlier today thousands of Microsoft Teams users reported receiving mysterious “FCM Messages – Test Notification” on their devices. This is similar to the notifications sent to Google Hangouts users just a couple of days back.

Microsoft has acknowledged the issue and Microsoft 365 Status confirmed that they are investigating the cause of the notifications.

The issue could be related to the Firebase exploit which was reported last week. Microsoft hasn’t confirmed if Firebase exploit was behind the notifications so it’s hard to say at the moment. Microsoft has noted that more information surrounding the issue will be available to Office 365 users under the SI TM221041 in the admin center.