If you have the Microsoft Teams app installed on a mobile device, you may have recently received several odd push messages. These push messages say some variation of “FCM Messages: test notification.”
This appears to be an in-the-wild exploit of the Firebase Cloud Messaging (FCM) system, which was first exploited with Google Hangouts a few days before. It is now being used to send push notifications to Microsoft Teams, and it seems likely other apps could follow.
At this point, the investigation into this issue, its impact, and a fix is still ongoing. It is not known if there is potential harm by a malicious actor. We advise that you dismiss any notifications along the lines of “FCM Messages: test notifications”. Do not tap or click on these notifications.
More info on this issue can be found here: https://code2care.org/q/fcm-messages-test-notification-microsoft-teams-google-hangouts-push-alert-firebase