Do you operate an Exchange Server? Is it patched and up to date?

There is an actively exploited zero-day flaw being reported, that Microsoft has not yet patched – so have a look at GTSC’s blog post:

Warning: New attack campaign utilized a new 0-day RCE vulnerability on Microsoft Exchange Server | GTSC (gteltsc.vn)

The post also includes some temporary mitigation whilst waiting for the Microsoft patch.