Microsoft warned of "active attacks" targeting some users of its SharePoint software over the weekend and released an emergency patch that fixes only some versions of the vulnerable server software. Why it matters: It remains unclear how many businesses were already hacked prior to Microsoft's warning,

Microsoft said the company has been "coordinating closely with CISA, DOD Cyber Defense Command, and key cybersecurity partners around the world throughout our response."

The term "zero-day" attack refers to when a previously unknown vulnerability is targeted. Tens of thousands of servers are said to be at risk. While the issue is serious, it differs from several previous vulnerabilities related to Microsoft. The attack only affects on-premises servers; cloud-based servers are unaffected.