Microsoft is investigating whether a leak from its early alert system for cybersecurity companies allowed Chinese hackers to exploit flaws in its SharePoint service before they were patched, Bloomberg News reported on Friday.

Microsoft has released security patches for the zero-day vulnerability chain dubbed ToolShell, capable of remote code execution on SharePoint, resulting in the exploitation of at least 54 organizations worldwide.

Microsoft contained a major SharePoint security flaw, amid fresh questions about the future of its legacy on-premises software.

New estimates regarding the recently-exploited Microsoft SharePoint vulnerabilities now evaluate that as many as 400 organizations may have been targeted.

One of the hacked organizations reportedly includes the U.S. agency responsible for maintaining the country's stockpile of nuclear weapons. China-backed hackers have been observed carrying out the hacks targeting SharePoint servers.

The zero-day vulnerability — which was first disclosed late Saturday — has been exploited by several Chinese state-aligned groups, according to Microsoft.

Microsoft confirms Chinese hackers exploited a SharePoint flaw; Patches now available. Cloud-based Microsoft 365 not affected.

Victims of the recent global hacking campaign include the National Institutes of Health and the National Nuclear Security Administration, officials said.

The name was coined by Dinh Ho Anh, a researcher from Khoa of Viettel Cyber Security, who developed the exploit. The researcher said he picked the name because it exploited ToolPane.aspx, a component for assembling the side panel view in the SharePoint user interface.