U.S. National Nuclear Security Administration was among those breached by a hack of Microsoft's SharePoint document management software, Bloomberg News reported on Tuesday, citing a person with knowledge of the matter.

The tech giants have evidence that Chinese hackers are exploiting the new bug, but warned "multiple actors" are also hacking into affected SharePoint systems.

Microsoft has issued an alert about “active attacks” on server software used by government agencies and businesses to share documents within organizations, and recommended security updates that customers should apply immediately.

Hackers have been exploiting a vulnerability to attack SharePoint and connected Microsoft services in what will be a big problem for corporate Mac users this week.

The hackers behind the initial wave of attacks exploiting a zero-day in Microsoft SharePoint servers have so far primarily targeted government organizations, according to researchers and news reports.

The U.S. agency responsible for maintaining and designing the nation’s cache of nuclear weapons was among those breached by a hack of Microsoft Corp.’s SharePoint document management software, according to a person with knowledge of the matter.

Microsoft’s first patch for a major SharePoint bug failed, leading to a cyber espionage spree affecting up to 9,000 servers, with China-linked actors named among the groups exploiting it.