The US 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 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.

A critical vulnerability in on-premise SharePoint servers allowed state-backed hackers to breach governments and institutions worldwide. Experts are questioning why more hasn't been done or said.

Hours after Microsoft revealed hacking groups affiliated with the Chinese government have been exploiting a flaw in its SharePoint software, Bloomberg News reports that the National Nuclear Security Administration has also been breached in the attacks.

A global attack on Microsoft server software used by thousands of government agencies and businesses to share documents within organisations is likely the work of a single actor, a cybersecurity researcher said on Monday.

Dozens of organizations appear to have been affected over the past few days by hackers targeting Microsoft server software. Microsoft said in a post on its website on Saturday that it was “aware of active attacks targeting on-premises SharePoint Server customers by exploiting vulnerabilities.