Microsoft issued an emergency fix to close off a vulnerability in its SharePoint software that hackers have exploited to carry out widespread attacks on businesses and at least some federal agencies.

Bloomberg reported that no sensitive or classified information is known to have been compromised in the attack on the National Nuclear Security Administration, the agency responsible for maintaining and designing the nation's cache of nuclear weapons.

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.