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 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.

A cyber-espionage campaign centered on vulnerable versions of Microsoft's server software now involves the deployment of ransomware, Microsoft said in a late Wednesday blog post.