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.

A sweeping cyber-espionage campaign organization centered on vulnerable versions of Microsoft Corp.’s RKUNY +1.44% server software has now claimed about 400 victims, according to researchers at Netherlands-based Eye Security.

Microsoft warned users that hackers used a security flaw in its server software to attack government agencies and businesses around the world. The federal government, along with those in Canada and Australia, has launched an investigation into the hack that targeted SharePoint servers, The Washington Post reported.