The number of companies and organizations compromised by a security vulnerability in Microsoft Corp.’s SharePoint servers is increasing rapidly, with the tally of victims soaring more than six-fold in a few days,

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 issued an alert on Saturday that said the server software being targeted is used by government agencies and businesses to share documents within their organizations.