WASHINGTON (Reuters) -A cyber-espionage campaign centered on vulnerable versions of Microsoft’s SharePoint server hackers software now involves the deployment of ransomware, Microsoft said in a late Wednesday blog post.

In the post, citing “expanded analysis and threat intelligence,” Microsoft said a group it dubs “Storm-2603” is using the vulnerability to seed the ransomware, which typically works by paralyzing victims’ networks until a digital currency payment is made.

The disclosure marks a potential escalation in the campaign, which has already hit at least 400 victims, according to Netherlands-based cybersecurity firm Eye Security. Unlike typical state-backed hacker campaigns, which are aimed at stealing data, ransomware can cause widespread disruption depending on where it lands.

The figure of 400 victims represents a sharp rise from the 100 organizations cataloged over the weekend. Eye Security says the figure is likely an undercount.

SharePoint server hackers by Microsoft

“There are many more, because not all attack vectors have left artifacts that we could scan for,” said Vaisha Bernard, the chief hacker for Eye Security, which was among the first organizations to flag the breaches.

The details of most of the victim organizations have not yet been fully disclosed, but on Wednesday a representative for the National Institutes of Health confirmed that one of the organization’s servers had been compromised.

“Additional servers were isolated as a precaution,” he said. The news of the compromise was first reported by the Washington Post.

The spy campaign kicked off after Microsoft failed to fully patch a security hole in its SharePoint server software, kicking off a scramble to fix the vulnerability when it was discovered. Microsoft and its tech rival, Google owner Alphabet, have both said Chinese hackers are among those taking advantage of the flaw. Beijing has denied the claim.

(Reporting by Raphael Satter; Editing by Mark Porter and Christopher Cushing)