VMware warned on Sunday that more of its source code for its ESX hypervisor technology could become public after another batch of code was released by a hacker.

The source code dates from 2004 and is related to other code released in April, wrote Iain Mulholland, VMware's director of platform security. He did not indicate what risk the current release poses to customers.

"It is possible that more related files will be posted in the future," Mulholland wrote. He recommended that VMware customers apply the most current product updates and patches while also review their "security hardening guidelines."

VMware's ESX hypervisor is virtualisation software that sits on top of a server's hardware, enabling multiple guest operating systems to run.

A hacker nicknamed "Stun" posted a link on Twitter to a torrent leading to the file, which had also been posted elsewhere.

In April, VMware said the source code leak did not necessarily pose an increased risk to customers. But the leak did show how companies face increasing difficulty in protecting some of their most valuable intellectual property.

At that time, the source code leak was accompanied by internal VMware emails on Pastebin from "Hardcore Charlie," a hacker who indicated the material came from China National Electronics Import and Export (CEIEC), an engineering and electronics company that also works with China's military. The code was part of thousands of documents that Hardcore Charlie claimed to have obtained from the company's servers.

It's unclear if Hardcore Charlie had a hand in the latest release. He hasn't posted on Twitter since 6 July.