A new batch of stolen source code for VMware ESX, the company's enterprise-level virtualization product, has been posted to the Internet.

The code release was announced Sunday on Twitter by the hacker "Stun," who also posted a link to a 1.89-MB Torrent download file for what's labeled as "VMware ESX Server Kernel."

"VMware will try to make like this Kernel is old and isn't used in its recent products. But thanks god, there is still such as thing as reverse engineering that will prove it's true destiny," read the accompanying notes from Stun. In addition, he noted that the kernel dates from between 1998 and 2004. "But as we all know, kernels don't change that much in programs, they get extended or adapted but some core functionality still stays the same," he said.

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VMware Sunday confirmed that the published VMware ESX source code is genuine, and did highlight that the code dates from 2004.

This is the second time this year that stolen VMware ESX source code has been published on the Internet. The first such release occurred on April 23, when hacker "Hardcore Charlie" published a "sneak peak" of 300 MB of VMware ESX source code that he claimed to have obtained from China Electronics Import & Export Corporation (CEIEC), a defense contractor. At the time, CEIEC denied the hacker's claim, saying that "the information reported is totally groundless, highly subjective, and defamatory." It promised to take legal action against whoever had been responsible for the claims.

Interestingly, whoever released the VMware source code then might somehow have been involved in the latest release, despite the change of hacker handle. "This source code is related to the source code posted publicly on April 23, 2012," said Iain Mulholland, VMware's director of platform security, in a security bulletin, though he offered no additional details.

In addition, this might not be the last source code to be leaked. "It is possible that more related files will be posted in the future," Mulholland said. "We take customer security seriously and have engaged our VMware Security Response Center to thoroughly investigate."

He also urged all VMware ESX users to ensure that they've installed the latest patches, which would protect them against any publicly known vulnerabilities in the code. "By applying the combination of the most current product updates and the relevant security patches, we believe our customer environments will be best protected," he said.

But could the breach still leave VMware users unprotected? It's difficult to answer that question, and depends on whether attackers find new vulnerabilities in the published source code. In January, for example, when hacker Yama Tough released source code for Symantec's pcAnywhere -- after having failed to extort $50,000 from the antivirus company -- security experts warned that attackers reviewing the source code might discover unknown vulnerabilities that could be exploited via zero-day attacks.

Still, after Hacker Charlie -- who said he'd collaborated with Yama Tough -- released the first trove of VMware ESX source code, multiple security experts declined to speculate on whether the code release might put users at risk. Given that ESX serves as a guest operating system for virtualized environments, however, any working exploit could allow an attacker to launch an "escape to hypervisor" attack and gain access to any other virtual machine running on the same server.

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