VMware has confirmed that source code for its ESX hypervisor has leaked online.
The code was obtained by hackers who allegedly broke into a Chinese defence contractor where they also stole internal emails and documents.
VMware, which shares source code with select industry partners, said it did not belive the 300Mb leak would place customers at risk because the code related to software released around 2004 that had since been updated.
"The fact that the source code may have been publicly shared does not necessarily mean that there is any increased risk to VMware customers," security response centre director Iain Mulholland said in a statement.
"VMware proactively shares its source code and interfaces with other industry participants to enable the broad virtualisation ecosystem today.
"We take customer security seriously and have engaged internal and external resources, including our VMware security response centre, to thoroughly investigate."
VMware could not rule out a breach of its own source code repository.
However the hacker, known as Hardcore Charlie, claimed to have obtained the code by hacking defence contractor China National Electronics Import-Export Corporation (CEIEC) earlier this month, an allegation the company tersely refuted.
"The information reported is totally groundless, highly subjective and defamatory," it said in statement.
Hardcore Charlie published a series of documents which he describes in a rambling pastebin entry to show alleged collusion between CITEC and Western military and terrorist organisations.
Similar documents stolen by comrade hacker YamaTough which sought to bring disrepute on the Indian Government were found to be falsified.
Hardcore Charlie promised to release more data breach data on May 5.