Virtualisation is one of the hottest topics in the industry today, but the technology is not without security risks, as a new set of vulnerabilities in VMware's virtual machine software shows.

VMware has patched the bugs, but users who have not updated their software could face serious security risks thanks to a trio of flaws in the DHCP server that ships with VMware.

The DHCP component assigns IP addresses to the different virtual machines running within VMware, but IBM researchers discovered that it can be exploited to gain control of the computer.

That could be very bad news for someone running a lot of applications on the same VMware box, said Tom Cross, a researcher with IBM's Internet Security Systems group. "By exploiting this vulnerability you get complete control of any of the machines that are running on that virtual environment," he said.

IBM's researchers have developed exploit code for three separate flaws in the DHCP software, all of which are now patched, Cross said.

In order to attack a system, however, an attacker would first need to gain access to software running within the virtual machine. Typically VMware's DHCP server is not configured to be accessible to systems on other machines.

Enterprises are increasingly looking at virtualisation as a way to cut down on datacentre costs.

VMware lets a single computer act as if it is a kind of mini data-centre, running a number of separate virtual machines on the same box. These virtual machines act as if they are truly separate from one another. They can run different operating systems, and if one virtual machine crashes, it does not affect the other virtual machines on the server.

VMware is also extremely popular with security researchers, who set up virtual machines on their PCs to test potentially malicious code without putting their computers at risk.

Unfortunately, this architecture also gives attackers a single point of failure: the VMware software itself.

"This is important because servers often run a vulnerable machine in one VM and have super-secret information in another VM, isolated by VMware," said Dave Aitel, CTO with security vendor Immunity, in an email interview. "VMware ESX has been getting massively popular among hosting environments, so this sort of bug becomes a force multiplier if you can find a remote vulnerability in a [virtual machine]."

The DCHP flaws affect VMware's ACE, Player, Server, and Workstation products running on Linux and the Windows operating system, IBM said.

VMware, a division of EMC, has also patched a fourth serious flaw in its software, discovered by McAfee. This one could also be used to run unauthorised code on a VMware machine, but it would be difficult for an attacker to exploit, said David Marcus, security research and communications manager with McAfee's Avert Labs.

"The attacker has to pass a whole bunch of parameters to the vulnerable service," he said "You have to be engaging in some behaviour inside that VMware machine to get the [exploit code] to work. So it's certainly not easy to exploit."

Still, Marcus agreed with Cross that virtual machines will get more attention from security researchers in the near future. "If you have the ability to attack that virtual machine and get outside that virtual machine shell to the host OS, then you can gain control of every virtual machine that's on the box."