Voice-over-IP apps could be used to cloak networks of zombies, used to launch denial of service attacks, a Cambridge professor has warned.

Armies of ordinary PCs - "botnets" - that have been infected by a virus and put under malicious control, could be controlled and orchestrated by messages hidden in VoIP traffic generated by programs such as Skype, warned Jon Crowcroft, Marconi professor of communications systems at Cambridge University.

Denial-of-service (DoS) attacks are usually shut down by tracing control messages, normally sent by chat and IM programs. "If someone were to use a VoIP overlay as a control tool for attacks, it would be much harder to find affected computers and almost impossible to trace the criminals behind the operation," said Crowcroft, who revealed the technique at the Communications Research Network (CRN), a networking think-tank funded by the Cambridge-MIT Institute, a joint venture between the two universities.

The threat is dealt with in more detail here.

"It would be irresponsible to build something that could go out and be used," said Crowcroft, but he nevertheless built a demonstration system. "It was write-once, tear-up code. But it was very easy to do - unfortunately."

Although the attack has not been detected in actual use yet, Crowcroft warns it is only a matter of time. The CRN's working group on Internet Security has raised the issue with VoIP providers, before making the issue public.

"There isn't a protocol you can't use as a covert signalling channel," responded Kurt Sauer, director of security operations at Skype. "Some large commercial groupware products have encrypted XML streams - they may not be quite as good at firewall traversal, but that's still an opaque data stream."

The attack will add to the unease enterprise IT staff already feel about applications, particularly the very popular Skype service. Some IT managers do not want uncontrolled traffic punching holes in their firewalls, and using bandwidth, and security vendors have launched specific products to block Skype.

Crowcroft would like Skype to publish its routing specifications, so IT managers can work better with the application, tracking it and checking its behaviour. "Skype's routing specification is proprietary," he said. "There are a whole bunch of reasons why obfuscation is not helpful in the long run."

Although Skype still wants its proprietary edge, the issue is up for discussion: "The people who own networks and systems have a right to manage as they see fit," said Sauer. "To the extent that we make it difficult to do that, we want to address that in our products."