The threats against VoIP are numerous and seem to be growing, but in 2008 the technology probably won't suffer crippling attacks, analysts have said.

The potential danger is very real. VoIP is susceptible to the many exploits that networks generally are heir to - denial of service, buffer overflows and more. VoIP PBXs are servers on corporate networks and are only as secure as the networks themselves.

In addition, there are many voice-specific attacks and threats. These have been chronicled by researchers and vendors intending to alert users and suggest ways to guard against them.

For instance, two protocols widely used in VoIP - H.323 and Inter Asterisk eXchange - have been shown to be vulnerable to sniffing during authentication, which can reveal passwords that later can be used to compromise the voice network.

Implementations of Session Initiation Protocol (SIP), an alternative VoIP protocol, can leave VoIP networks open to unauthorised transport of data.

In addition, tools that can help find vulnerable deployments have been published online by a VoIPSA, an industry group dedicated to securing VoIP. The VoIPSA tools are intended to help businesses test and secure their networks, but these and other online tools can be used to probe for weaknesses as well.

Still, there have been few exploits so far and none that have been widespread or crippling to businesses. "We are not hearing about attacks. We don't think they are happening," says Lawrence Orans, an analyst with Gartner.

Part of the reason may be that the largest VoIP vendors use proprietary protocols, such as Cisco's Skinny, Nortel's Unistim and Avaya's variant of H.323, Orans says. That makes them difficult to obtain and study for potential security cracks. "These systems are not readily available to the bad guys," he says.

SIP, which is gaining popularity, is a mixed bag, Orans says, because it is readily available to those who might want to exploit it. "I would say that SIP is a good-news, bad-news story. It's easy to get your hands on, and that includes the bad guys. The good news is there are more options to protect SIP," he says. These options include firewalls and intrusion-prevention systems that support SIP.

Another reason for the lack of broad exploits is that there isn't enough ROI for attackers' development time. Attackers' motivation may improve, however, as VoIP increases in popularity, something it is doing relentlessly.

Hybrid PBX systems - which handle both VoIP and TDM voice - account for 64 percent of all PBX lines sold, according to a December 2007 Infonetics report. Pure IP systems account for another 18 percent.