Guide to VoIP security

New ways to hack VoIP aren't fatal if you're prepared for them

VoIP requires strict attention to security best practices

New exploits against VoIP continue to emerge, but experts say these demonstrations reveal the need for vigilant security and are not fatal flaws to the technology.

At Black Hat this month researchers released hacking tools against VoIP signaling protocols H.323 and AIX as well as tools to insert audio into VoIP calls. At Defcon, a tool that automatically probes the Session Initiation Protocol for vulnerabilities was released to enable the covert piggy-backing of data over VoIP streams.

The problem lies not in VoIP technology but in its implementation, says Barrie Dempster, a senior security consultant for Next Generation Security Software. "If you apply traditional network security logic to VoIP you can make it as secure as any other protocol," he says.

Much of the notoriety of VoIP vulnerabilities come because the technology is relatively new and its code wasn't necessarily written with security in mind - a problem that plagues many new technologies.

Dempster cites ways to exploit Asterisk, the open source PBX, including buffer overflows. He says this and other weaknesses can be dealt with by removing the code for unused features and performing security audits on the features that are used. "The problem is not the specific vulnerabilities themselves. It's the maturity of the software. There hasn't been enough security review yet," he says.

The problem is well recognized, and known exploits are publicized to help develop defenses against them. For example, the industry group VoIP Security Alliance publishes a set of hacking tools on its site that it promotes as security tools to test that VoIP gear can withstand real-world attacks.

Securing VoIP is not insurmountable, says Peter Thermos, CTO of security consulting firm Palindrome Technologies. He revealed vulnerabilities to media gateway control protocol (MGCP) that enable rerouting calls or cutting them off. He also showed a vulnerability to ZRTP, a pending-standard, encrypted VoIP protocol that didn't encrypt the sounds of tones made by pressing phone buttons. That potentially left credit card numbers being entered over VoIP lines open to being picked off, he says.

The MGCP problem will ultimately require a change to the protocol itself, but in the meantime users can shore it up by blocking unauthorized access to the ports MGCP uses, Thermos says. The ZRTP problem involved the implementation of the protocol and has been addressed with a patch.

The best route for businesses implementing VoIP is to set individual security requirements ahead of time, which differ among companies, he says. A financial institution or government agency may need confidentiality and therefore more encryption than other businesses, he says.

"The common mistake I see is that customers don't define their security requirements for their particular network, realize later that they need security, then perceive it as an additional cost," Thermos says. Getting security tools in place from the outset also better defends VoIP against threats not yet discovered, he says.

Despite the genuine possibilities of attack, some experts say that VoIP is more secure than the traditional public switched telephone network (PSTN).

"The VoIP system is much more secure than traditional systems," says Ari Takanen, founder and CTO of Codenomicon, which makes software security-testing tools. Speaking at the recent VON Europe 2007 conference, he acknowledged VoIP vulnerabilities, but said they were not insurmountable. "IP systems are more exposed, but you have more security that you can install," he says. "If you don't use it - that's stupid."

Cullen Jennings, a distinguished engineer at Cisco's VoIP group, who also spoke at the conference, notes that PSTN caller ID is easily spoofed, and toll fraud via traditional PBXs is still common.

Jennings says PSTN reliability - the availability of dial tone nearly all the time - is one highly touted measure of the quality of service. But that does not mean the PSTN is invulnerable or even better than VoIP. "I'm not claiming the PSTN does not meet its [reliability] goals," he says, but that has no bearing on whether, for example, caller ID can be spoofed. "If the core network went down doesn't matter if the threat was to caller ID," he says.

The top threats to VoIP listed by the VON panel were:

  • Zero day problems for which vendors have not yet issued a fix.
  • Security not being turned on because it is too complex.
  • Vendor-specific vulnerabilities that are not addressed by best practices.

Ultimately businesses will not turn their backs on VoIP because they are worried about security, says Akif Arsoy, a VoIP product manager for Verisign. They will adopt it for integrated voice and data in a converged network. "End users make decisions on what am I getting [with VoIP] that I'm not getting today [with traditional voice]?" Arsoy says.

Even so, expect more VoIP exploits to emerge over the near-term, says Thermos, who says he has already identified more signaling protocol weaknesses and implementation vulnerabilities. "We're just touching on the beginnings of many exploits that will be coming down the road," he says.

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Network World staff

Network World
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