VOIP: Don't overlook security

Corporations that are implementing voice over IP (VOIP) technologies in a bid to cut communications costs shouldn't overlook the security risks that can crop up when the voice and data worlds converge, users and analysts say.

Most users implementing VOIP these days are primarily concerned about voice quality, latency and interoperability. All are fundamental quality-of-service considerations that companies need to deal with before they can even begin justifying the move to VOIP.

But some security organizations are cautioning users about the dangers of unsecured VOIP services. For instance, in an August 2001 paper on its Web site, the Bethesda, Md.-based SANS Institute warned of privacy- and authentication-related issues stemming from VOIP services and urged users to apply the same precautions they've used to protect their data services.

"With the convergence of the voice and data worlds, the real similarities of the security concerns will become apparent," the SANS report said, urging users to take measures such as encrypting voice services, building redundancy into their VOIP networks, locking down their VOIP servers and performing regular security audits.

Without a sharp focus on security as well, VOIP will never make it into corporate use, say users and analysts.

With VOIP, voice traffic is carried over a packet-switched data network via Internet Protocol. VOIP networks treat voice as another form of data but use sophisticated voice-compression algorithms to ensure optimal bandwidth utilization. As a result, VOIP networks are able to carry many more voice calls than traditional switched circuit networks. VOIP also enables enhanced services such as unified communications.

Voice as Data

Securing voice traffic on such networks isn't very different from securing any data traffic on an IP network, says David Krauthamer, director of IT at Advanced Fibre Communications Inc. (AFC), a Petaluma, Calif.-based manufacturer of telecommunications equipment. AFC is using limited VOIP communications internally and may use it for external communications as well.

"VOIP security needs to be handled in the overall context of data security," Krauthamer says.

But there are some aspects of VOIP networks that users need to pay close attention to, says Christopher Kemmerer, an analyst at NexTiraOne Inc., an integrator of voice and data networks in Houston.

In a VOIP world, private branch exchanges (PBX) are replaced by server-based IP PBXs running on Microsoft Corp.'s Windows NT or a vendor's proprietary operating system. Such call management boxes, which are used both for serving up VOIP services and for logging call information, are susceptible to virus attacks and hackers. Break-ins of these servers could result in the loss or compromise of potentially sensitive data, Kemmerer says.

Consequently, it's important that such equipment is properly locked down, placed behind firewalls, patched against vulnerabilities and frequently monitored using intrusion-detection systems, he says.

The University of Houston went one step further and made sure that its call manager and its entire VOIP network aren't directly accessible from the Internet. The school has put its IP PBXs in a different domain than its other servers and has limited administration access to the servers.

"As a university, the potential for being hacked or coming under a denial-of-service attack is a huge concern for us," says Charles Chambers, the university's manager of network planning and development.

Trouble at the Gateways

VOIP gateway technologies are also a potential weak point. When VOIP is used externally, gateway technologies convert data packets from the IP network into voice before sending them over a public switched telephone network. When VOIP is used internally, the gateways basically route packetized voice data between the source and the destination.

The concern here is that such gateways can be hacked into by malicious attackers in order to make free telephone calls, Chambers says. The trick to protecting against this lies in having strict access-control lists and making sure the gateway is configured in such a fashion that only the people on this list are permitted to make and receive VOIP calls, he says.

"We are mainly trying to make sure that the scope and access is restricted to a minimal number of people so that our exposure [to threats] is reduced," Chambers says.

As with traditional telephony, eavesdropping is a concern for organizations using VOIP—and the consequences can be greater, says Charlie Rabie, a vice president at Aspect Communications Corp. in San Jose. Aspect is a provider of software and services for implementing VOIP, traditional telephony and other communication services.

Because voice travels in packets over the data network, hackers can use data-sniffing and other hacking tools to identify, modify, store and play back voice traffic traversing the network, Kemmerer says.

A hacker breaking into a VOIP data stream has access to a lot more calls than he would with traditional telephone tapping. As a result, "one of the big differences is that a hacker has a much higher probability of getting intelligent information" from tapping a VOIP data stream than from monitoring traditional phone systems, Rabie says.

Encryption Helps

Separating and isolating voice traffic onto a virtual LAN is one way of mitigating that risk, Kemmerer says.

So is encrypting VOIP traffic and running it over a virtual private network (VPN) when dealing with external communications, Krauthamer says. Some of AFC's salespeople and remote workers use VOIP to communicate with the head office. All of this communication takes place under the security of a VPN using multiple encryption layers, Krauthamer says.

It might be a good idea to encrypt VOIP traffic flowing internally over a corporate network to prevent insider attacks, Rabie notes.

The use of desktop-based soft phones to make and receive VOIP-based telephone calls can also result in dangerous holes being punched into the corporate firewall that hackers could exploit, warns Kemmerer.

Once again, the best way to address this problem is to restrict use through access lists and to ensure that all inbound VOIP traffic that flows through a corporate firewall is routed through a gateway server to eliminate a direct connection to the Internet, he says.

VOIP security is a challenge that is "inextricably linked" with issues such as interoperability with data networks and quality of service, says Rabie.

But ultimately, it's important to remember that securing a VOIP infrastructure involves nothing that is "drastically different" from the measures corporations have always taken to protect their data, Kemmerer says.

"Security issues relating to VOIP have only begun to surface over the last one year," he says. "But this has to be a major consideration. Chances are, you are unlikely to get hacked. But once you do, you'll never forget it."

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Tips for Securing VOIP Traffic

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Encrypt VOIP traffic and run it over a VPN.

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Make sure you've properly configured your firewalls. Check to see if your networking and security vendors have support for Session Initiation Protocol and the International Telecommunication Union's H.323 voice protocol.

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Consider segmenting voice and data traffic by using a virtual LAN. This will limit the threat posed by packet-sniffing tools and minimize disruption in the event of an attack.

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Think about using proxy servers in front of corporate firewalls to process incoming and outgoing voice data.

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Make sure that server-based IP PBXs are locked down and protected against viruses and denial-of-service attacks.

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VOIP Vulnerabilities
VOIP Vulnerabilities

Copyright © 2002 IDG Communications, Inc.

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