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BANKINFO SECURITY
Voice Over IP
Security
March 18, 2004
By Matthew
Tanase
Introduction
As information technology
professionals, we are constantly bombarded with new products and ideas claiming
to be revolutionary. And for a brief amount of time, a few of these technologies
seem to grab all the headlines, in trade magazines, on tech sites and at
industry conferences. The technology darling of late is VoIP. Short for Voice
over IP, you've likely come across it, probably considered it and possibly
deployed it. The hype, deservedly so, is reaching a crescendo as excess
bandwidth, improved software and better hardware drive this technology forward.
For the right situations, it's a truly wonderful solution. Lower phone bills,
virtual offices, centralized management and rapid deployment are just a few of
the benefits. And the success of companies such as Vonage and Skype, combined
with the growth of wireless networks will move this technology from enterprises
to smaller businesses and the SOHO market. Unfortunately, new technologies bring
new security concerns. Suddenly, you have the burden of protecting two
infrastructures - voice and data. This article will introduce voice over IP
concepts and the new security concerns they raise.
Is it Better?
Before jumping into the security factors of a voice over IP network,
let's examine the rationale behind it. The traditional telephone network, known
as POTS (plain old telephone service) or the PSTN (public switched telephone
network), served us well for many years. Unfortunately, it was costly, managed
by only a few companies and inefficient. Each voice call over POTS uses a unique
connection, allotted 64K of bandwidth. We're all familiar with a T1 connection,
which has 24 channels and 1.5M of bandwidth (64K *24). In reality, a quality
voice call on one of these channels requires a fraction of the 64K granted to it
by POTS. Moreover, a silent moment, or lapses in speech still consume the 64K.
VoIP deployments capitalize on the inefficiency of this design. The analog voice
signal is digitized, compressed, chunked into packets and sent over a data
network. Advanced compression algorithms reduce the bandwidth necessary for a
quality voice call to a fraction of the 64K required by POTS. The silence and
background noise transmission of POTS can be eliminated as well (although some
deployments build this feature in to comfort end users!). As if the bandwidth
savings weren't enough, VoIP deployments also reduce cost and enhance
scalability by employing standard data networking components (routers, network
switches), instead of expensive, complicated telephone switches. Now the same
team handling the data network can manage a voice network - great news for all
of you overworked IT staffers.
How Does It Work?
The
process of VoIP is dependent on signaling and media transport. A signaling
protocol, such as SIP (session initiation protocol), performs the legwork:
locating users, call parameters, modifications and building or ending a session.
Media transport protocols, like RTP (real time transport protocol) handle the
voice portion of a call: digitizing, encoding and ordering. Networking
protocols, such as IP, are wrapped around the VoIP packets when they are
transmitted to the proper servers.
VoIP calls can take place between
LANs or on WANs, as is the case with internal calls on a corporate network. If a
VoIP user wishes to call a destination on POTS, a special gateway is used. These
devices act as connectors between the data network and the SS7 network used by
POTS. They translate the incoming data into a format the recipient, be it IP or
SS7, can understand.
Onto Security
With an understanding
of VoIP technology and its benefits in place, let's consider the security
ramifications. In the process of saving money and increasing efficiency, two
crucial portions of any infrastructure, voice and data, were combined. Suddenly,
an IT staff is not responsible for securing only standard servers (database,
mail, web), workstations and routers. As if these data security concerns weren't
enough, VoIP servers acting as gateways, special routers, phones, new protocols
and operating systems are now thrown into the mix. The burden of voice and
telecommunications security has been shifted from the carrier to the IT team. It
has moved from an obscure PSTN, to an IP network every cracker is familiar with.
Let's examine the risks and how you can mitigate them.
What Are The
Threats?
Unfortunately, there are numerous threats to a VoIP
network, many of which aren't obvious to newcomers. The networking devices, the
servers and their operating systems, the protocols, the phones and their
software are all vulnerable.
Information about a call is almost as
valuable as the voice content. For instance, a compromised signaling server used
to setup and manage calls, might yield the following: a list of incoming and out
going calls, their durations and parameters. Using just this information, an
attacker could map all of the calls on your network, creating complex
conversation records and user tracking.
The conversation itself is also
at risk and the most obvious target of a VoIP network. By breaching a key part
of the infrastructure, such as a VoIP gateway, an attacker could capture and
reassemble packets in order to eavesdrop on the conversation. Or even more
nefariously, record everything, and replay all conversations occurring on your
network. On the PSTN, this would be an impressive feat, since few are skilled
enough on or have access to the huge switches managing calls. That's obviously
not the case on a data network, as legions of script kiddies prove every day.
And if your VoIP packets traverse the Internet to reach a destination, a number
of attackers have a shot at your voice data.
The calls are also
vulnerable to hijacking or a man in the middle attack. In such a scenario, an
attacker would intercept a connection and modify call parameters. This is an
especially scary attack, since the participants likely wouldn't notice a change.
The ramifications include spoofing or identity theft and call redirection,
making data integrity a major risk.
The availability of the VoIP network
is also a major concern. On the PSTN, availability is rarely a problem.
Attackers would need to overload some very large circuits or cut a connection.
It's much easier to thwart a VoIP network. All of us are familiar with the
crippling effects of distributed denial of service attacks. If directed at key
points of your network, it would disrupt your ability to communicate via voice
or data.
The phones and servers are targets themselves. Although as a
whole they mimic phones, they are, at the core, computers with software.
Obviously, this software is vulnerable to the same types of bugs and exploits
that hamper every operating system and application available today. Code could
be inserted to perform any number of malicious actions.
VoIP Security
And Defense
How anticlimactic. I point out the wonders of voice over
IP and follow them up with major security problems! Fortunately, the situation
is not without remedy. The risks outlined above, while specific to VoIP, are all
issues we deal with on regular IP networks. Unfortunately, in the initial
rollouts and designs of voice related hardware, software and protocols, security
was not a major concern. But that's usually the case with every new technology,
a fact that we are all working to change. Let's examine some of the tried and
true workarounds that can alleviate the threats outlined above.
The
first thing that should come to mind reading about VoIP is encryption. While
it's not easy to capture, reassemble and decode voice packets, it can definitely
be done. Encryption is the only way to prevent such an attack. Unfortunately, it
adds overhead, eating up or eliminating altogether the bandwidth reductions from
traditional voice calls. This, in turn, affects throughput and performance -
which can introduce dreaded jitter into the call. So what can you do? There are
multiple encryption options - VPN setups, the IPSec protocol and other protocols
such as SRTP (secure RTP- though it does not offer any authentication features
like VPNs, it does encrypt voice packet payloads). The key however, is to choose
a fast, efficient encryption algorithm and employ a dedicated encryption
processor. This should alleviate any performance concerns. Another option would
be strict QoS standards for VoIP packets on your network and powerful hardware.
Such QoS requirements will ensure that voice is always handled in a timely
manner, reducing the chance of degraded quality.
Next, as should be
expected, would be the process of securing all elements of a VoIP network.
You'll be dealing with call servers, routers, switches (you're not still using a
hub that can be sniffed!), workstations and phones. You need to perform regular
assessments on each of these devices to ensure they are in line with your
security demands. The servers should have minimal jobs running and only the
necessary ports open. The routers and switches should be configured properly,
with access control lists and filters in place. All of the devices should be up
to date in terms of patches and upgrades. These are the same types of
precautions you would take when adding new elements to your existing data
network, just extend the process to the VoIP portion as well.
As
mentioned, the availability of your VoIP network is a concern as well. Unlike
POTS, a power loss will bring your network down -- so make sure prolonged
redundancy options are in place. DdoS attacks are always difficult to defend
against. Aside from proper router configurations, make sure you have an
escalation process in place with your IP carrier, since many times they are
needed to assist. Remember, such an attack would not only halt your data
services, but voice as well.
Lastly, you can employ a firewall and an
IDS to help protect your voice network. A VoIP firewall is a difficult beast to
manage due to the ever-changing requirements. Call servers are constantly
opening and closing ports for new connections. This dynamic element makes rule
management difficult, even on stateful devices. But the costs are far outweighed
by the benefits, so spend some time perfecting your access controls. An IDS can
assist in monitoring the network for any anomalies or potential abuses. Early
warnings are key to preventing larger attacks.
Conclusion
As evidenced by the explosive growth of the voice over IP market, this
technology will work its way into your businesses and networks. It carries with
it however, the new burden of voice security. Careful planning and architecture,
borrowed from our data security experiences, can help mitigate the risks and
amplify the returns.
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