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VoIP security

A look at some of the security threats to VoIP and where to look for help.

Security has never been one of the overriding concerns of setting up VoIP in an organization. Reliability and interoperability have always been of much greater concern to administrators. Usually when technology is new, getting it up and running is the main goal, with security coming in as a secondary or even tertiary goal. Well, VoIP has been around long enough, it is time to start thinking about security.

But what threats are exclusive to VoIP networks? According to Cullen Jennings, PhD. of Cisco Systems the threats are not so different from other Internet-based threats. In a talk he gave at the 2004 RSA Conference in San Francisco, Jennings outlined many of the possible threats that administrators should be aware of. Among them:

  • Toll fraud
  • Impersonation
  • Hijacking of calls
  • Learning private information
  • Eavesdropping
  • Session Replay
  • Faking identities
  • Media tampering
  • Denial of Service
  • Spam

Complications of securing VoIP lay with its communication protocol, SIP. SIP, or Session Initiation Protocol, is an IETF standard for initiating a user session that involves multimedia elements, voice being one of them. Since SIP deals with different mid-points and end-points, all with different trust relationships, establishing a secure transmission can be tricky. Not to mention the fact that your voice traffic has to travel through firewalls, over different transport protocols (TCP, UDP, TLS, SCTP, DCCP) and be highly reliable.

The solution involves breaking the voice transmission down into segments and securing each in turn by integrating identity authentication and encryption into SIP. The best way to understand how to implement VoIP security is to take a look at how it's done. Open source VoIP security is available at At you will find an open source stack with documentation and links to developer mailing lists.

Benjamin Vigil is a technical editor at

This was last published in March 2004

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