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Skype: Its dangers and how to protect against them

Skype may be free for end users but it could be costing your enterprise its security. This tip outlines the free VoIP solution's security risks and offers tips for keeping Skype off of the network.

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Unless you've been hiding under a technology rock for the past two years, you've undoubtedly heard about Skype – the free voice over IP (VoIP) telephony solution that's spreading around the Internet like wildfire. With the promise of free calling, Skype's gathered an international following of over 100 million users.

However, while Skype might be a fun toy for home use, it may represent a definite threat to your enterprise security. Users attempting to make personal calls on company time or save a few bucks on their personal telephone bill might unwittingly jeopardize your data. Let's take a look at a few of the more alarming features of Skype:

  • Skype is a closed-source VoIP solution. Skype does not disclose the cryptographic protocols used by its software. This could lead to "security through obscurity" concerns. Indeed, a white paper by Simson Garfinkel pointed this out in March 2005. Skype reacted quickly by commissioning Anagram Laboratories to conduct an evaluation of the VoIP system. The evaluation, published a month later, granted Skype a clean bill of health. That said, I'd be much happier if Skype either opened to public scrutiny the source code for their security features or relied upon an accepted cryptographic package.

  • Some Skype traffic may take place in the clear. Garfinkel's analysis identified unencrypted communications related to call setup that may make it possible for an eavesdropper to perform traffic analysis (i.e. determine who calls whom and for how long). Anagram's evaluation doesn't refute this finding.

    More information on VoIP security

    Learn the ins and outs of VoIP security, from implementing VoIP standards and protocols to defending against VoIP threats, with our Learning Guide.


  • Skype traffic bypasses audit controls. By their nature, VoIP calls placed on the Skype network evade local call auditing systems. If you operate in a regulated environment, this may pose an unacceptable risk or require the use of specialized controls designed specifically to audit Skype traffic.

  • Skype's EULA grants Skype the use of the system on which it is installed. Article 4 of the Skype end user license agreement states, "You hereby acknowledge that the Skype Software may utilize the processor and bandwidth of the computer (or other applicable device) You are utilizing, for the limited purpose of facilitating the communication between Skype Software users." Enough said.

Blocking Skype

If you decide that Skype poses a threat to your organization, you may choose to block it entirely. However, that's not an easy task with modern security technology. You're probably thinking that you can just block the Skype destination port or, if that fails, simply block all access to the Skype server IP addresses. Unfortunately, these techniques are completely ineffective. Skype is capable of tunneling over a standard HTTPS connection, preventing port filters from distinguishing it from other port 443 traffic. Additionally, Skype servers (a.k.a. supernodes) are moving targets – there simply isn't a master list that you can block outright.

There are solutions for those willing to invest a little time and money. First, some firewall manufacturers now include explicit code that's able to detect and filter Skype communications. For example, SonicWall security appliances introduced Skype detection/filtering capabilities with version of SonicOS. In addition, Packeteer's Packet Shaping appliances are able to detect and either throttle or completely block Skype communications.

If these solutions don't fit your budget, you'll need to get more creative. For example, you can implement strict egress filtering on your network to block all unspecified outbound traffic, but that's likely to be an extremely awkward solution on all but the most well-defined networks. Another possibility is using your existing intrusion prevention system (IPS) to block Skype traffic. Skype activity signatures are floating around the Internet for a variety of IDS/IPSes.

Perhaps the easiest way to control Skype usage is to strictly manage desktops in your enterprise through the use of Windows Group Policy or similar controls. If users are unable to install the Skype client, you won't need to spend as much time worrying about how to block Skype traffic on your network.

About the author
Mike Chapple, CISSP is an IT Security Professional with the University of Notre Dame. He previously served as an information security researcher with the National Security Agency and the U.S. Air Force. Mike is a frequent contributor to SearchSecurity, a technical editor for
Information Security magazine and the author of several information security titles including the CISSP Prep Guide and Information Security Illuminated.

This was last published in May 2006

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