Gary Audin
Gary Audin is the President of Delphi, Inc. He has more than 40 years of computer, communications and security...
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Gary Audin | July 29, 2010 |


Packet Inspection for Security: The Challenges

Packet Inspection for Security: The Challenges What if you were told that someone can invade your data network, steal information and intellectual property and access resources by pretending to be a VoIP call?

What if you were told that someone can invade your data network, steal information and intellectual property and access resources by pretending to be a VoIP call?

What if you were told that someone can invade your data network, steal information and intellectual property and access resources by pretending to be a VoIP call? Would you even have guessed this would happen?

VoIP calls have two data streams, call signaling like the Session Initiation Protocol (SIP) and the voice stream using the Real Time Protocol (RTP). RTP is a standard protocol for carrying digital voice and digital video. The SIP stream occurs at the beginning and end of a call. The RTP stream operates continuously during the call. The RTP packets may arrive at a constant rate or sporadically if silence suppression is used. The digitized voice can be a standard like G.711, compressed and/or encrypted. The RTP stream content is difficult to analyze for security threats.

This new threat will only become more common as enterprises add VoIP calls that pass through their firewalls and session border controllers (SBC). VoIP traffic may be diverted around the firewall and SBC defenses because the packet inspection of the RTP streams degrades voice quality. Therefore the rigorous inspection that occurs for data and signaling streams is not imposed on the RTP steam. When the RTP stream is encrypted, it becomes even more difficult to perform packet inspection.

Attackers can spoof the firewall and SBC into determining that the RTP stream is safe to relay. Passing the attacks through the RTP stream is called Vunneling. The alternative is to inspect the RTP packets which can slow down the transmission and distorts the voice.

The US Defense Information Services Agency (DISA) recognizes Voice and Video over IP (VVoIP) as a means for data exfiltration in the DISA Voice and Video over IP (VVoIP) Security Technical Implementation Guide (STIG) Version 3 Release 1, released in December 23, 2009. The purpose of this document is to provide guidance for protecting the IP network and its resources. Some of the recommended actions will be difficult to implement. The document also presents the risks if the recommendations are not implemented. The document authors concluded that it takes much more effort to make an IP-based telephone system as reliable, available, secure and of as good a quality as the TDM systems being replaced.

The Illinois Institute of Technology (IIT) discovered the Vunneling problem and is the process of patenting packet inspection techniques to securely analyze the RTP stream. The RTP traffic is about 97% of the VoIP traffic according to Salare Security.The typical security technique for the RTP stream is to check the size of the packets. They should be of constant length. Salare Security has developed appliances to mitigate this problem.

The Salare solution , vPurity software, relies on a number of techniques to solve the Vunneling problem. Network Behavior Analysis (NBA) is employed by Salare. The passive NBA technique is well known for producing many false positive and false negative alerts. Salare's Active NBA virtually eliminates false positives. This is accomplished by introducing stimulus events and observing the reaction or non-reaction This provides accurate and precise recognition of the traffic types passing through the network.

The Salare technique inserts distortion in the packet that destroys embedded data and executable transmissions; this distortion is not perceptible by the listener. The insertion does not impact the quality of the voice conversation. Salare has several white papers on this subject at


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