Voice over IP Security: VoIP Threat Taxonomy

In an excerpt from Voice over IP Security, author Patrick Park reviews VoIP threats, including spam, phishing and other threats against social context.

The following is an excerpt from the book Voice Over IP Security. In this section of Chapter 2: VoIP Threat Taxonomy (.pdf), author Patrick Park reviews spam calls, phishing tactics and other Voice over IP attacks that threaten social context and often misrepresent identity and authority. 

Threats against social context
A threat against social context (as known as "social threat") is somewhat different from other technical threats against availability, confidentiality, or integrity, as previously discussed, in terms of the intention and methodology. It focuses on how to manipulate the social context between communication parties so that an attacker can misrepresent himself as a trusted entity and convey false information to the target user (victim). The typical threats against social context are as follows:


  • Misrepresentation of identity, authority, rights, and content
  • Spam of call (voice), IM, and presence
  • Phishing

NOTE A call with misrepresentation is initiated by an attacker who is a communication entity, which is different from the threats in the "Threats Against Integrity" section, which are based on interception and then modification.

The general meaning of spam is unsolicited bulk email that you may see every day. It wastes network bandwidth and system resources, as well as annoying email users. The spam exists in VoIP space as well, so-called VoIP spam, in the form of voice, IM, and presence spam. This section looks into each type of VoIP spam with SIP protocol. The content refers to RFC 5039.1

Phishing is becoming popular in the VoIP world these days as a method of getting somebody's personal information by deceiving the identity of an attacker.

The following sections give more details about these social threats.

NOTE These same types of attacks are equally available in today's PSTN environment.

Misrepresentation is the intentional presentation of a false identity, authority, rights, or content as if it were true so that the target user (victim) or system may be deceived by the false information. These misrepresentations are common elements of a multistage attack, such as phishing.

Identity misrepresentation is the typical threat that an attacker presents his identity with false information, such as false caller name, number, domain, organization, email address,or presence information.

Authority or rights misrepresentation is the method of presenting false information to an authentication system to obtain the access permit, or bypassing an authentication system by inserting the appearance of authentication when there was none. It includes presentation of password, key, certificate, and so on. The consequence of this threat could be improper access to toll calls, toll calling features, call logs, configuration files, presence information of others, and so on.

Content misrepresentation is the method of presenting false content as if it came from a trusted source of origin. It includes false impersonation of voice, video, text, or image of a caller.

Call spam (SPIT)
Call (or voice) spam is defined as a bulk unsolicited set of session initiation attempts (for example, INVITE requests), attempting to establish a voice or video communications session. If the user should answer, the spammer proceeds to relay their message over realtime media. This is the classic telemarketer spam, applied to VoIP, such as SIP. This is often called SPam over IP Telephony, or SPIT.

The main reason SPIT is becoming popular is that it is cost-effective for spammers. As you know, legacy PSTN-call spam already exists in the form of telemarketer calls. Although these calls are annoying, they do not arrive in the same kind of volume as email spam. The difference is cost; it costs more for the spammer to make a phone call than it does to send email. This cost manifests itself in terms of the cost for systems that can perform telemarketer calls, and in cost per call. However, the cost is dramatically dropped when switching to SPIT for many reasons: low hardware cost, low line cost, ease of writing a spam application, no boundary for international calls, and so on. Additionally, in some countries, such telemarketing calls over the PSTN are regulated.

In some cases, spammers utilize computational and bandwidth resources provided by others, by infecting their machines with viruses that turn them into "zombies" that can be used to generate call spam.

Another reason SPIT is getting popular is its effectiveness, compared to email spams. For email spams, you may already realize that there is a big difference between turning on and off a spam filter for your email account. In fact, most spam filters for email today work very well (filter more than 90 percent of spams) because of the nature of email; store and forward. All emails can be stored and examined in one place before forwarding to users. Even though users may still receive a small percentage of email spams, they usually look at profiles (for example, sender name and subject) and delete most of them without seeing the contents. However, the method of filtering emails does not work for SPIT because voice is real-time media. Only after listening to some information initially can users recognize whether it is a spam or not. So, spammers try to put main information in the initial announcement so that users may listen to it before hanging up the phone. There is a way to block those call attempts based on a blacklist (spammers' IP address or caller ID), but it is useless if spammers spoof the source information.

You can find more information on SPIT and mitigation methods in Chapter 6, "Analysis and Simulation of Current Threats."

The next topic is a different type of VoIP spam, IM spam.

IM spam (SPIM)
IM spam is similar to email. It is defined as a bulk unsolicited set of instant messages, whose content contains the message that the spammer is seeking to convey. This is often called Spam over Instant Messaging, or SPIM.

SPIM is usually sent in the form of request messages that cause content to automatically appear on the user's display. The typical request messages in SIP are as follows:

  • SIP MESSAGE request (most common)
  • INVITE request with large Subject headers (since the Subject is sometimes rendered to the user)
  • INVITE request with text or HTML bodies



1 RFC 5039, "SIP and Spam," J. Rosenberg, C. Jennings, http://www.ietf.org/ rfc/rfc5039.txt, January 2008. 

Reproduced from the book Voice Over IP Security Copyright [2009], Addison Wesley Professional. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other users.

This was last published in December 2008

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