VoIP Threat Taxonomy

Chapter Description

This chapter categorizes the main threats against VoIP service (threats against availability, confidentiality, integrity, and social context) and explains their impact and possible methods of protection.

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Voice over IP Security

Voice over IP Security


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

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.


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 real-time 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

Example 2-4 shows examples with SIP INVITE and MESSAGE.

Example 2-4. IM Spam

INVITE sip:Bob1@ SIP/2.0
Via: SIP/2.0/UDP;branch=z9hG4bK00002000005
From: Spammer <sip:spammer1@>;tag=2345
To: Bob <sip:Bob1@>
Call-Id: 9252226543-0001
Subject: Hi there, buy a cool stuff in our website www.spam-example.com
Contact: <sip:spammer1@>
Expires: 1200
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 143

MESSAGE sip:Bob1@ SIP/2.0
Via: SIP/2.0/UDP;branch=z9hG4bK00002000005
From: Spammer <sip:spammer1@>;tag=2345
To: Bob <sip:Bob1@>
Call-Id: 9252226543-0001
Max-Forwards: 70
Content-Type: test/plain
Content-Length: 25

Hi there, buy a cool stuff in our website www.spam-example.com

SPIM is very much like email, but much more intrusive than email. In today's systems, IMs automatically pop up and present themselves to the user. Email, of course, must be deliberately selected and displayed.

Presence Spam (SPPP)

Presence spam is similar to SPIM. It is defined as a bulk unsolicited set of presence requests (for example, SIP SUBSCRIBE requests) in an attempt to get on the "buddy list" or "white list" of a user to subsequently send them IM or INVITEs. This is occasionally called SPam over Presence Protocol, or SPPP.

The cost of SPPP is within a small constant factor of IM spam, so the same cost estimates can be used here. What would be the effect of such spam? Most presence systems provide some kind of consent framework. A watcher that has not been granted permission to see the user's presence will not gain access to their presence. However, the presence request is usually noted and conveyed to the user, allowing them to approve or deny the request. This request itself can be spam, as shown in Example 2-5.

In SIP, this is done using the watcherinfo event package. This package allows a user to learn the identity of the watcher, in order to make an authorization decision. This could provide a vehicle for conveying information to a user; Example 2-5 shows the example with SIP SUBSCRIBE.

Example 2-5. Presence Spam

SUBSCRIBE sip:bob@example.com SIP/2.0
Event: presence
To: sip:bob@example.com
From: sip:buy-cool-dvds-and-games@spam-example.com
Contact: sip:buy-cool-dvds-and-games@spam-example.com
Call-ID: knsd08alas9dy@
Expires: 3600
Content-Length: 0

A spammer in Example 2-5 generates the SUBSCRIBE request from the identity (sip:buy-cool-dvds-and-games@spam-example.com), and this brief message can be conveyed to the user, even though the spammer does not have permission to access presence. As such, presence spam can be viewed as a form of IM spam, where the amount of content to be conveyed is limited. The limit is equal to the amount of information generated by the watcher that gets conveyed to the user through the permission system.


The general meaning of phishing is an illegal attempt to obtain somebody's personal information (for example, ID, password, bank account number, credit card information) by posing as a trust entity in the communication. In VoIP, phishing is typically happening through voice or IM communication, and voice phishing is sometimes called "vishing."

The typical sequence is that a phisher picks target users and creates request messages (for example, SIP INVITE) with spoofed identities, pretending to be a trusted party. When the target user accepts the call request, either voice or IM, the phisher provides fake information (for example, bank policy announcement) and asks for personal information. Some information like user name and password may not be directly valuable to the phisher, but it may be used to access more information useful in identity theft.

Here are a couple of phishing examples:

  1. A phisher makes a call to a target user and leaves a voice message like: "This is an important message from ABC Bank. Because our system has changed, you need to change your password. Please call back at this number: 1-800-123-4567." When the target user calls the number back, the phisher's Interactive Voice Response (IVR) system picks up the call and acquires the user's password by asking "Please enter your current password for validation purposes...."
  2. A phisher sends an instant text message to a smart phone (for example, PDA phone) or softphone (for example, Skype client) users, saying "This message is from ABC Bank. Your credit card rate has been increased. Please check it out on our website: http://www.abcbank.example.com." When the users click the URL, it goes to a phisher's website (example.com) that appears to have exactly the same web page that ABC Bank has. The fake website collects IDs and passwords that the users type in.

In this section, you have learned about VoIP threats in a social context, such as misrepresentation, call spamming, IM spamming, presence spamming and phishing. For more detailed information about VoIP spamming, refer to Chapter 6, "Analysis and Simulation of Current Threats."