Voice over IP Security: Security Best Practices Derived from Deep Analysis of the Latest VoIP Network Threats


Voice over IP Security

Security best practices derived from deep analysis of the latest VoIP network threats

Patrick Park

VoIP security issues are becoming increasingly serious because voice networks and services cannot be protected from recent intelligent attacks and fraud by traditional systems such as firewalls and NAT alone. After analyzing threats and recent patterns of attacks and fraud, consideration ...

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Voice over IP Security: Security Best Practices Derived from Deep Analysis of the Latest VoIP Network Threats

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Voice over IP Security

Security best practices derived from deep analysis of the latest VoIP network threats

Patrick Park

VoIP security issues are becoming increasingly serious because voice networks and services cannot be protected from recent intelligent attacks and fraud by traditional systems such as firewalls and NAT alone. After analyzing threats and recent patterns of attacks and fraud, consideration needs to be given to the redesign of secure VoIP architectures with advanced protocols and intelligent products, such as Session Border Controller (SBC). Another type of security issue is how to implement lawful interception within complicated service architectures according to government requirements.

Voice over IP Security focuses on the analysis of current and future threats, the evaluation of security products, the methodologies of protection, and best practices for architecture design and service deployment. This book not only covers technology concepts and issues, but also provides detailed design solutions featuring current products and protocols so that you can deploy a secure VoIP service in the real world with confidence.

Voice over IP Security gives you everything you need to understand the latest security threats and design solutions to protect your VoIP network from fraud and security incidents.

Patrick Park has been working on product design, network architecture design, testing, and consulting for more than 10 years. Currently Patrick works for Cisco® as a VoIP test engineer focusing on security and interoperability testing of rich media collaboration gateways. Before Patrick joined Cisco, he worked for Covad Communications as a VoIP security engineer focusing on the design and deployment of secure network architectures and lawful interception (CALEA). Patrick graduated from the Pusan National University in South Korea, where he majored in computer engineering.

Understand the current and emerging threats to VoIP networks

Learn about the security profiles of VoIP protocols, including SIP, H.323, and MGCP

Evaluate well-known cryptographic algorithms such as DES, 3DES, AES, RAS, digital signature (DSA), and hash function (MD5, SHA, HMAC)

Analyze and simulate threats with negative testing tools

Secure VoIP services with SIP and other supplementary protocols

Eliminate security issues on the VoIP network border by deploying an SBC

Configure enterprise devices, including firewalls, Cisco Unified Communications Manager, Cisco Unified Communications Manager Express, IP phones, and multilayer switches to secure VoIP network traffic

Implement lawful interception into VoIP service environments

This IP communications book is part of the Cisco Press® Networking Technology Series. IP communications titles from Cisco Press help networking professionals understand voice and IP telephony technologies, plan and design converged

networks, and implement network

solutions for increased productivity.

Category: Networking—IP Communication

Covers: VoIP Security

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Product Details

Meet the Author

Patrick Park has been working on product design, network architecture design, testing, and consulting for more than 10 years. Currently, Patrick works for Cisco as a VoIP test engineer focusing on the security and interoperability testing of rich media collaboration gateways. Before Patrick joined Cisco, he worked for Covad Communications (a VoIP service provider) as a VoIP security engineer focusing on the design and deployment of secure network architecture and lawful interception (under the Communications Assistance for Law Enforcement Act [CALEA]) with various tools and solutions. Patrick graduated from Pusan National University in South Korea, where he majored in computer engineering. While attending graduate school, he wrote the book Web Server Programming with PHP. Patrick lives with his wife and children in Los Gatos, California.

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Table of Contents


Part I: VoIP Security Fundamentals 3

Chapter 1: Working with VoIP 5

VoIP Benefits 6

VoIP Disadvantages 8

Sources of Vulnerability 10

IP-Based Network Infrastructure 10

Open or Public Networks 11

Open VoIP Protocol 11

Exposed Interface 11

Real-Time Communications 11

Mobility 11

Lack of Security Features and Devices 11

Voice and Data Integration 12

Vulnerable Components 12

Myths Versus Reality 14

Legacy Versus VoIP Systems 14

Protecting Networks Using Strict Authentication and Encryption 14

Protecting Networks Using a Data Security Infrastructure 15

Summary 15

End Notes 16

References 16

Chapter 2: VoIP Threat Taxonomy 19

Threats Against Availability 20

Call Flooding 20

Malformed Messages (Protocol Fuzzing) 22

Spoofed Messages 24

Call Teardown 25

Toll Fraud 26

Call Hijacking 26

Registration Hijacking 27

Media Session Hijacking 27

Server Impersonating 28

QoS Abuse 29

Threats Against Confidentiality 30

Eavesdropping Media 30

Call Pattern Tracking 32

Data Mining 33

Reconstruction 34

Threats Against Integrity 34

Message Alteration 35

Call Rerouting 35

Call Black Holing 36

Media Alteration 37

Media Injection 37

Media Degrading 38

Threats Against Social Context 38

Misrepresentation 39

Call Spam (SPIT) 39

IM Spam (SPIM) 40

Presence Spam (SPPP) 41

Phishing 42

Summary 43

End Notes 44

References 44

Chapter 3: Security Profiles in VoIP Protocols 47

H.323 48

Overview 48

Components 49

Basic Call Flow 50

Security Profiles 52

H.235 Annex D (Baseline Security) 54

H.235 Annex E (Signature Security) 55

H.235 Annex F (Hybrid Security) 56

SIP 57

Overview 58

Components 58

Basic Call Flow 60

Session Setup Example 61

Security Profiles 67

Digest Authentication 68

Identity Authentication 69

Secure/Multipurpose Internet Mail Extensions (S/MIME) 70

Secure RTP 71

TLS 71

IPSec 73


Overview 74

Basic Call Flow 75

Security Profiles 75

Summary 78

End Notes 79

References 80

Chapter 4: Cryptography 83

Symmetric (Private) Key Cryptography 84

DES 85

3DES 87

AES 89

SubBytes 89

ShiftRows 90

MixColumns 91

AddRoundKey 92

Asymmetric (Public) Key Cryptography 92

RSA 93

Digital Signature 95

Hashing 96

Hash Function (MD5) 97

SHA 98

Message Authentication Code 99

MAC Versus Digital Signature 100

Key Management 100

Key Distribution 101

Summary 103

End Notes 104

References 104

Chapter 5: VoIP Network Elements 107

Security Devices 108

VoIP-Aware Firewall 108

NAT 109

Session Border Controller 113

Lawful Interception Server 114

Service Devices 116

Customer Premise Equipment 116

Call Processing Servers 117

PAP Versus CHAP 119


Summary 120

End Notes 121

References 122

Part II: VoIP Security Best Practices 125

Chapter 6: Analysis and Simulation of Current Threats 127

Denial of Service 128

Intentional Flooding 129

Simulation 129

Analysis 135

Mitigation 137

Unintentional Flooding 138

Analysis 139

Mitigation 141

Malformed Messages 143

Simulation 144

Analysis 150

Mitigation 154

Sniffing/Eavesdropping 154

Simulation 154

Analysis 158

Mitigation 161

Spoofing/Identity Theft 162

Simulation 162

Prespoofing Scan 162

Identity Theft 163

Analysis 164

Mitigation 165

VoIP Spam 165

Voice Spam 165

IM Spam 167

Presence Spam 167

Mitigation 168

Content Filtering 168

Turing Test 168

Reputation System 169

Address Obfuscation 170

Limited-Use Address 171

Consent-Based Black/White List 171

Summary 172

End Notes 173

References 173

Chapter 7: Protection with VoIP Protocol 175

Authentication 175

User-to-Proxy Authentication 176

User-to-User Authentication 179

Encryption 182

Message Encryption (S/MIME) 183

S/MIME Certificates 184

S/MIME Key Exchange 185

Formatting S/MIME Bodies 186

Media Encryption 188

Key Derivation 188

SRTP Packet Processing 190

SRTP Test 191

Transport and Network Layer Security 193

Transport Layer Security 194

IPSec (Tunneling) 195

Threat Model and Prevention 195

Registration Hijacking 195

Impersonating a Server 196

Tearing Down Sessions 196

Denial-of-Service and Amplification 197

Limitations 198

Digest Authentication Limitations 198

S/MIME Limitations 198

TLS Limitations 199

SIPS URI Limitations 199

Summary 200

End Notes 200

References 201

Chapter 8: Protection with Session Border Controller 203

Border Issues 204

Between Access and Core Networks 206

Between Core and Peer Networks 207

Access and Peer SBCs 208

SBC Functionality 208

Network Topology Hiding 208

Example of Topology Hiding 209

DoS Protection 213

Policy-Driven Access Control 213

Hardware Architecture 215

Overload Prevention 216

Registration Timer Control 217

Ping Control 220

Load Balancing 220

NAT Traversal 222

Lawful Interception 224

Other Functions 226

Protocol Conversion 226

Transcoding 226

Number Translation 227

QoS Marking 228

Service Architecture Design 228

High Availability 229

Active-Standby 230

Active-Active 231

Network Connectivity 232

Service Policy Analysis 234

Virtualization 237

Optimization of Traffic Flow 239

Deployment Location 239

Media Control 240

Summary 245

End Notes 246

References 246

Chapter 9: Protection with Enterprise Network Devices 249

Firewall 249

ASA and PIX Firewalls 251

Routed Mode 251

Transparent Mode 252

TLS Proxy Feature 253

Configuration Example 254

FWSM Firewall 256

Routed Mode 256

Transparent Mode 256

Configuration Example 257

Limitations 258

Unified Communications Manager Express 259

Access Control 259

Phone Registration Control 261

Secure GUI Management 263

Class of Restriction 264

After-Hours Call Blocking 266

Unified Communications Manager 267

Security Features and Certificates 267

Integrity and Authentication 269

Image Authentication 270

Device Authentication 270

File Authentication 270

Signaling Authentication 271

Digest Authentication 271

Authorization 272

Encryption 273

Signaling Encryption 273

Media Encryption 274

Configuration File Encryption 275

Configuration Guideline 275

Access Devices 277

IP Phone 278

Switch 278

Mitigate MAC CAM Flooding 278

Prevent Port Access 279

Prevent Network Extensions 280

Prevent Fraudulent DHCP Server 280

Mitigate DHCP DoS Attacks 281

Limit ARP Responses 282


Deployment Example 284

Summary 286

End Notes 287

References 287

Part III: Lawful Interception (CALEA) 289

Chapter 10: Lawful Interception Fundamentals 291

Definition and Background 292

Requirements from Law Enforcement Agents 293

Reference Model from an Architectural Perspective 294

AF (Access Function) 295

DF (Delivery Function) 295

CF (Collection Function) 296

SPAF (Service Provider Administration Function) 297

LEAF (Law Enforcement Administration Function) 297

Request and Response Interfaces 297

Operational Considerations 300

Detection by the Target Subscriber 300

Address Information for Call Content Interception 301

Content Encryption 302

Unauthorized Creation and Detection 303

Call Forwarding or Transfer 303

Capacity 304

Summary 304

End Notes 305

Chapter 11: Lawful Interception Implementation 307

Intercept Request Interface 308

SIP P-DCS Header 309

Intercept Process Flow for Outbound Call 310

Intercept Process Flow for Inbound Call 311

Cisco SII 313

Device Interfaces 314

Intercept Process Flow for Standard Call 316

Intercept Process Flow for Forwarding Call 319

Intercept Process Flow for Conference Call 322

Predesign Considerations 325

Security Considerations 326

Configuration Example 327

Call Data and Content Connection Interfaces 329

Call Content Connection Interface 330

Call Data Connection Interface 333

CDC Messages 333

Interface Between MD and LEA 339

Summary 341

End Notes 342

References 342

Index 345

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Voice over Internet Protocol (VoIP) has been popular in the telecommunications world since its emergence in the late 90s, as a new technology transporting multimedia over the IP network. In this book, the multimedia (or rich media) includes not only voice, but also video, instant message, presence data, and fax data over the IP network.

Today people commonly make phone calls with IP phones or client software (such as Skype or iChat) on their computer, or send instant messages to their friends. This gives them convenience and cost savings. Many telecommunications companies and other organizations have been switching their legacy phone infrastructure to a VoIP network, which reduces costs for lines, equipment, manpower, and maintenance.

However, the benefits of VoIP are not free. There are disadvantages to using VoIP. The integrated rich media makes it difficult to design the network architecture. Multiple VoIP protocols and different methods of implementation create serious interoperability issues. Integration with existing data networks creates quality of service issues. The fact that so many network elements are involved through open (or public) networks creates serious security issues, because each element and network has vulnerable factors.

The security issues especially are becoming more serious because traditional security devices (such as firewalls) and protocols (such as encryption) cannot protect VoIP services or networks from recent intelligent threats.

This book focuses on the important topic of VoIP security by analyzing current and potential threats to demonstrating the methods of prevention.

Goals and Methods

The most important goal of this book is to give you correct and practical answers for the following questions:

  • What are the current and potential threats?
  • What are the impacts of those threats?
  • Why are current data security devices not able to protect against recent intelligent threats?
  • How can you protect VoIP services and networks from those threats?
  • What is lawful interception and how do you implement it?

One key methodology used in this book is to give you hands-on experience of current well-known threats by simulating them with publicly available tools. Through the simulation, you can realize the characteristics and impacts of those threats and have a better understanding of mitigation.

Another key methodology is to give you detailed examples of protection methods with protocols, products, and architecture so that you may apply them to real VoIP service environments.

This book also gives you clarification of VoIP security concepts, definitions, standards, requirements, limitations, and related terms.

Who Should Read This Book

This book is NOT designed to give you information about VoIP in general which is available almost everywhere. Instead, this book focuses on VoIP security and gives practical information to people like those in the following list:

  • Managers or engineers who are planning to employ VoIP systems in their organizations
  • System engineers or architects who design and implement VoIP networks
  • Network administrators who administer, upgrade, or secure networks that include VoIP elements
  • Security consultants who perform security assessments for VoIP environments
  • Developers who implement VoIP products or solutions
  • Researchers and analysts who are interested in VoIP security

This book assumes that the readers have some minimal knowledge of networking (such as TCP/IP), operating systems, and VoIP in general (such as IP phones).

How This Book Is Organized

Although this book could be read cover to cover, it is designed to be flexible and allow you to easily move between chapters and sections of chapters to cover just the material that you need more work with.

This book consists of three parts. Part I, "VoIP Security Fundamentals," contains Chapters 1 through 5 and covers VoIP security fundamentals that are essential to understand current threats and security practices. Part II, "VoIP Security Best Practices," contains Chapters 6 through 9 and demonstrates VoIP security best practices with the detailed analysis and simulation of current threats. Part III, "Lawful Interception (CALEA)," contains Chapters 10 through 11 and covers another aspect of VoIP security, Lawful Interception, from basic concept to real implementation.

Chapter 1, "Working with VoIP," provides an overview of VoIP and its vulnerability in general. Chapters 2 through 11 are the core chapters and can be read in any order. If you do intend to read them all, the order in the book is an excellent sequence to use.

The core chapters, Chapters 2 through 11, cover the following topics:

  • Chapter 2, "VoIP Threat Taxonomy"—This chapter defines VoIP threat taxonomy, based on four different categories: threats against availability, confidentiality, integrity, and social context. This chapter is not intended to provide exhaustive lists of current and potential threats, but to define the taxonomy for identifying the threat in the first place, measuring the current and potential impact, and helping implementers to develop protection methods and secure service architecture. Twenty-two typical threats are introduced with examples and features.
  • Chapter 3, "Security Profiles in VoIP Protocols"—This chapter introduces the security profiles of VoIP protocols: SIP, H.323, and MGCP. The content shows how each protocol defines specific security mechanisms and recommends combined solution with other security protocols, such as IPSec, TLS, and SRTP.
  • Chapter 4, "Cryptography"—This chapter provides a high-level understanding of cryptographic algorithms with comprehensible figures, avoiding mathematical details. Well-known cryptographic algorithms are introduced, such as DES, 3DES, AES, RAS, DSA, and hash functions (MD5, SHA, and HMAC). This chapter also covers the mechanism of key management, focusing on key distribution.
  • Chapter 5, "VoIP Network Elements"—This chapter covers what devices are involved in the VoIP network architecture, and how they work for secure services. Session Border Controller, VoIP-aware firewalls, NAT servers, lawful interception servers, customer premise equipment, call processing servers, and media gateways are introduced.
  • Chapter 6, "Analysis and Simulation of Current Threats"—This chapter covers two main topics: detailed analysis and hands-on simulation of most common threats, and the guidelines for mitigation. For the analysis, it examines the detailed patterns, usage examples, and impacts of the threats. For the simulation, it introduces negative testing tools that are available on the Internet so that you can have hands-on experience. The threats that this chapter covers are DoS, malformed messages, sniffing (eavesdropping), spoofing (identity theft), and VoIP spam (voice, instant message, and presence spam).
  • Chapter 7, "Protection with VoIP Protocol"—This chapter demonstrates the details of how to make VoIP service secure with SIP and other supplementary protocols. It focuses on the methodology of protection in these five categories: authentication, encryption, transport and network layer security, threat model and prevention, and limitations.
  • Chapter 8, "Protection with Session Border Controller"—This chapter examines security issues on the VoIP network borders, and provides the methodology of preventing the issues with an SBC. This chapter includes the details of SBC functionality (such as network topology hiding, DoS protection, overload prevention, NAT traversal, and lawful interception), as well as the method of designing service architecture with an SBC in terms of high availability, secure network connectivity, virtualization, and optimization of traffic flow.
  • Chapter 9, "Protection with Enterprise Network Devices"—This chapter demonstrates how to protect the enterprise VoIP network with Cisco devices for practical information. Cisco firewalls, Unified Communications Manager, Unified Communications Manager Express, IP phone, and multilayer switches are used. This chapter includes security features, usage examples, and configuration guidelines for those devices.
  • Chapter 10, "Lawful Interception Fundamentals"—This chapter covers the fundamentals of lawful interception. The topics are definition, background information, requirements from law enforcement agents, the reference model from an architectural perspective, functional specifications, request/response interface, and operational considerations.
  • Chapter 11, "Lawful Interception Implementation"—This chapter demonstrates how to implement lawful interception into the VoIP service environment. It focuses on how the interception request and response work between functional modules, based on industry specifications.

© Copyright Pearson Education. All rights reserved.

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  • Posted December 13, 2008

    more from this reviewer

    VoIP Security made easy

    I was really excited to take a look at a book on this topic. It seems to me that while we all knew that there are security issues with this type of technology, no one really wanted to discuss the gritty details in a way that made them easy to understand. I think that `Voice over IP Security¿ is a great start to understanding just what those details are. Just keep in mind while reading it just who the target audience is.<BR/><BR/>In the intro section, the `who should read this book¿ section addresses a very broad audience, everyone from managers to engineers to security people to developers. This is an ambitious lot to try and satisfy in less than 400 pages. However, I think that the book makes a noble attempt to in fact meet the requirements of these various groups. Perhaps not in the depth that each of the groups would want to see, but I think it¿s a good foundation for anyone trying to learn the technology.<BR/><BR/>The areas that I most appreciated (being a information security manager type who has to look at technology like this from many dimensions: policy, technical configuration recommendations, audit) were it breaks done the many vulnerable areas, actually specifying the components and their weaknesses in the context of confidentiality, availability and integrity. The book also has a detailed discussion on the protocols of VoIP and how they work. I found the diagrams and other illustrations very useful in these areas. <BR/><BR/>The last section I wanted to point out was the discussion on lawful interception. I don¿t think that a lot of organizations consider this issue when they implement this type of system, so I particularly found this helpful and well explained. I will definitely use this as a reference as I prepare to write some policies for a VoIP implementation.<BR/><BR/>I noticed that the author did very well at taking some very technical topics and made them easy to understand. Well written, I think that `Voice over IP Security¿ is a great read to better understand the components of a VoIP system, the threats, and how best to protect your organization from such threats.

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  • Anonymous

    Posted September 13, 2008

    security weaknesses in VoIP

    Voice over Internet Protocol has emerged as a very popular way to do cheap (often free) long distance phone calls. But there is a huge amount of complexity beneath VoIP, that most users and even sysadmins are blissfully unaware of. The authors of this book perform a valuable service by educating the reader about current and, perhaps more importantly, possible future attacks. A major source of weakness in VoIP is shown to be due to interoperability issues between different underlying protocols or applications. In turn, a major reason for this is that when the protocols were defined, the authors of the defining documents unwittingly left ambiguities in the specifications. Then when vendors implemented VoIP products based on those protocols, different vendors might reasonably have interpreted the documents differently. Another source of weakness in security, as compared to traditional phone calls, is that tapping the latter often requires physical access to a phone line or a switching exchange. But VoIP at a low enough level is just like anything else that uses the Internet. Packets are routed through arbitrary third parties on the Internet. Those might have been subverted via remote attacks, so the VoIP cracker could be anywhere in the world. The book then spends most of its time suggesting protective measures. Including, most interestingly, how to simulate current and possibly future threats. This gives you practical hands on experience in role playing the adversary. Something necessary to fully devise technical solutions. But even if you do not do the latter, the book is useful simply in making you aware of the danger. So that for 'sensitive' conversations, you might advise users to minimise the use of VoIP, perhaps by using standard land lines.

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