ISO/IEC 27033-5:2013

INTERNATIONAL ISO/IEC
STANDARD 27033-5
First edition
2013-08-01
Information technology — Security
techniques — Network security —
Part 5:
Securing communications across
networks using Virtual Private
Networks (VPNs)
Technologies de l’information — Techniques de sécurité - Sécurité de
réseau —
Partie 5: Sécurité des communications au travers des réseaux utilisant
des réseaux privés virtuels (VPNs)
Reference number
©
ISO/IEC 2013
© ISO/IEC 2013
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ii © ISO/IEC 2013 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviations. 2
5 Document structure . 2
6 Overview . 2
6.1 Introduction . 2
6.2 Types of VPNs . 3
7 Security Threats . 4
8 Security Requirements . 5
8.1 Overview . 5
8.2 Confidentiality . 5
8.3 Integrity . 6
8.4 Authenticity . 6
8.5 Authorization . 6
8.6 Availability . 6
8.7 Tunnel Endpoint Security . 6
9 Security Controls . 6
9.1 Security aspects . . 6
9.2 Virtual circuits . 7
10 Design Techniques . 7
10.1 Overview . 7
10.2 Regulatory and legislative aspects . 8
10.3 VPN management aspects . 8
10.4 VPN architectural aspects . 8
10.5 VPN technical considerations .11
11 Guidelines for Product Selection .12
11.1 Carrier protocol selection .12
11.2 VPN appliances .12
Bibliography .14
© ISO/IEC 2013 – All rights reserved iii

Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are
members of ISO or IEC participate in the development of International Standards through technical
committees established by the respective organization to deal with particular fields of technical
activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other international
organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the
work. In the field of information technology, ISO and IEC have established a joint technical committee,
ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the national bodies
casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
ISO/IEC 27033 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 27, IT security techniques.
This first edition cancels and replaces ISO/IEC 18028-5:2006, which has been technically revised.
ISO/IEC 27033 consists of the following parts, under the general title Information technology — Security
techniques — Network security:
— Part 1: Overview and concepts
— Part 2: Guidelines for the design and implementation of network security
— Part 3: Reference networking scenarios — Threats, design techniques and control issues
— Part 4: Securing communications between networks using security gateways
— Part 5: Securing communications across networks using Virtual Private Networks (VPNs)
— Part 6: Securing wireless IP network access
(Note that there may be other parts. Examples of possible topics to be covered by parts include local area
networks, wide area networks, broadband networks, web hosting, Internet email, and routed access to
third-party organizations. The main clauses of all such parts should be Risks, Design Techniques, and
Control Issues.)
iv © ISO/IEC 2013 – All rights reserved

INTERNATIONAL STANDARD ISO/IEC 27033-5:2013(E)
Information technology — Security techniques —
Network security —
Part 5:
Securing communications across networks using Virtual
Private Networks (VPNs)
1 Scope
This part of ISO/IEC 27033 gives guidelines for the selection, implementation, and monitoring of
the technical controls necessary to provide network security using Virtual Private Network (VPN)
connections to interconnect networks and connect remote users to networks.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 27001:2005, Information technology — Security techniques — Information security management
systems — Requirements
ISO/IEC 27002:2005, Information technology — Security techniques — Code of practice for information
security management
ISO/IEC 27005:2011, Information technology — Security techniques — Information security risk management
ISO/IEC 27033-1:2009, Information technology — Security techniques — Network security — Part 1:
Overview and concepts
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/IEC 7498 (all parts),
ISO/IEC 27000, ISO/IEC 27001, ISO/IEC 27002, ISO/IEC 27005, and ISO/IEC 27033-1 apply.
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4 Abbreviations
For the purposes of this document, the abbreviated terms given in ISO/IEC 27033-1 and the following apply.
AH Authentication Header
ESP Encapsulating Security Payload
IKE Internet Key Exchange
IPsec Internet Protocol Security
ISAKMP Internet Security Association and Key Management Protocol
L2F Layer Two Forwarding (Protocol)
LDP Label Distribution Protocol
MPPE Microsoft Point-to-Point Encryption
MPLS Multi-protocol Label Switching
NAS Network Area Storage
OSI Open Systems Interconnection
PPP Point-to-Point Protocol
PPTP Point-to-Point Tunneling Protocol
SSL Secure Sockets Layer
VPLS Virtual Private LAN Service
VPWS Virtual Private Wire Service
WAN Wide Area Network
5 Document structure
The structure of ISO/IEC 27033-5 comprises:
— an overview of VPNs (see clause 6),
— security threats associated with VPNs (see clause 7),
— security requirements derived from threat analysis for VPNs (see clause 8),
— security controls associated with typical network scenarios and network technology areas using
VPNs (see clause 9),
— various design techniques for VPNs (see clause 10).
6 Overview
6.1 Introduction
VPNs have developed rapidly as a means of inter-connecting networks and as a method of connecting
remote users to networks.
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There exists a broad range of definitions for VPNs. In their simplest form, they provide a mechanism for
establishing a secure data channel or channels over an existing network or point-to-point connection.
They are assigned to the exclusive use of a restricted user group, and can be established and removed
dynamically, as needed. The hosting network may be private or public.
An example representation of a VPN, with the secure data channel connecting an end user to a gateway
across a public network and a secure data channel connecting two gateways across a public network, is
shown in Figure 1 below.
Attacker 1
VPN between User
Public Network
and Gateway
Gateway
User
Attacker 2
Figure 1 — Example representations of a VPN
Remote access using a VPN is implemented over the top of a normal point-to-point connection. The
normal point-to-point connection between the local user and the remote locations is established first.
Some VPNs are provided as a managed service, in which secure, reliable connectivity, management
and addressing, equivalent to that on a private network, are provided on a shared infrastructure.
Additional security controls, as indicated in this standard, may therefore need to be taken into account
to strengthen the VPN.
The data and code transiting a VPN should be restricted to the organization using the VPN and should
be kept separate from other users of the underlying network. It should not be possible for data and
code belonging to other users to access the same VPN channel. The level of trust in the confidentiality
and other security aspects of the organization owning or providing the VPN should be taken into
consideration when evaluating the extent of additional security controls that may be required.
6.2 Types of VPNs
As stated above, there are multiple ways of expressing types of VPN.
Architecturally, VPNs comprise of either:
— a single point-to-point connection (e.g. client device remotely accessing an organization’s network
via a site gateway, or a site gateway connecting to another site gateway), or
— a point-to-cloud connection (e.g. implemented by MPLS technology).
From an OSI Basic Reference Model perspective, there are three main types of VPN:
— Layer 2 VPNs offer a simulated LAN facility, using VPN connections running over a hosting network
(e.g. a provider’s network) to link sites of an organization or to provide a remote connection to an
organization. Typical provider offerings in this area include Virtual Private Wire Service (VPWS),
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which provides a simulated “wires only connection”, or Virtual Private LAN Service (VPLS), which
provides a more complete simulated LAN service.
— Layer 3 VPNs offer a simulated WAN facility, again using VPNs running over a network infrastructure.
These offerings provide sites with simulated “OSI Network Layer” connectivity. A basic attraction
here is the ability to use private IP addressing schemes over a public infrastructure, a practice
that would not be permitted over a “normal” public IP connection. Whilst private addresses can be
used over public networks via NAT (Network Address Translation), this can complicate IPsec VPN
establishment and use, although there are work-arounds available.
— Higher Layer VPNs are used for securing transactions across public networks. They typically
provide a secure channel between communicating applications, thus ensuring data confidentiality
and integrity during the transaction. This type may also be known as a Layer 4 VPN because the
VPN connection is usually established over TCP which is a Layer 4 protocol.
7 Security Threats
For the foreseeable future, organizations can expect increasingly sophisticated attacks to be mounted
against their systems. Attempts at unauthorized access can be malicious, for example leading to a Denial-
of-Service attack, the misuse of resources, or the access to valuable information.
Generally speaking threats against a VPN can be in the form of Intrusions or Denial of Service (DoS).
Intrusions happen when an outsider or malicious perpetrator takes control over part of your network;
this can be a computer or other networking device (including mobile devices).
Intrusions may come from any location that has connectivity to your/the network. These attacks can
come from other VPNs, the internet or the service provider core itself. The protection against these
types of attacks comes from the ability to filter unwanted traffic from unwanted sources on network’s
ingress points. One of the typical examples of intrusion is the unauthorized access to the secure tunnel
by an unauthorized entity.
This can be difficult in some VPN design models which lack centralization as all sites connect to each
other without traffic control.
DoS attacks are another type of threat against a VPN. Both DoS attacks and intrusions can come from another
VPN, the internet or the service provider core. The main difference between the two types of attacks is that
for DoS attacks the attacker needs to get access or have control over one of your pieces of equipment.
DoS attacks against the service provider devices can also cause a denial of service to some parts of
your VPN. Although it might be hard to sometimes protect your network against DoS attacks, the main
protection against them lies in the good network design of the VPN.
Security issues for VPNs include:
— address space and routing separation between VPNs carried over the label switched network;
— ensuring that the internal structure of the label switched network core is not visible to outside
networks (e.g. to limit information available to a potential attacker);
— providing resistance to denial of service attacks;
— providing resistance to unauthorized access attacks;
— protecting against label spoofing (although whilst it may be possible to insert wrong labels into a
label switched network from the outside, because of address separation the spoofed packet would
only harm the VPN from which the spoofed packet originated).
4 © ISO/IEC 2013 – All rights reserved

8 Security Requirements
8.1 Overview
The primary security objective of a VPN is protection from unauthorized access. VPNs could therefore
be used to fulfill wider network security objectives:
— to safeguard information in networks, in systems connected to networks, and the services used by them,
— to protect the supporting network infrastructure,
— to protect network management systems.
To achieve the objectives outlined in the paragraph above, VPNs should be implemented in a way
that ensures the:
— confidentiality of data in transit between VPN end-points,
— integrity of data in transit between VPN end-points,
— authenticity of VPN users and administrators,
— authorization of VPN users and administrators,
— availability of VPN end-points and network infrastructure.
This in turn implies that the underlying tunnels used to construct the VPN should be implemented in
such a way that the security objectives are met. These objectives are summarized in Figure 2.
Figure 2 — Generic security requirements of VPNs mapped onto the underlying tunnel
Each of these requirements is discussed in detail below.
Clause 9 also discusses the types of security controls used to implement secure VPNs.
8.2 Confidentiality
The confidentiality of data and code in transit in the tunnel should not be compromised. Use of tunnel
technologies may imply that data and code in transit are not visible to other users of the network. However,
this does not mean that the traffic is kept confidential. In particular data and code flowing in tunnels
are not protected from determined inspection using data analyzers or interceptors. The preservation
of confidentiality of data and code whilst in transit in tunnels is therefore crucially dependent upon the
likelihood of such inspection occurring. This in turn is a factor of the degree of trust that exists in the
underlying network(s) supporting the VPN(s), which will vary depending upon the ownership of the transit
network. If the transit network is not in a trusted domain (see ISO/IEC 27033-1 for more information on
domains of trust), or if the data and code to be transmitted are considered sensitive, additional security
© ISO/IEC 2013 – All rights reserved 5

controls may need to be taken to further protect confidentiality. In such cases, the tunnel mechanism(s)
employed should support encryption, or items to be sent should be encrypted off-line before transmission
over the VPN. The security of the tunnel endpoints should also not be neglected (see 8.7).
8.3 Integrity
The integrity of data and code in transit in the tunnel should not be compromised. The mechanisms
used to implement the VPN tunnel should support integrity checking of data and code in transit,
using techniques such as message verification codes, message authentication codes and anti-replay
mechanisms. If such protection is not available from the tunnel implementation, or if the data or code to
be transmitted is particularly sensitive, then integrity protection controls should be implemented in the
end-systems, such that integrity protection is provided end-to-end.
8.4 Authenticity
Authenticity of information crossing public IP networks should be provided between participating
peers in a VPN. The tunnel establishment and operating pr
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