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ITL Bulletin for July 2008




ITL Bulletin for July 2008
ITL Bulletin for July 2008



Forwarded from: Elizabeth Lennon 

ITL BULLETIN FOR JULY 2008

GUIDELINES ON IMPLEMENTING A SECURE SOCKETS LAYER (SSL) VIRTUAL PRIVATE 
NETWORK (VPN)

By Sheila Frankel
Computer Security Division
Information Technology Laboratory
National Institute of Standards and Technology
U.S. Department of Commerce
 
The protection of sensitive information that is transmitted across 
interconnected networks is critical to the overall security of an 
organization's information and information systems. The Information 
Technology Laboratory of the National Institute of Standards and 
Technology (NIST) recently issued guidelines to assist organizations in 
strengthening their network security and in lessening the risks 
associated with the transmission of sensitive information across 
networks. NIST Special Publication (SP) 800-113, Guide to SSL VPNs, 
offers practical guidelines on implementing a Secure Sockets Layer (SSL) 
virtual private network (VPN).

SSL VPNs provide secure remote access to an organization's resources. A 
VPN is a virtual network, built on top of existing physical networks, 
which can provide a secure communications mechanism for data and other 
information transmitted between two endpoints. Because a VPN can be used 
over existing networks such as the Internet, it can facilitate the 
secure transfer of sensitive data across public networks. An SSL VPN 
consists of one or more VPN devices to which users connect using their 
Web browsers. The traffic between the Web browser and the SSL VPN device 
is encrypted with the SSL protocol or its successor, the Transport Layer 
Security (TLS) protocol. This type of VPN may be referred to as either 
an SSL VPN or a TLS VPN. This guide uses the term SSL VPN.

SSL VPNs provide remote users with access to Web applications and 
client/server applications, and connectivity to internal networks. 
Despite the popularity of SSL VPNs, they are not intended to replace 
Internet Protocol Security (IPsec) VPNs. The two VPN technologies are 
complementary and address separate network architectures and business 
needs. SSL VPNs offer versatility and ease of use because they use the 
SSL protocol, which is included with all standard Web browsers, so the 
client usually does not require configuration by the user. SSL VPNs 
offer granular control for a range of users on a variety of computers, 
accessing resources from many locations. There are two primary types of 
SSL VPNs:

* SSL Portal VPNs. This type of SSL VPN allows a user to use a single 
  standard SSL connection to a Web site to securely access multiple 
  network services. The site accessed is typically called a portal 
  because it is a single page that leads to many other resources. Remote 
  users access the SSL VPN gateway using any modern Web browser, 
  identify themselves to the gateway using an authentication method 
  supported by the gateway, and are then presented with a Web page that 
  acts as the portal to the other services. These other services might 
  be links to other Web servers, shared file directories, Web-based 
  email systems, applications that run on protected servers, and any 
  other services that can be channeled through a Web page.

SSL portal VPNs work with essentially any modern Web browser. 
Specifically, they work with browsers whether or not the browsers allow 
(or support) active content. Thus, SSL portal VPNs are accessible to 
more users than SSL tunnel VPNs.


* SSL Tunnel VPNs. This type of SSL VPN allows a user to use a typical 
  Web browser to securely access multiple network services, including 
  applications and protocols that are not Web-based, through a tunnel 
  that is running under SSL. SSL tunnel VPNs require that the Web 
  browser be able to handle specific types of active content, which 
  allows them to provide functionality that is not accessible to SSL 
  portal VPNs, and that the user be able to run them. Examples of active 
  content include Java, JavaScript, Active X, or Flash applications or 
  plug-ins.

The tunneling in an SSL tunnel VPN allows a wide variety of protocols 
and applications to be run through it. For example, essentially any 
protocol that runs over Transmission Control Protocol (TCP) or User 
Datagram Protocol (UDP) can be tunneled through such a gateway, making 
the remote user's experience of the protected network very similar to 
being directly on the network. To the user, an SSL tunnel VPN may appear 
quite different from a typical Web site because the tunneling plug-in or 
application needs to be loaded into the user's browser before the user 
can access the VPN. This might involve warning messages about the 
software being loaded, and it could also prevent users from entering the 
VPN if their Web browsers are instructed not to allow such programs to 
run. Because of the active content requirement, SSL tunnel VPNs may be 
accessible to fewer users than SSL portal VPNs.

 

NIST Special Publication 800-113, Guide to SSL VPNs

Written by Sheila Frankel of NIST, Paul Hoffman of the Virtual Private 
Network Consortium (VPNC), and Angela Orebaugh and Richard Park of Booz 
Allen Hamilton, this publication discusses the fundamental technologies 
and features of SSL VPNs. It describes SSL and how it fits within the 
context of layered network security. The guide presents a phased 
approach to SSL VPN planning and implementation that can help in 
achieving successful SSL VPN deployments. It also compares the SSL VPN 
technology with IPsec VPNs and other VPN solutions. This information is 
particularly valuable for helping organizations to determine how best to 
deploy SSL VPNs within their specific network environments. The document 
is available at 
http://csrc.nist.gov/publications/nistpubs/800-113/SP800-113.pdf. 



NIST Recommendations for Implementing SSL VPNs

Implementing the following recommendations should assist in facilitating 
more efficient and effective SSL VPN use for federal departments and 
agencies.

Organizations planning SSL VPN deployments should identify and define 
requirements, and evaluate several products to determine their fit into 
the organization.

SSL VPN products vary in functionality, including protocol and 
application support. They also vary in breadth, depth, and completeness 
of features and security services. Some recommendations and 
considerations include the following:

* SSL VPN manageability features such as status reporting, logging, and 
  auditing should provide adequate capabilities for the organization to 
  effectively operate and manage the SSL VPN and to extract detailed 
  usage information.

* The SSL VPN high availability and scalability features should support 
  the organization's requirements for failover, load balancing, and 
  throughput. State and information sharing is recommended to keep the 
  failover process transparent to the user.

* SSL VPN portal customization should allow the organization to control 
  the look and feel of the portal and to customize the portal to support 
  various devices such as personal digital assistants (PDAs) and smart 
  phones.

* SSL VPN authentication should provide the necessary support for the 
  organization's current and future authentication methods and leverage 
  existing authentication databases. SSL VPN authentication should also 
  be tested to ensure interoperability with existing authentication 
  methods.

* The strongest possible cryptographic algorithms and key lengths that 
  are considered secure for current practice should be used for 
  encryption and integrity protection unless they are incompatible with 
  interoperability, performance, and export constraints. Federal 
  agencies have more stringent requirements.

* SSL VPNs should be evaluated to ensure that they provide the level of 
  granularity needed for access controls. Access controls should be 
  capable of applying permissions to users, groups, and resources, as 
  well as integrating with endpoint security controls.

* Implementation of endpoint security controls is often the most diverse 
  service amongst SSL VPN products. Endpoint security should be 
  evaluated to ensure that it provides the necessary host integrity 
  checking and security protection mechanisms required for the 
  organization.

* Not all SSL VPNs have integrated intrusion prevention capabilities.  
  Those that do should be evaluated to ensure that they do not introduce 
  an unacceptable amount of latency into the network traffic.


Federal agencies deploying SSL VPNs must configure them to only allow 
Federal Information Processing Standards (FIPS)-compliant cryptographic 
algorithms, cipher suites, and versions of SSL.

Some organizations, such as federal agencies, have strict requirements 
for encryption and integrity protection. SSL VPNs should support the 
required algorithms for symmetric encryption, key exchange, and hash 
functions. For government agencies, traffic that requires protection 
must employ FIPS-compliant cryptographic algorithms and modules. Many of 
the cryptographic algorithms used in some SSL cipher suites are not 
FIPS-approved, and therefore are not allowed for use in SSL VPNs that 
are to be used in applications that must conform to FIPS 140-2, Security 
Requirements for Cryptographic Modules. This means that to be run in 
FIPS-compliant mode, an SSL VPN gateway must only allow cipher suites 
that are allowed by FIPS 140-2.

Some of the cryptographic requirements, including allowable hash 
functions and certificate key lengths, will change at the end of 2010. 
Therefore, federal agencies who want to provide SSL VPN services after 
2010 must ensure that their systems are upgradeable to the new 
FIPS-compliant cipher suites and key lengths before the end of 2010, and 
that their SSL VPN vendors guarantee that such upgrades will be 
available early enough for testing and deployment in the field.

Organizations should use a phased approach to SSL VPN planning and 
implementation.

A successful SSL VPN deployment can be achieved by following a clear, 
step-by-step planning and implementation process. The use of a phased 
approach can minimize unforeseen issues and identify potential pitfalls 
early in the process. The five phases of the recommended approach are:

1. Identify Requirements. Identify the requirements for remote access 
   and determine how they can best be met.

2. Design the Solution. Make design decisions in five areas: access 
   control, endpoint security, authentication methods, architecture, and 
   cryptography policy.

3. Implement and Test a Prototype. Test a prototype of the designed 
   solution in a laboratory, test, or production environment to identify 
   any potential issues.

4. Deploy the Solution. Gradually deploy the SSL VPN solution throughout 
   the enterprise, beginning with a pilot program.

5. Manage the Solution. Maintain the SSL VPN components and resolve 
   operational issues.  Repeat the planning and implementation process 
   when significant changes need to be incorporated into the solution.

 
Organizations should be familiar with the limitations of SSL VPN 
technology.

SSL VPNs, although a maturing technology, continue to face several 
challenges. These include limitations on their ability to support a 
large number of applications and clients, the methods of implementing 
network extension and endpoint security, the ability to provide 
clientless access, the use of the SSL VPN from public locations, and 
product and technology education.

Organizations should implement other measures that support and 
complement SSL VPN implementations.

These measures help to ensure that the SSL VPN solution is implemented 
in an environment with the technical, management, and operational 
controls necessary to provide sufficient security for the SSL VPN 
implementation. Examples of supporting measures include:

* Establishing and maintaining control over all entry and exit points 
  for the protected network, which helps to ensure its integrity;

* Incorporating SSL VPN considerations into organizational policies 
  (e.g., identity management, remote access); and

* Ensuring that all SSL VPN endpoints are secured and maintained 
  properly to reduce the risk of SSL VPN compromise or misuse.

Although SSL VPNs are flexible enough to meet many needs, there are 
certain cases when other types of VPNs may provide a better solution. 
Network layer VPN protocols, primarily IPsec; data link layer VPN 
protocols, such as Point-to-Point Tunneling Protocol (PPTP), Layer 2 
Tunneling Protocol (L2TP), and Layer 2 Forwarding (L2F); and application 
layer security protocols, including OpenPGP and Secure Shell (SSH), are 
all effective alternatives to SSL VPNs for particular needs and 
environments.

Disclaimer: Any mention of commercial products or reference to 
commercial organizations is for information only; it does not imply 
recommendation or endorsement by NIST nor does it imply that the 
products mentioned are necessarily the best available for the purpose.

 
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