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SSL VPN Design Considerations

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jun 10, 2008.

Chapter Description

This chapter discusses design issues you should consider when you build a Secure Socket Layer (SSL) Virtual Private Network (VPN) solution.

Security Considerations

A remote access VPN extends the perimeter of your network to the remote endpoints. An SSL VPN has been an entry point for security threats to enter the network. The ubiquity, versatility, and clientless nature of the SSL VPN provide significant business benefits and cost savings, but they also pose additional security challenges compared to traditional remote access VPNs.

The following sections first examine the security threats that need to be addressed in SSL VPN security design. The sections then cover some of the security design measures you can take that help to mitigate those threats.

Security Threats

The following sections look at the common security risks that are associated with SSL VPNs.

Lack of Security on Unmanaged Computers

As mentioned earlier, SSL VPNs can support users coming from any computer on the Internet, such as public domain machines (for example, kiosk PCs) that are not controlled by the corporate IT department. This department ensures that the machines have proper service packs and security software, such as antivirus software. This poses a major threat to security. If, for example, SSL VPN users sign in to the SSL VPN from a compromised or infected PC, they can become a source for spreading viruses, worms, network attacks, and Trojan horses into the corporate network.

Several other security risks mentioned in the sections that follow are also related to these security threats. In general, as you deal with uncontrolled endpoints, you face increased security risks.

Data Theft

Several types of security threats lead to data theft or password theft:

  • Sensitive data left in a browser's cache: Web browsers cache the various web objects that users downloaded during browsing. This caching helps browsers to improve the browsing experience. The browser cache files are physically stored on the user's computer in predefined directories. For example, the Temporary Internet Files folder is used for Internet Explorer browsers. After users finish browsing and leave the computer, the browser cache is left on the computer and can be accessed by other users who later log on to the same computer. This can be a security risk in a kiosk scenario that uses SSL VPN clientless web access. In this case, a VPN user logs in to the SSL VPN portal from a kiosk machine to access corporate resources, such as e-mail or other business applications. During the session, the user can access sensitive documents through the web browser that caches the document on a local hard drive. After the user signs off and leaves, attackers can easily use the kiosk computer and collect the browser cache to retrieve the sensitive information.
  • Browser histories: Similar to the browser cache, browser histories are stored by the browser to enhance the user experience. The browser histories reveal the user activities and internal web server structure. Similar to the browser cache, browser histories saved on unmanaged computers are vulnerable to data theft.
  • Browser cookies: A cookie is a text-only string sent by a web server to a web browser. The cookie can reside in the browser's memory or be stored on a local hard drive. A cookie is often used for purposes such as authentication, tracking, and personalization, such as site preference. Depending on their usage and content, cookies could contain sensitive information about users. Similar to a browser cache, cookies saved on unmanaged computers are vulnerable to data theft.
  • Brower-saved forms and user passwords: Similarly, browser-saved forms and user passwords are vulnerable to data theft and password theft.
  • Documents on unmanaged computers: More generally, documents and other types of sensitive data left on the unmanaged computers are vulnerable to data theft. For example, it is common for a VPN user to temporarily download a sensitive document to the local computer for reading or editing and later forget to delete the sensitive document before logging off. Furthermore, even if the user deletes the files before the VPN logoff, it is fairly easy for attackers to recover the deleted files by using common file-recovery utilities that are readily available on the Internet.
  • Data theft and password theft using keystroke loggers or other Trojan horse programs: In the SSL VPN web-based clientless mode, users can access corporate resources from an already compromised computer that contains malware. For example, loggers that are preinstalled by the attackers can capture user input, such as e-mail IDs and passwords, and take screen shots of the e-mails.

Man-in-the-Middle Attacks

There have been known man-in-the-middle (MITM) attacks to the SSL protocol, and this is how they can work. The attacker first launches an Address Resolution Protocol (ARP) spoofing attack or Domain Name System (DNS) spoofing attack to the SSL VPN user. The success of the attack will redirect the SSL traffic to the attack host that is configured with SSL proxy software. The attack host then acts as the destination web server by establishing an SSL connection with the user on one side and another SSL connection with the true destination web server on the other side, proxying the traffic back and forth. Because the attack host serves as the endpoint of the two SSL tunnels, it has the proper keys to decrypt the SSL traffic. In this attack, the attack host would need to present a spoofed digital certificate to the end users. In most cases, the web browser prompts the end user with a security alert. However, users often simply ignore the warning and proceed.

Web Application Attack

SSL and SSL VPNs do not have built-in mechanisms to detect application attacks such as SQL injections, buffer overflow attacks of web applications, directory traversal attacks, or cross-site scripting.

Spread of Viruses, Worms, and Trojans from Remote Computers to the Internal Network

Corporate networks are vulnerable to the spread of viruses, worms, and Trojans when the SSL VPN users connect using the tunnel client mode. With the tunnel client mode, the endpoints are directly connected to the corporate network with full network-layer access. Endpoints might not be compliant with corporate security policy, which can require, for example, a proper Windows patching level or up-to-date antivirus DAT files. In this case, a high possibility exists that the endpoints will forward their infection to the internal network.

Split Tunneling

In a remote access VPN deployment, split tunneling gives the user direct access to a public network and VPN access to a private network simultaneously. The end user's computer becomes an extended Internet entry point to the corporate network. If no proper security measures are in place on the end user's computer, attackers have opportunities to compromise the computer from the Internet and gain access to the internal network through the VPN tunnel. For this reason, many organizations choose to disable split tunneling in their remote access VPN deployment. Figure 3-1 illustrates the split tunneling topology.

Figure 3-1

Figure 3-1 Split Tunneling

When split tunneling is disabled, one common issue is that users can no longer access the local LAN for tasks such as printing. The solution is to disable split tunneling but enable local LAN access. This way, the local LAN traffic will not be tunneled to the head-end SSL VPN gateway.

Password Attacks

The password attack is one of the most effective attacks. Common practices such as weak passwords and simple authentication methods such as static passwords are vulnerable to various attacks through password cracking or eavesdropping.

Security Risk Mitigation

The following sections detail the design considerations and security measures that you should consider when implementing an SSL VPN deployment.

Strong User Authentication and Password Policy

Using a strong user authentication mechanism is critical to the security of a remote access VPN. If possible, consider using two-factor authentication techniques, such as hardware tokens and smart cards. If static passwords are used, enforce strong password policy.

Choose Strong Cryptographic Algorithms

The SSL VPN device normally allows you to choose SSL/TLS protocol versions and cipher suites. Consider enforcing SSLv3 or Transport Layer Security (TLS) rather than SSL version 2. Also, choose strong cipher suites for data encryption and integrity. For example, choose Triple DES (3DES) or AES instead of RC4.

Session Timeout and Persistent Sessions

On the SSL VPN device, configure a short session timeout to prevent potential piggybacking unauthorized access to your internal network through a public computer.

Some vendors support persistent sessions that keep the SSL VPN session even after the user closes the browser without signing off. End users might think that closure of the browser is equal to termination of the session. This could lead to unauthorized access to the internal network from a public computer.

Endpoint Security Posture Assessment and Validation

A thorough preconnect security assessment is necessary. As discussed earlier, this helps prevent viruses, worms, and Trojan horses from spreading into the internal network and helps administrators make intelligent decisions on what access privilege to grant to the VPN users based on the endpoint security posture. The preconnect security posture validation can include the following aspects:

  • Location checking: Using information such as IP address, Windows registries, or even PC screen banners, the SSL VPN device can figure out whether the user is coming from the Internet or from a corporate LAN, using corporate-owned PCs or a kiosk PC.
  • Security posture checking: This refers to a checklist that can be used to determine whether the endpoint has proper antivirus protection, a personal firewall, or other required security agents that are installed and enabled with up-to-date policies.
  • Malware scans: A malware scan can detect items such as keystroke loggers, spyware, and other Trojan horse programs on the endpoints.

VPN Session Data Protection

Although SSL VPNs provide secure communication, the session data is not encrypted on the endpoints and can be vulnerable to various malicious programs already on the compromised endpoints, such as a compromised kiosk computer. To protect the VPN session data, consider deploying secure desktop technology. This secure desktop is typically protected from other processes on the computer and has an "on-the-fly" encrypted file system. Malicious codes, even if they are present on the computer, might not be able to access the content stored on the secure desktop. This type of implementation also helps ensure that data will be erased in a secure manner at the end of the session. Later chapters describe the Cisco Secure Desktop in more detail.

Techniques to Prevent Data Theft

To prevent the previously mentioned data theft, consider the following techniques:

  • Cookie management: Many SSL VPN vendors support cookie management so that user cookies are not passed down to the endpoint.
  • Browser cache control: HTTP (RFC 2616) offers several cache-control headers that can be used to control the caching behavior of the browser.
  • Cache cleaner: Many SSL VPN vendors offer a cache cleaner that cleans the browser cache at the end of the session when users log off. The cache cleaner can delete the browser cache and browser histories. When deploying a cache cleaner, consider the following:
    • — Which folders does the cache cleaner clean? Some applications leave the cache in a different folder from the standard browser cache folder. Make sure that the cache cleaner can clean those locations. For example, the popular web-based e-mail application iNotes leaves its cache in a different folder than the standard browser cache folder. In this situation, the cache cleaner could miss the cache.
    • — Which operating systems does the cache cleaner support?
    • — Does the cache cleaner perform secure data sanitization? As mentioned earlier, a simple file delete is not secure. The secure cache cleaner complies to higher data sanitization standards than simple file delete to ensure that the deleted files can not be recovered later.
  • Secure desktop: Because the secure desktop provides a sandbox that traps all the session data, deletion of the secure desktop at the end of the session provides a thorough data cleanup. This method is more thorough than the cache cleaner, which cleans only specific locations.
  • User education and security awareness: These are also important aspects of the company security efforts. For SSL VPN security, focus on the following:
    • — Educate users on the potential security risks associated with accessing corporate resources from a public system.
    • — Encourage users to exercise security precautions when they use an SSL VPN on a public computer. This includes terminating the VPN session before leaving the computers, not leaving sensitive documents on the local hard drive of the public computer, and carefully examining certificate messages to guard against MITM attacks.

Web Application Firewalls, Intrusion Prevention Systems, and Antivirus and Network Admission Control Technologies

As mentioned earlier, the SSL protocol does not have a built-in mechanism to defend against web application attacks, and the SSL VPN tunnel client mode makes it easier for viruses and worms to spread into the Internet network from infected endpoints. Consider integrating threat defense technologies and security compliance technologies with your SSL VPN solution to mitigate these security risks:

  • Web application firewalls: These are OSI Layer 7 firewalls that can understand and analyze HTTP application traffic to detect protocol conformance violations and attacks. They normally use a combination of protocol conformance enforcement and attack signature pattern-matching techniques to either prevent application anomalies or look for specific attack patterns.
  • Intrusion prevention systems (IPS) and gateway-level antivirus systems: These systems offer a broader level of threat prevention. They help to prevent network attacks, viruses, worms, Trojan horses, spyware, and other security threats from entering the internal networks.
  • Network Admission Control (NAC): This is an emerging technology that addresses security compliance enforcement issues. The basic idea is to make sure that the endpoints are compliant with corporate security policies, such as having proper antivirus software and Windows patching level, before the network devices grant users access to network resources. The endpoint security integrity checking that we just discussed is a form of Network Admission Control, and it can be integrated with the overall NAC framework to provide a consistent security validation for all types of network access methods.
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