IPSec (Internet Protocol Security) is a framework for a set of protocols for security at the network or packet processing layer of network communication. Earlier security approaches have inserted security at the application layer of the communications model. IPSec is said to be especially useful for implementing virtual private networks and for remote user access through dial-up connection to private networks. A big advantage of IPSec is that security arrangements can be handled without requiring changes to individual user computers. Cisco has been a leader in proposing IPSec as a standard (or combination of standards and technologies) and has included support for it in its network routers. IPSec provides two choices of security service: Authentication Header (AH), which essentially allows authentication of the sender of data, and Encapsulating Security Payload (ESP), which supports both authentication of the sender and encryption of data as well. The specific information associated with each of these services is inserted into the packet in a header that follows the IP packet header. IPSec is a successor of the ISO standard Network Layer Security Protocol (NLSP). NLSP was based on the SP3 protocol that was published by NIST, but designed by the Secure Data Network System project of the National Security Agency (NSA).

The IPSec suite is a framework of open standards. IPSec uses the following protocols to perform various functions: A security association (SA) set up by Internet Key Exchange (IKE and IKEv2) or Kerberized Internet Negotiation of Keys (KINK) by handling negotiation of protocols and algorithms and to generate the encryption and authentication keys to be used by IPSec. Authentication Header (AH) to provide connectionless integrity and data origin authentication for IP datagram and to provide protection against replay attacks. Encapsulating Security Payload (ESP) to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality.


VPN uses encryption to provide data confidentiality. Once connected, the VPN makes use of the tunneling mechanism described above to encapsulate encrypted data into a secure tunnel, with openly read headers that can cross a public network. Packets passed over a public network in this way are unreadable without proper decryption keys, thus ensuring that data is not disclosed or changed in any way during transmission. VPN can also provide a data integrity check. This is typically performed using a message digest to ensure that the data has not been tampered with during transmission.
By default, VPN does not provide or enforce strong user authentication. Users can enter a simple username and password to gain access to an internal private network from home or via other insecure networks. Nevertheless, VPN does support add-on authentication mechanisms, such as smart cards, tokens and RADIUS.


While providing significant business benefits and cost savings, VPN technologies (SSL VPN included) come with their own security issues. These issues must be dealt with appropriately to ensure the confidentiality and integrity of data and information, as well as overall corporate network security. The following discussion first addresses the general security risks associated with using computers via VPN to access a company‘s internal network, and then addresses SSL VPN security risks.


User-credential-related risks
VPNs provide easy access from the Internet into a corporate network and its internal resources. VPN security is only as strong as the methods used to authenticate the users (and the devices) at the remote end of the VPN connection. Simple authentication methods based on static passwords are subject to password ―cracking‖ attacks, eavesdropping, or even social engineering attacks. Two-factor authentication, which consists of something you know and something you have, is a minimum requirement for providing secure remote access to the corporate network. In some cases, three-factor authentication may be necessary; this form of authentication adds one more requirement—something you are (a biometric such as fingerprint or iris scan, for example).

Spread of viruses, worms, and Trojans from remote computers to the internal network
Remote access is a major threat vector to network security. Every remote computer that does not meet corporate security requirements may potentially forward an ―infection‖ from its local network environment to an organization‘s internal network. Up-to-date antivirus software on the remote computer is required to mitigate this type of risk.

Split tunneling
Split tunneling takes place when a computer on the remote end of a VPN tunnel simultaneously exchanges network traffic with both the shared (public) network and the internal (private) network without first placing all of the network traffic inside the VPN tunnel.

This provides an opportunity for attackers on the shared network to compromise the remote computer and use it to gain network access to the internal network. A host-based firewall is an effective way to defend against network-based attacks. Furthermore, many organizations have chosen to disallow split tunneling.


Security risks more specific to SSL VPN are discussed below. Many of these risks are related to the fact that SSL VPN can be used on public machines. Lack of required host security software on public machines SSL VPN makes it easy and convenient to connect from anywhere on the Internet to a corporate internal network. However, public machines used for SSL VPN may not have the required antivirus software installed and properly maintained; also, they typically do not have a host-based firewall installed and enabled. These public machines cause a major threat when used for SSL VPN. They may spread viruses, worms, and Trojan horses—and may even become a back door for malicious attackers. Even strong user authentication will fail to protect the network if a remote computer has been compromised, because an attacker can ―piggyback‖ onto a live session via the Trojan and target the internal resources.

Physical access to shared machines
If a remote computer has an established network connection to your internal network, and the user leaves the session open, your internal network is now exposed to people who have physical access to the machine. Unauthorized personnel may use this computer to explore and attack your internal resources. SSL VPN significantly increases this type of risk—a connection can be started from any Internet-based machine. The physical access nature of shared machines adds numerous risks besides providing unauthorized network connection to the corporate internal network.

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