Why companies should PAD their networks

Once upon a time, network security infrastructure consisted only of a firewall deployed at the perimeter. This worked fairly well when there was limited interaction between internal and external networks, the internal users were trusted and the value of the network-available assets was limited.

However, there has been considerable change in recent years. Network-aware applications and interactions between networks have greatly increased, and while access is being granted on a greater scale to these business-critical functions, attackers and their tools have become more sophisticated.

Fortunately, many organizations have augmented their security infrastructures to handle these changes. Using a number of tools, including virus-detection systems, vulnerability-assessment scanners, and encryption- and intrusion-detection systems (IDS), companies have made an effort to both detect and prevent security threats to their networks. Early versions of these security tools had trouble detecting certain types of threats and were unnecessarily complex, IDSs in particular. Such challenges rendered IDSs difficult to deploy, frustrating to use and possible to evade.

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Brian Hernacki is an architect at the Symantec Research Lab.
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Brian Hernacki is an architect at the Symantec Research Lab. He can be reached at brian_hernacki@symantec.com.
How anomaly detection works

To address these shortcomings, several products now support a technique known as anomaly detection. While anomaly detection is far from new, there has been considerable confusion over what it is and how it works.

By definition, an anomaly is something that's different, abnormal or not easily classified. Therefore, the concept of anomaly detection in computer security involves the discovery of an abnormality in something (a network, host, set of users, for example) when compared against expected behavior.

One of the key differences between anomaly detection and other forms of detection is that, rather than defining what is either not allowed or bad, it defines what is allowed or good. Many traditional forms of detection rely on comparing observed behavior and noting when something known to be "bad" is seen. These are often referred to as misuse-detection systems; however, this nomenclature can be confusing since anomaly systems also detect misuse. A more accurate name, perhaps, is explicit-detection systems. These systems operate well when the number of possible bad behaviors is small and doesn't change very rapidly. However, in larger systems with greater variation, these two conditions often do not hold. It then becomes an onerous task to maintain the list of what is bad.

Anomaly detection is a more proactive system that relies on having some definition of allowed behavior and then noting when observed behaviors differ. This operates well when it's easier or more efficient to define what is allowed rather than what is not. In these cases, the definition of what is allowed tends to be much shorter. It also tends not to require changes as new problems are created or discovered.

Anomaly-detection systems monitor networks for two primary criteria: characteristic and statistical deviations. Characteristic deviations are more qualitative in nature and thus are often unable to identify quantitative anomalies. For example, "User joe123 does not normally transfer files outside of the company." Statistical deviations are more quantitative and less likely to pick up on qualitative anomalies. An example of a statistical deviation would be "This site's ICMP traffic never exceeds 15% of capacity."

However, protocol anomaly detection (PAD) systems, which are primarily characteristic (qualitative) in nature, can also use many of the strict model system attributes to identify anomalies. This type of design takes advantage of the fact that protocols by nature are usually very restrictive. Therefore it's possible to construct a very strict model of what should occur and easily note any deviations from it.

What differentiates a PAD system from a traditional explicit matching system (which is based on signatures), is the kind of patterns used. In most cases, the PAD system also requires some sort of stateful, protocol-aware matching system, without which it can be prone to false positives.

PAD provides some powerful capabilities that make it an excellent mechanism for performing network intrusion detection. First and foremost, because it doesn't require any prior signature to detect certain classes of attacks, it provides the ability to detect attacks before signatures are published. This eliminates the vulnerability window that exists during the first hours or days after an exploit is published.

In addition, PAD is resistant to evasion by polymorphic attacks. Since it doesn't rely on matching explicit patterns, variations in the attack generally are caught, unlike the failure that can occur when the form of the attack changes slightly to escape detection by signature-based systems. And since signature updates are not needed, there is lower administrative overhead.

There is, however, a caveat to such intrusion-detection technology. Because PAD systems are not explicit, they generally provide less specific information than comparable signature-matching systems. For example, a PAD system monitoring HTTP traffic may report observing a questionably encoded URL; while a signature system may report the same event by its exact name helping security administrators know which particular threat they are dealing with.

However, through various forms of classification work, a PAD system can be structured so that once anomalies are identified, additional work is performed to more specifically identify the event and provide additional reference information.

PAD provides a powerful, scalable and maintainable intrusion-detection mechanism. It's a core technology around which to build a detection infrastructure, and it provides unique capabilities, such as detection of zero-day attacks, which are not available with other methods. While it isn't a panacea for all security needs, it provides a valid and effective solution to some of the more critical limitations exhibited by current security systems.

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Copyright © 2003 IDG Communications, Inc.

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