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Intrusion Prevention: Signatures and Actions

Attack signatures have been around for long enough that the definition should be universally understood, but that's not the case. Simply put, an IPS signature is any distinctive characteristic that identifies something. Using this definition, all IPS products use signatures of some kind, regardless of what the product descriptions claim. To find something and stop it, you must be able to identify it, and for you to identify it, it must display a distinct characteristic. This chapter introduces you to the concept of signatures.
This chapter is from the book

This chapter is from the book

Before you decide to buy a product of any kind, you usually want to know exactly what it is you're buying. That way, you don't get something you don't want. This seems like a reasonable goal, but achieving this goal isn't always easy. What's challenging is the vocabulary used to describe what the product does and how it works. Two different products might have a feature with the same name, but the feature in each product might actually be completely unrelated. Without an industry agreed-upon set of definitions, product marketing can use terminology to make each product appealing to customers, even if this usage makes it difficult for customers to compare the functionality between different IPS products.

Take the purchase of a new vehicle, for example. Three different automobiles claim to have drive stabilization systems. That sounds great, but does the system work in the same way for each car? Is one more suitable for your needs than the other? How is the system implemented? Close examination might show how the system in one car reduces vibration when driving over bumpy roads whereas in another car it helps control the vehicle's balance during sharp turns. The name for the feature is exactly the same, but what it actually does is very different.

Seeing through the fog of feature names and marketing buzzwords is especially difficult when the product of interest is in a new technology, such as Intrusion Prevention. Intrusion Prevention System (IPS) product data sheets and websites tend to use vague product descriptors like deep packet inspection, anomaly detection, innate defense models, signatures, and behavior-based and advanced network intelligence. The descriptors might be accurate, but the functionality behind the words is often not consistent from product to product.

The way to see through the words and discern the product functionality is to create clear definitions for commonly used feature names. One feature commonly associated with IPS is signatures. Attack signatures have been around for long enough that the definition should be universally understood, but that's not the case. Simply put, an IPS signature is any distinctive characteristic that identifies something. Using this definition, all IPS products use signatures of some kind, regardless of what the product descriptions claim. To find something and stop it, you must be able to identify it, and for you to identify it, it must display a distinct characteristic. Signatures are distinguished by the following characteristics:

  • Signature types
  • Signature trigger
  • Signature actions

Signature Types

Signatures fall into one of the following two basic categories depending on their functionality:

  • Atomic signatures
  • Stateful signatures

This section examines these signature types in further detail. Furthermore, the triggering mechanisms explained later in this chapter can be used with both of these base signature types. The major distinction between these two base signature types is whether or not the inspection process requires the IPS device to maintain state about previous actions that have been observed.

Atomic Signatures

Atomic signatures represent the simplest signature type. For an atomic signature, a single packet, activity, or event is examined to determine if the signature should trigger a signature action. Because these signatures trigger on a single event, they do not require your intrusion system to maintain state. The entire inspection can be accomplished in an atomic operation that does not require any knowledge of past or future activities.

Atomic Signature Considerations

One drawback with atomic signatures is that you have to know all the atomic events that you want to look for. For each of these events, you then have to create the appropriate signature. As the number of atomic signatures increases, just managing the different signatures can become overwhelming.

Another drawback is that these signatures can be applied only to situations in which the context of the event is not important. For example, assume that you have a simple string match signature that triggers an alert action whenever the traffic that it is analyzing contains /etc/passwd. If you apply this simple string signature to monitor TCP traffic, an attacker can generate alerts by sending a flood of TCP packets with the /etc/passwd string in payload. The alerts are generated even if the connection is not part of a valid TCP connection (because it is an atomic signature). Furthermore, analyzing the alerts can minimize the time that your security staff spends identifying more serious attacks that represent valid attacks against your network. Generating a large number of bogus alerts can also impact the performance of your monitoring applications and devices.

Nevertheless, atomic signatures have their advantages. First, these signatures consume minimal resources (such as memory) on the IPS/IDS device. These signatures are also easy to understand because they search only for a specific event. Finally, traffic analysis for these atomic signatures can usually be performed very quickly and efficiently.

Host-Based Examples

Host-based IPS examines many operations on the system, including function calls, files accessed, and so on. One common method for detecting anomalous user behavior is to establish a baseline of the operations that a user normally performs on the system. Then by monitoring deviations from the baseline, you can detect potentially malicious activity. For example, if a function call is never invoked normally (except in connection with malicious activity), then triggering a signature action whenever it is called is a simple example of a host-based atomic signature. Another example of this is an application that you consider a problem. For example, you might want to trigger a signature action whenever a command shell is invoked on the local system.

Network-Based Examples

A good example of a network-based atomic signature is the LAND attack. By inspecting a single packet, your Network-based (or Host-based) IPS can identify this attack. Because everything is contained in a single packet, no state information is needed to identify this attack.

Stateful Signatures

Unlike atomic signatures, stateful signatures trigger on a sequence of specific events that requires the IPS device to maintain state. The length of time that the signatures must maintain state is known as the event horizon. Configuring the length of the event horizon is a tradeoff between consuming system resources and being able to detect an attack that occurs over a long period of time.

Stateful Signature Considerations

The main limitation to stateful signatures is that maintaining state consumes memory resources on your IPS/IDS device. Usually, however, this is not a significant problem if the IPS product is designed to efficiently use its resources. If your IPS does not efficiently manage resources when maintaining state, then the large consumption of resources (such as memory and CPU) can lead to a slow response time, dropped packets, missed signatures, and so on, which adversely impacts the effectiveness of your IPS.

Requiring a specific event to be detected in a known context increases the likelihood that the activity represents legitimate attack traffic. This minimizes the false positives generated by the stateful signatures.

Host-Based Examples

For a host-based example, we are going to use a commonly used Windows command shell called cmd.exe. As opposed to our atomic host-based example earlier in this chapter, in this situation, we do not want to trigger a signature action whenever cmd.exe is invoked (because our users use this program frequently). Our examination, however, reveals that many attacks invoke cmd.exe remotely. To remotely execute cmd.exe, the attacker must make a network connection to the host. This information can be used to refine our atomic signature by adding state. The stateful signature triggers a signature action when cmd.exe is invoked, but only if the application invoking cmd.exe first accepted a network connection.

The Host-based IPS must remember which applications have accepted network connections. This state information can then be examined whenever cmd.exe is invoked.

Network-Based Examples

Often, Network-based IPS signatures are stateful signatures because the information needed can usually be distributed across multiple packets. Even a simple string match signature is usually stateful because the string can occur across multiple packets (because the IPS must examine the data from all the packets until the successful match is made). For example, if you want to search for the string /etc/password in an HTTP URL, you might have to check multiple packets because the string can be distributed across more than one packet (although it can occur in a single packet as well).

Other examples of stateful signatures are the signatures used to monitor TCP traffic. To minimize the ability of an attacker to generate a large number of bogus alarms, most TCP attack signatures are valid only if the signature trigger is observed on a valid TCP connection. For example, suppose your signature triggers on the string /etc/password in a Telnet connection. Telnet uses TCP port 23, so the first thing that the IPS needs to track is established connections to TCP port 23. Then it also needs to track the sequence numbers for the established Telnet connections. Finally, whenever the string /etc/password is observed on an established TCP connection with the correct sequence numbers, then the signature triggers. Without maintaining this state, an attacker can generate a flood of invalid alarms by sending a flood of TCP packets to port 23 containing the string /etc/password (without ever actually establishing any valid TCP connections to port 23).

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