15th USENIX Security Symposium

Pp. 273–288 of the Proceedings

Behavior-based Spyware Detection

Engin Kirda and Christopher Kruegel
Secure Systems Lab
Technical University Vienna
{ek,chris}@seclab.tuwien.ac.at

Greg Banks, Giovanni Vigna, and Richard A. Kemmerer
Department of Computer Science
University of California, Santa Barbara
{nomed,vigna,kemm}@cs.ucsb.edu

Abstract:

Spyware is rapidly becoming a major security issue. Spyware programs are surreptitiously installed on a user's workstation to monitor his/her actions and gather private information about a user's behavior. Current anti-spyware tools operate in a way similar to traditional anti-virus tools, where signatures associated with known spyware programs are checked against newly-installed applications. Unfortunately, these techniques are very easy to evade by using simple obfuscation transformations.

This paper presents a novel technique for spyware detection that is based on the characterization of spyware-like behavior. The technique is tailored to a popular class of spyware applications that use Internet Explorer's Browser Helper Object (BHO) and toolbar interfaces to monitor a user's browsing behavior. Our technique uses a composition of static and dynamic analysis to determine whether the behavior of BHOs and toolbars in response to simulated browser events should be considered malicious. The evaluation of our technique on a representative set of spyware samples shows that it is possible to reliably identify malicious components using an abstract behavioral characterization.

Keywords: spyware, malware detection, static analysis, dynamic analysis.

1 Introduction

Spyware is rapidly becoming one of the major threats to the security of Internet users [20,23]. A comprehensive analysis performed by Webroot (an anti-spyware software producer) and Earthlink (a major Internet Service Provider) showed that a large portion of Internet-connected computers are infected with spyware [1], and that, on average, each scanned host has 25 different spyware programs installed [7].

Different from other types of malware, such as viruses and worms, the goal of spyware is generally not to cause damage or to spread to other systems. Instead, spyware programs monitor the behavior of users and steal private information, such as keystrokes and browsing patterns. This information is then sent back to the spyware distributors and used as a basis for targeted advertisement (e.g., pop-up ads) or marketing analysis. Spyware programs can also ``hijack'' a user's browser and direct the unsuspecting user to web sites of the spyware's choosing. Finally, in addition to the violation of users' privacy, spyware programs are also responsible for the degradation of system performance because they are often poorly coded.

A number of anti-spyware products, whose goal is the identification and removal of unwanted spyware, have been developed. These tools are mostly based on the same technology used by anti-virus products. That is, they identify known spyware instances by comparing the binary image of these programs with a number of uniquely-characterizing signatures. These signatures are manually generated by analyzing existing samples of spyware. As a consequence, these anti-spyware tools suffer from the same drawbacks as signature-based anti-virus tools, including the need for continuous updating of their signature set and their inability to deal with simple obfuscation techniques [4].

This paper presents a novel spyware detection technique that overcomes some of the limitations of existing anti-spyware approaches. Our technique is based on an abstract characterization of the behavior of a popular class of spyware programs that rely on Internet Explorer's Browser Helper Object (BHO) and toolbar interfaces to monitor a user's browsing behavior. More precisely, our technique applies a composition of static and dynamic analysis to binary objects to determine if a component monitors users actions and reports its findings to an external entity. This characterization is independent of the particular binary image and therefore can be used to identify previously unseen spyware programs, and, in addition, it is resilient to obfuscation.

The main contributions of this paper are as follows:

We introduce a novel characterization of the behavior of spyware components that are implemented as Browser Helper Objects or toolbars.
We present novel static and dynamic analysis techniques to reliably identify malicious behavior in Browser Helper Objects and toolbar components.
We present experimental results on a substantial body of spyware and benign samples that demonstrate the effectiveness of our approach.

The remainder of this paper is structured as follows. In Section 2, we present related work in the field of behavior-based malware detection in general and spyware detection in particular. Section 3 provides some background information on Browser Helper Objects and toolbars. It also shows how they are exploited by spyware programs to monitor user behavior and to hijack browser actions. In Section 4, we describe our abstract characterization of spyware-like behavior. In Section 5, we motivate the use of static and dynamic analysis for spyware detection, and we provide the details of our technique in sections 6 and 7. Section 8 discusses possible limitations of our proposed system. In, Section 9 we provide an experimental evaluation of the effectiveness of our technique. Finally, Section 10 briefly concludes and outlines future work.

2 Related Work

Spyware is difficult to define. There are many types of spyware that behave in different ways and perform actions that represent different levels of ``maliciousness.'' For example, ``adware'' programs that present targeted advertisements to the user are considered less malicious than other forms of spyware, such as key-loggers, which record every single key pressed by the user. Regardless of the type of privacy violation performed, spyware is generally undesirable code that the user wants to remove from his/her system.

The increasing sensitivity of consumers to the spyware problem prompted a number of anti-spyware commercial products. For example, both AdAware [2] and SpyBot [22] are popular tools that are able to remove a large number of spyware programs. Recently, Microsoft released a beta version of an anti-spyware tool, aptly called Windows AntiSpyware [14].

Current spyware detection tools use signatures to detect known spyware, and, therefore, they suffer from the drawback of not being able to detect previously unseen malware instances. This is a deficiency shared by other malware-detection tools, such as anti-virus products and many network-based intrusion detection systems [17,19]. Recently, researchers have tried to overcome these limitation by proposing behavior-based malware detection techniques. These techniques attempt to characterize a program's behavior in a way that is independent of its binary representation. By doing this, it is possible to detect entire classes of malware and to be resilient to obfuscation and polymorphism.

For example, in [5] the authors characterize different variations of worms by identifying semantically equivalent operations in the malware variants. Another approach is followed in [11], which characterizes the behavior of kernel-level rootkits. In this case, the authors use static analysis to determine if a loadable kernel module is accessing kernel memory locations that are typically used by rootkits (e.g., the system call table).

There are many advantages to representing malware in an abstract way. For instance, by using behavioral characterizations to detect malicious applications, one can obviate the need for a large data base of signatures to identify each known piece of malware. Another important benefit is that the characterization is resilient to malware variants and allows for the detection of previously unseen malware instances. An example of using behavior characterization is Microsoft's Strider Gatekeeper [24]. This tool monitors auto-start extensibility points (ASEPs) to determine if software that will be executed automatically at startup is being surreptitiously installed on a system.

Our approach is similar to the one pursued by Strider Gatekeeper, because our goal is to model spyware-like behavior. Consequently, our behavioral classifications are not specifically tailored to a single spyware program, but, instead, they are able to detect entire classes of spyware applications. However, our technique is more powerful than the one used by Strider Gatekeeper, because it identifies a more general behavior pattern, which is the acquisition of private information and the leaking of this information outside the boundaries of the application. In addition, our technique uses a combination of static and dynamic analysis, which allows for a very precise characterization of the behavior of an application in reaction to browser events, thus, reducing the chance of false positives.

The use of static analysis to analyze the behavior of malware has been proposed previously in [5]. However, the technique proposed by Christodorescu et al. is tailored to detect different variations of the same malware (e.g., different versions of the NetSky worm). Our technique, instead, focuses on abstract, spyware-like behavior and is not limited to detecting just one spyware program and its variants.

The technique presented in this paper, however, is not completely general. We focus explicitly on one type of spyware, that is, malware that exploits the hooks provided by Microsoft's Internet Explorer to monitor the actions of a user. This is done by using the Browser Helper Object (BHO) interface or by acting as a browser toolbar object. Our initial focus is justified by the fact that the overwhelming majority of spyware has a component based on one of these two technologies. This is confirmed not only by our own experience in analyzing various spyware components but also by a recent study [24], which found that out of 120 distinct spyware programs, just under 90 used BHOs as an entry point to monitor user activity and approximately 46 used the IE toolbar mechanism (note that some spyware programs used both mechanisms). In addition to these findings, a US-CERT report [8] names BHOs as one of the more frequently used techniques employed by spyware along with browser session hijacking and stand-alone applications. Other forms of spyware (e.g., stand-alone applications) are not currently being addressed but are open questions for future work. Additionally, we recognize that spyware does not only affect the Microsoft Windows platform or the Internet Explorer browser. Mozilla products also have their problems [15], and have not been completely free from spyware [9,26]. Other platforms also contain ASEP hooks similar to those found in Windows as described in [24]. However, as of today the occurrence of spyware affecting other platforms and browsers is significantly lower than that affecting both Microsoft Windows and Internet Explorer. Still, the growing popularity of alternatives make this an important consideration for the future.

Because of the relevance of the Component Object Model (COM) architecture and the hooks that Internet Explorer provides, the next section presents some background material to help the reader who is not familiar with these concepts. The following sections then present the details of our detection technique.

3 Spyware, Browser Helper Objects, and Toolbars

Spyware authors have many options on a Windows host when it comes to looking for good vantage points from which to glean personal information about users. For example, Layered Service Providers, which sit between the application-level network APIs (Application Programming Interfaces) and the kernel, can filter network traffic and/or collect information about users. Another example is represented by background processes that are automatically executed at startup to monitor user actions. Because sensitive information is often accessed through web-based interfaces, browser plug-ins are another popular mechanism to collect sensitive data and monitor user actions. Internet Explorer plug-ins, and in particular Browser Helper Objects (BHOs) and toolbars, as mentioned previously, are used in the majority of spyware programs as a mechanism to access information about a user's browsing habits or to control the browser's behavior.

Browser Helper Objects and toolbars are binary objects that conform to the Component Object Model (COM). COM is a binary standard developed by Microsoft to support, among other things, a component-based software market [25]. Every COM object implements a set of interfaces, each of which is a well-defined contract that describes what functionality the object provides. The COM standard guarantees that the virtual tables of interfaces remain the same across compilers, allowing COM objects to be implemented and used by any language that supports calling functions through a table of function pointers. The IUnknown interface must be implemented by all COM objects. It contains reference-counting functionality and the function QueryInterface, which allows one to query for the other interfaces that an object might implement.

A Browser Helper Object is in essence a simple COM object that implements the IObjectWithSite interface. Toolbar objects work in a way similar to BHOs and, in addition, implement a few more interfaces and include a graphical component.

At startup, Internet Explorer loads all the BHOs that are registered as COM servers and whose Class Identifiers (CLSIDs) are included under the registry key HKLMSOFTWAREWindowsCurrentVersionExplorerBrowser Helper Objects. Toolbars are loaded in a similar fashion with their CLSIDs being present under several other keys. Then, for each loaded BHO or toolbar, the browser calls the SetSite method exported by the IObjectWithSite interface, passing a reference to the browser's IUnknown interface as a parameter. This reference can then be used by a BHO or toolbar to query for other interfaces implemented by the browser. Interfaces of interest include IWebBrowser2, which allows a BHO or toolbar to access the current document and Uniform Resource Locator (URL) as well as to load specific pages, and IConnectionPoint, which allows a BHO to monitor the browser events specified in DWebBrowserEvents2. Toolbar objects access browser interfaces in a similar way.

By invoking the methods provided by the browser interfaces, BHOs and toolbars can completely control the browser's behavior and access sensitive data entered by the user during navigation. Because of this, BHOs and toolbars are often used as the core components in spyware applications.

4 Spyware Characterization

A distinctive characteristic of spyware is the fact that a spyware component (or process) collects data about user behavior and forwards this information to a third party. Since we restrict our focus to spyware that is implemented as a BHO or toolbar, we have to identify the mechanisms that these components employ (i) to monitor user behavior and (ii) to leak the gathered data to the attacker.

The most straightforward mechanism to monitor user behavior is to subscribe to browser events (using the browser's IConnectionPoint interface). When subscribing to these events, a component is notified in response to almost all browser actions or state changes. For example, events are generated when a new URL is opened, when a requested page cannot be found, or when the download of a resource has completed. In response to an event, the browser extension can request more information using the COM interfaces offered by the browser. In particular, the BHO or toolbar can request a handle to the document that was accessed, so that it can perform further analysis. In addition, a BHO or toolbar can react to an event by directing the browser to another page or opening a pop-up window. In summary, we expect to see the spyware component interact with the web browser by invoking browser functions in response to events.

After the spyware component has extracted the desired information, its next task is to transmit the data to a third party (the attacker). To this end, the information must be either directly transfered over the network, passed to a cooperating process running on the same host, or stored locally (for example, in a file on disk or in the Windows Registry). In any case, the spyware component has to interact with the operating system to be able to deliver the data to the intended recipient. ¹ Even when data is temporarily kept in memory, eventually an operating system service has to be invoked to leak the data.

Because the interaction with the operating system is necessary for a spyware component, we analyze the operating system services that the component requests. In particular, we focus on the Windows API ² calls that a component can use to leak information from the current process. Again, we are only interested in the Windows API calls that are performed in response to events. The reason is that for a Browser Helper Object, event handling code is the only code that is executed after the component's startup phase. With a toolbar, code can also be executed when the user clicks on a user interface element belonging to the toolbar. However, there is the significant risk that the web browser will be closed without ever making use of the toolbar. In this case, all information would be lost because the spyware code is simply not run. Thus, information can only be reliably leaked in event handling code (the last chance at which this is possible being in response to the event that signals the browser is being closed).

There is one important exception to the assumption above: A spyware component could attempt to set up an additional path through which events can be leaked by starting another thread or by registering a timer with an appropriate callback function. In response to a browser event, the collected user information would not be leaked immediately, but it would first be stored in a globally accessible data structure. Later, the second thread or the timer callback function could flush out this information without being detected. To eliminate this possibility for evading detection, we chose a conservative approach. That is, whenever a component can create either a thread or a timer, all Windows API calls that this component can invoke are included in the analysis, not only those invoked in response to events.

We realize, of course, that benign components may interact with the web browser in response to events to provide some service to the user, such as automatically displaying the page source in a separate frame. Benign components may also interact with the operating system (via Windows API calls). For example, a component could read some configuration parameters from a file, write entries to a log-file, or download updates to the software at startup. The key insight here is that each of the two characteristics on their own does not generally warrant suspicion, but together they are strong indicators of malicious behavior. Therefore, we classify a browser helper object or a toolbar as spyware if the component, in response to browser events,

monitors user behavior by interacting with the web browser and
invokes Windows API calls that can potentially leak information about this behavior (e.g., calls to save the data to a file or transmit information to a remote host).

Note that our classification is more general than the one used by virus scanners and signature-based intrusion detection systems, as we are looking for intrinsic behavioral characteristics of spyware instead of byte strings specific to particular malware instances.

Our detection approach can take advantage of the proposed spyware classification in one of two ways. First, we can compare the results of our analysis (that is, the identification of BHO/toolbar-to-browser and BHO/toolbar-to-OS interactions in response to events) to an a priori assembled list of browser COM functions and Windows API calls that we deem malicious. In this way, we define our behavioral characterization based on prior experience and use this characterization to detect previously unknown instances of spyware. The second method is to automatically generate a behavioral characterization by comparing the behavior of known benign components to the behavior of known malicious ones. More precisely, the characterization is automatically derived by identifying the browser functions and Windows API calls performed by malicious BHOs and toolbars that are not also executed by benign samples. This characterization would then be used to identify previously unseen spyware samples.

In the following, we apply the first approach described above. We then use the second approach as a way to validate our choice of characterizing browser functions and Windows API calls.

5 Component Analysis

Given our characterization of spyware, the task of the analysis phase is to extract the behavior of an unknown BHO or toolbar. That is, we are interested in the interaction of an unknown sample with the browser and with the operating system in response to browser events. Based on the results of the analysis, we can then classify the sample appropriately.

As a first step, we propose a dynamic analysis technique that exposes a suspicious component to crafted browser events (which simulate user activity) and analyzes the component's response. In particular, we dynamically record both the browser COM functions and the Windows API functions that the component calls. This approach exhibits some commonalities with black-box testing in that data is sent to a component under examination and its behavior is analyzed, without knowing anything about the component's implementation.

An interesting problem is to determine events and input that are suitable to capture the behavior of a component. When performing black-box testing, it is typically a significant challenge to devise a set of test inputs that exercise the bulk of the functionality (or code paths) of the object under test. In our case, the situation is exacerbated by the fact that we have no a priori knowledge about the functionality of the (potentially malicious) component. Thus, it may be difficult to generate input that will reveal spyware-like behavior with a sufficiently high probability. Consider, for example, a spyware component that scans all web pages that are fetched by the browser for the occurrence of certain keywords (e.g., ``car insurance''). The corresponding URL is logged to a file only when these keywords appear on the page. Thus, we would observe suspicious behavior (in the form of a file system call) only if one of the test pages actually contains the words ``car insurance.'' Another problem is that spyware often does not react consistently to identical events. For example, a spyware developer might decide that a user would be exceedingly annoyed if a pop-up window containing an advertisement appeared every time a page with a specific keyword was accessed. Therefore, the pop-up would be opened only occasionally, and, as a consequence dynamic black-box testing alone might not be enough to observe all interesting reactions.

Before discussing appropriate extensions to our analysis, an important point in the previous discussion is the fact that the lack of coverage in the dynamic test affects (almost) exclusively the interaction of the component with the operating system and not its interaction with the web browser. The reason is that a component has to monitor the user behavior by interacting with the web browser before any decision can be made. For example, before a page can be scanned for the occurrence of a certain keyword, the spyware component has to first request the page source from the browser. Also, a spyware component might not necessarily log all URLs that a user visits. However, before the decision to write the URL to a log-file can be made, the visited URL must be retrieved from the browser. A spyware component might decide to record only certain pieces of information or engage with the environment only under certain circumstances. However, as a first step, it is always necessary to extract information about the current document from the web browser. Thus, the results of the dynamic analysis with regards to the interaction with the web browser are sufficiently comprehensive.

To capture all possible reactions of a component with regards to its interaction with the operating system, we complement the dynamic testing with a static analysis step. To be more precise, we use dynamic analysis to locate the entry point into the code of the component that is responsible for handling events (i.e., the object's Invoke function). Starting from this entry point, the static analysis step extracts the control flow graph of all code regions that are responsible for handling events. The key observation is that this control flow graph contains (or encodes) all possible reactions of the component to events. As a consequence, there is no need to confine the analysis to the API calls that are actually observed during dynamic testing. Instead, we can analyze all Windows API calls that this component can possibly invoke when receiving events. Following up on the example of the spyware that scans for ``car insurance'' in fetched pages, we extract the control flow graph of the code that handles a ``web page received'' event during our analysis and determine that the component might perform a file system access under certain circumstances. Note that it is necessary to confine the static analysis to those code regions that are responsible for handling events. Otherwise, the analysis could end up including API function invocations that cannot be used for leaking information (e.g., API calls during program startup). Taking into account these additional function calls could lead to incorrect classification results.

As static analysis is already required to identify the interaction of a component with the operating system, one might consider dropping the dynamic analysis step. That is, one could attempt to solely rely on static analysis to recover the interaction of a component both with the web browser and with the operating system. This is difficult for a number of reasons. One problem is that Browser Helper Objects or toolbars often contain a number of different COM objects. Usually, it is not a priori evident which of these objects will register for browser events. Even when the correct COM object can be identified, locating the code regions that are responsible for handling events is not straightforward. Finally, COM function invocations are implemented as indirect function calls (via the COM object's virtual function table). Thus, it would be significantly more difficult to statically recover the interaction of the BHO or toolbar with the web browser.

To summarize, both static and dynamic analysis techniques have their advantages. The dynamic analysis step can precisely pinpoint the COM object and the code regions that handle browser events. In addition, the interactions between a component and the web browser can be accurately captured. Static analysis, on the other hand, is more comprehensive in identifying possible interaction of a component with the operating system. Thus, we believe that a combination of dynamic and static analysis is most suitable to analyze the behavior of unknown browser helper objects and toolbars.

In the previous discussion, we have not considered the fact that a spyware component might react differently to different events. That is, the dynamic analysis step produces a single list of COM browser functions that are called by the component under test, and the static analysis step yields a single list of Windows API calls that can be reached in response to events. However, it might be beneficial to distinguish between responses associated with different events. For example, a component might perform a suspicious Windows API call only in response to a certain event that conveys no relevant information about a user's actions (e.g., an event that signals that the browser window has been resized). In this case, the suspicious API call may not leak any information, and excluding it from the list could reduce false positives. Unfortunately, restricting automatic analysis to certain relevant events offers spyware a way to evade detection. To see this, consider a spyware component that monitors user behavior (via browser COM functions) in response to events, but, instead of immediately leaking this information, stores it temporarily in memory. Later, the collected information is flushed in response to an event that is not considered by our analysis. Therefore, we decide to take the more conservative approach in order to be more resilient to evasion.

As was mentioned in the previous section, our technique makes use of a more general characterization of malware behavior than that used by current signature-based systems. However, the methods used to realize this detection strategy are not without their drawbacks. While both our tool and the signature-based systems can be classified as detection engines, the latter are much faster and are practical for common desktop usage, while our technique is more appropriate for analysts who are looking to classify unknown instances of malware in an automated fashion.

In the following sections, we explain in detail the dynamic and static analysis steps that we perform to determine behavioral characterizations for spyware. We then discuss possible methods of evasion for our technique. Finally, we present experimental data that demonstrates that our detection technique is capable of effectively distinguishing malicious and benign components.

6 Dynamic Analysis Step

The basis for our detection technique, as discussed in the previous sections, is to extract behavioral characterizations based on how spyware interacts with (i) the browser and (ii) the underlying operating system in response to events. To this end, we make use of both dynamic and static analysis techniques.

The goal of the dynamic analysis step is twofold. First, it has to monitor the interaction of the component with the browser and record all the browser's COM functions that are invoked in response to events. Second, it has to determine the code regions that are responsible for handling events, thereby providing the necessary starting points for the static analysis step. These tasks are accomplished with the help of three core elements.

The first element is a ``fake'' WebBrowser COM object, which provides the component under analysis with an environment similar to the one that would be present when being hosted by an instance of Internet Explorer. The second element is our COM object host application, which properly instantiates all involved components and sends the relevant browser events to the BHO or toolbar component under evaluation. The final element is a program that traces the execution of our host application to extract those code regions that handle the various browser events that are delivered.

6.1 Recording Browser Function Calls

The ``fake'' WebBrowser and the host application provide a controlled environment in which we can instantiate a suspicious component, send events, and monitor the component's reaction. The purpose of the ``fake'' WebBrowser COM object is to host the component under analysis. This involves the provision of an environment that is ``convincing'' to a BHO or toolbar. To this end, the WebBrowser element must offer all Internet Explorer functionality expected by the browser extensions. Otherwise, a BHO or toolbar might fail during initialization, preventing any further analysis. Therefore, the WebBrowser COM object implements several key interfaces expected by a BHO or toolbar. Most importantly, it implements IConnectionPointContainer, IConnectionPoint, and IWebBrowser2. The two interfaces related to ConnectionPoint are required so that a BHO or toolbar is able to notify our WebBrowser COM object about its interest in receiving events (using the IConnectionPoint::Advise() function). The IWebBrowser2 interface is the main interface used to interact with the browser. More precisely, a browser component invokes the functions of this interface to collect browsing information such as the current page or URL and to influence browser behavior. Since we are interested in the interaction between a possible spyware component and the browser, we pay particular attention to calls to the IWebBrowser2 interface.

The WebBrowser element also implements several other interfaces that are expected by a toolbar. These include IOleWindow, IInputObjectSite, IOleCommandTarget, and IServiceProvider. Note that implementing an interface does not necessarily imply that it is necessary to faithfully simulate the functionality of all its procedures. Instead, we usually provide a stub for every function. This stub always returns success and logs the invocation. However, for certain frequently-used functions (e.g., those that request the document or the location of the current page), appropriate objects are returned.

The host application provides the ``glue'' that will hold the various components together. To this end, the program registers the component under analysis with the Windows operating system, initializes the COM library, and instantiates both our WebBrowser and the BHO or toolbar.

Before events can be sent to a component, its SetSite method has to be called (with a pointer to the IUnknown interface of our WebBrowser as an argument). If the component is actually interested in receiving events, it will respond by querying for the WebBrowser's IConnectionPoint interface and call its Advise function. At his point, the host application can obtain a reference to the IDispatch interface implemented by the browser extension and start to send events. Note that all events are delivered through a single function (the Invoke method of the IDispatch interface), using the first parameter of the function to indicate the type of the event.

To be able to generate events that are as realistic as possible, we recorded an actual event stream created by Internet Explorer over the course of five days. To this end, we developed our own BHO that logged all relevant event information while the browser was used. These events were then replayed to perform our dynamic tests.

6.2 Locating Event-Handling Code

Given the infrastructure to send browser events to a Browser Helper Object or toolbar, the next task is to determine the regions of the code that handle events. Moreover, we are interested in determining the separate code regions associated with each event. Then, we can use static analysis to extract the control flow graphs that correspond to these events.

If each different type of event would be passed to a separate function, the start address for the static analysis process could be easily determined as the start address of the respective function. Unfortunately, as mentioned in the previous section, all events are delivered through the single Invoke function. Thus, if we were to use the start address of the Invoke function, we would be unable to determine which API calls are associated with which events. To obtain the API calls made in response to a specific event, we have to look deeper into the component under analysis and find the first instruction in the code that is responsible for handling the event. We call this instruction the first event-specific instruction. Of course, it is also possible that a component is not interested in a certain event and provides no special handling code. The execution typically runs through a default path, ignoring the information contained in the event. In this case, there is no event-specific code.

To determine the first event-specific instructions, we collect execution traces of the BHO or toolbar when processing different events. That is, we send one event of each type to the component and record the corresponding sequences of machine instructions that are executed in response. To record the machine instruction traces, we use the Windows Debug API [21]. The Windows Debug API offers an interface that is comparable to the ptrace mechanism provided by some UNIX implementations, and it provides a parent process with complete control over the execution of a child process. This includes the possibility to read and write the registers as well the address space of the child process, which allows one to set breakpoints or run a process in single-step mode. By switching to single-step mode before sending a browser event, we can record each executed machine instruction and obtain the desired traces.

The tracing component collects an execution trace for each of the events being analyzed. Then the application performs a pairwise comparison between all traces. The idea is to identify the first event-specific instruction for each event by checking for the first instruction that is unique to the corresponding trace. More formally, a trace for an event can be considered as a string whose symbols are the (addresses of the) instructions that are executed. To identify the first event-specific instruction in , we determine the longest common prefixes between and all other traces $t_i : 0 \le i < n, e \ne i$ . Assuming that the longest of the prefixes has a length of , then the $(l + 1)^{\mbox{st}}$ instruction in trace is the first event-specific one. The rationale behind this approach is that we search for the first instruction for which the trace deviates from all other traces. As a consequence, when two or more traces contain the same sequence of instructions, then these traces have no event-specific instructions and are considered to represent the default path (as the different types of events had no influence on the execution).

**Figure 1:** Dynamic traces for different types of events.
$\scalebox{0.7}{\includegraphics{traces}}$

Consider the example shown in Figure 1. Note that in this case, we demonstrate the identification of event-specific instructions using source code. However, the real analysis is done on binary code. The figure shows the traces generated for five events. As expected, the correct event-specific instructions are found for the first three events (line 4 for event A, line 6 for event B, and line 8 for event C), while the last two events (D and E) represent the default path. Note that even though the first instruction in trace B that is different from trace A is on line 5, there is a longer common sequence of this trace with trace C (as well as D and E). Thus, the event-specific instruction for event B is determined to be on line 6.

When collecting traces for Browser Helper Objects or toolbars, only the instructions that are executed in the context of the component itself are used to determine event-specific instructions. Thus, we remove all instructions that belong to dynamically loaded libraries from the traces. The reasons are twofold: First, we are interested in finding the first unique instruction within the component for the static analysis process. Second, a library can contain initialization code that is executed when one of its functions is used for the first time. This introduces spurious deviations into the traces that do not correspond to actual differences in the code executed by the BHO or toolbar component.

In addition to restricting our analysis to code within the BHO or toolbar, subsequent repetitions of identical instruction sequences that are executed as part of a loop are collapsed into a single instance of this sequence. The reason is that we occasionally observed that the traces for two events were identical before and after a loop, while the loop itself was executed for a different number of times in each case. This happened, for example, with a spyware component that was going through an array of identifiers to determine whether the current event (given its identifier) should be processed. For different event identifiers, the loop terminated after a different number of iterations because the respective event identifiers were found at different positions in the array. However, for both traces execution continued on the same path for a number of instructions until control flow eventually branched into the event-specific parts. In such situations, collapsing multiple loop iterations into one allows us to identify the actual event-specific handling code.

7 Static Analysis Step

The goal of the static analysis step is to determine the interaction of a BHO or toolbar component with the operating system. To this end, we statically examine certain code regions of a component for the occurrence of operating system calls.

Before the component is actually analyzed, we check its API function import table for the occurrence of calls relevant to the creation of threads or timers (e.g., CreateThread or SetTimer). As explained in Section 4, if a component could launch threads or create timers, we have to conservatively assume that any imported API function can be invoked in response to an event. In this case, no further analysis of the binary is necessary because we can directly use the calls listed by the function import table. When neither of these functions is present, however, the static analysis step is required to identify those API functions that can be called in response to events.

The first task of the static analysis step is to disassemble the target binary and generate a control flow graph from the disassembled code. A control flow graph (CFG) is defined as a directed graph in which vertices $u,v \in V$ represent basic blocks and an edge $e \in E: u \rightarrow v$ represents a possible flow of control from to . A basic block describes a sequence of instructions without any jumps or jump targets in the middle. We use IDA Pro [6] to disassemble the binary. Since IDA Pro is a powerful disassembler that already provides comprehensive information about the targets of control flow instructions, the CFG can be generated in a straightforward manner using a custom-written IDA Pro plug-in. Note that if our detection technique were to be deployed in the general public, the disassembly and CFG generation would be done using a custom disassembler optimized for our task. During our experiments, we encountered a number of spyware samples that were compressed with UPX [16], a packer tool for executables. If this was the case, we uncompressed the samples prior to performing static analysis (using the available UPX unpacking utility). Otherwise, IDA Pro would not be able to extract any valid instructions.

Based on the CFG for the entire component, the next step is to isolate those parts of the graph that are responsible for handling events. In particular, we are interested in all subgraphs of the CFG that contain the code to handle the different events. To this end, we use the event-specific addresses collected during dynamic analysis and traverse the entire subgraph reachable from each of those addresses. While traversing the graph, the static analysis process inspects all instructions to identify those that represent operating system calls. More specifically, we make a list of all possible Windows API calls that can be reached from each event-specific address. Finally, the event specific lists are merged to obtain a list of all API calls that are invoked in response to events. At this point, the analysis process has collected all the information necessary to characterize the component (i.e., browser COM functions and Windows API calls executed in response to events).

Note that while the Windows API is the common way to invoke Microsoft Windows services, current versions of Windows (starting with Windows NT and its successors) also offer a lower-level interface. This interface is called the Windows NT Native API, and it can be compared to the system call interface on UNIX systems. Both the Native API kernel interface and the Windows API are offered to accommodate the micro-kernel architecture of Windows. That is, instead of providing one single operating system interface, Windows NT offers several different operating system interfaces (e.g., OS/2, DOS, POSIX). This allows one to execute applications that were developed for different operating systems. The different OS interfaces are implemented by different operating environment subsystems, which are essentially a set of system-specific APIs implemented as DLLs that are exported to client programs. All subsystems are layered on top of the Native API, with the Windows API being the most popular subsystem. Because applications typically use the Windows API and not the Native API, we monitor calls to the Windows API to capture the behavior of components under analysis. However, to assure that no spyware can bypass our detection technique by relying directly on the Native API, any direct access to this interface is automatically characterized as suspicious.

8 Evading Detection

In this section, we discuss the limitations of our detection technique. In particular, we explore possible mechanisms that a spyware author can use to evade detection and countermeasures that can be taken in order to prevent such evasion.

Before revisiting our technique, it is important to note that due to the nature of the component object model a component that ``plugs'' into our WebBrowser component must use the interfaces it exposes (or the ones it expects Internet Explorer to expose) in order to extract information about the user at runtime. These interfaces are well documented and are essentially a contract between the COM client and the COM server. Since we control our WebBrowser component, we see all the interface calls and the queries for different interfaces. Just as our a priori list of suspicious Windows API calls is subject to change as we discover new suspicious calls, so are the various COM functions and interfaces used. This contract also applies to events. If a component wants to receive events from the WebBrowser it must call the Advise function on the WebBrowser's IConnectionPoint interface. Since we are in control of our WebBrowser object we can monitor calls to this interface and reliably discover if a component is interested in events and, if it is, the address of the function that handles those events.

Recalling our behavioral characterization of spyware, we note that a BHO or toolbar component must both collect user data via browser functions and leak this information to the adversary via Windows API calls. Thus, to evade detection, a malware author could either attempt to hide the fact that the BHO monitors user data via browser COM calls, or disguise the fact that the collected data is leaked.

Covering the footprints that indicate user data is being collected is likely the more difficult task. We use dynamic analysis to monitor all the functions that the BHO component invokes in our web browser. To avoid invoking the browser functions, a spyware component could attempt to read interesting user data directly from memory. This is possible because both the BHO and the web browser share the same address space. However, this is difficult because a non-standard access to memory regions in a complex and undocumented COM application, such as Internet Explorer, is not likely to yield a robust or portable monitoring mechanism. Thus, reading data directly from memory is not considered to be a viable approach.

A more promising venue for a spyware component to evade our detection is to attempt to conceal the fact that data is leaked to a third-party via API calls. We have previously mentioned the possible existence of covert channels, and concluded that their treatment is outside the scope of this paper. However, a spyware component could attempt to leak information using means other than API calls, or it could prevent the static analysis process from finding their invocations in the code of the BHO.

One possible way to leak information without using the Windows API is to make use of the functionality offered by Internet Explorer itself. For example, a spyware component could use the Internet Explorer API to request a web resource on a server under the control of the attacker. Sensitive information could be transmitted as a parameter of the URL in this request. The current limitation of not taking browser calls into account can be addressed in two ways. First, we could extend the static analysis step to also flag certain COM calls to the browser as suspicious. The problem with this solution is that COM calls are invoked via function pointers and, thus, are not easily resolvable statically. The second possibility would be to extend the dynamic analysis step. We already record the browser functions that a BHO invokes to determine when user data could be leaked. Thus, it would be straightforward to additionally take into consideration browser calls that a component invokes after user data has been requested. However, for this, one has to enlarge the set of test inputs used for the dynamic analysis step to ensure better test coverage.

As mentioned previously, another evasion venue is to craft the BHO code such that it can resist static analysis. Static analysis can be frustrated by employing anti-disassembling mechanisms [13], or code obfuscation. If these techniques are used, then our static analysis step could be forced into missing critical Windows API calls that must be recognized as suspicious. Again, we have two options to deal with this problem. First, the static analysis step could be made more robust to tolerate obfuscation (e.g., by using a disassembler that handles anti-disassembler transformations [12]). Also, strong obfuscation typically leads to disassembly errors that in itself can be taken as sufficient evidence to classify a component as spyware. A second approach is to expand the dynamic analysis step to also monitor Windows API functions. This could be achieved by hooking interesting API calls [10] before the spyware component is executed. Using these hooks, all Windows API calls made by the spyware component could be observed. Again, the set of test data would have to be enlarged to improve test coverage.

While there are a number of possible ways that a spyware component could attempt to evade our current detection system, we have shown how to counter these threats. Furthermore, in the next section we show how in its current form our system was successful in correctly identifying all spyware components that we were able to collect. Thus, our proposed techniques significantly raise the bar for spyware authors with respect to traditional signature based techniques.

9 Evaluation

In order to verify the effectiveness of our behavior-based spyware detection technique, we analyzed a total of 51 samples (33 malicious and 18 benign); 34 of them were BHOs and 17 were toolbars. The process of collecting these samples in the ``wild'' is both a tedious and non-trivial task. This is confirmed by a recent study [3] in which the authors traversed 18,237,103 URLs discovering 21,200 executables, of which there were just 82 unique instances of spyware as identified by popular spyware scanners. The problem is further exacerbated by the fact that popular spyware dominates the set of infected files, making it hard to obtain a well rounded collection. Thus, we obtained all of the malicious samples in our final test set from an anti-virus company and collected all of the benign samples from various shareware download sites. Note that we picked all samples (benign and malicious) that we collected and that either registered themselves as BHOs or as toolbars. While collecting the benign samples, we verified that the applications were indeed benign by checking both anti-spyware vendor and software review web sites. Furthermore, we selected samples from different application areas, including anti-spyware utilities, automated form-fillers, search toolbars, and privacy protectors. Note that our tool was developed while analyzing only seven (two benign and five malicious) samples from our final test set. The remaining samples were effectively unknown, with respect to our tool, thereby validating the effectiveness of our characterization on new and previously unseen malware components. Given the difficulty of collecting samples, we consider this to be a well rounded and significant sample set with which to evaluate our technique.

Table 1: Results for different detection strategies.

Detection Strategy	Spyware Components		Benign Components
	Correct	Incorrect	Correct	Incorrect
1. All Windows API calls (static)	33	0	0	18
2. Windows API calls in response to events (static)	33	0	13	5
3. Subscription to browser events (dynamic)	33	0	10	8
4. Browser COM method invocations (dynamic)	33	0	15	3
5. Combined static and dynamic analysis	33	0	16	2

Table 1 presents our detection results in terms of both correctly and incorrectly classified samples. In addition to the detection results for our proposed combined approach, this table also includes the results that are achievable when taking into account the information provided by only the static analysis or only the dynamic analysis step. In particular, we show detection results when the classification is solely based on statically analyzing all API calls invoked by the sample (Strategy 1) or only those API calls in response to events (Strategy 2). Moreover, we present the results obtained when a BHO or toolbar sample is classified as spyware if it subscribes to browser events (Strategy 3) or solely based on its interaction with the browser via COM functions (Strategy 4). Finally, Strategy 5 implements our proposed detection technique, which uses a composition of static and dynamic analysis. The aim is to demonstrate that the combined analysis is indeed necessary to achieve the best results.

Given our detection results, it can be seen that malicious spyware samples are correctly classified by all five strategies, even the most simple one. Since every strategy focuses on the identification of one behavioral aspect present in our characterization of spyware, these results indicate that the proposed characterization appears to accurately reflect the actual functioning of spyware. However, simple strategies also raise a significant number of false alarms. The reason is that certain behavioral aspects of spyware are also exhibited by benign samples. In the following paragraphs, we discuss in more detail why different detection strategies incorrectly classify certain samples as malicious. The discussion sheds some light on the shortcomings of individual strategies and motivates the usage of all available detection features.

**Figure 2:** Excerpt of *a priori* assembled list of suspicious Windows API calls.
$\scalebox{0.4}{\includegraphics{listing.eps}}$

As mentioned in Section 4, we need a list of Windows API calls that contains all suspicious functions that can be used by a spyware component to leak information to the attacker. As a first step, we manually assembled this list by going through the Windows API calls, in particular focusing on functions responsible for handling network I/O, file system access, process control, and the Windows registry. Figure 2 shows an excerpt of the 59 suspicious calls that were selected. The calls that are depicted are representative of commonly used registry, file access, and networking functions.

The first detection strategy (Strategy 1) uses the list of suspicious API functions to statically detect spyware. To this end, static analysis is used to extract all API calls that a sample could invoke, independent of events. This can be done in a straightforward fashion, using available tools such as PEDump [18]. Then, the extracted API calls are compared to the list of suspicious functions. A sample is classified as spyware if one or more of the sample's API calls are considered suspicious.

Using the first strategy, all benign samples are incorrectly detected as spyware. In many cases, samples require Windows registry, file, or network access during the startup and initialization phase. In other cases, benign samples such as the Google search toolbar use suspicious calls such as InternetConnectA to connect to the Internet (in the case of the Google toolbar, the sample sends search queries to Google). However, such calls are typically not done in response to events; in fact, many samples do not even register for browser events.

If we restrict the static analysis to only those Windows API calls that are invoked in response to browser events, only five of the 18 benign samples are incorrectly classified (Strategy 2). Two of these false positives are easy to explain. One is a BHO called Airoboform, a tool that supports users by filling in web forms automatically. In response to every event that signals that a new page is loaded, this tool scans the page for web forms. If necessary, it loads previously provided content from a file to fill in forms or it stores the current form content to this file. Because web forms can also contain sensitive information (such as passwords), one can argue that Airoboform actually behaves in a way that is very similar to a spyware application. The only exception is that in the case of spyware, the file content would probably be transmitted to an attacker through an additional helper process.

Besides Airoboform, the benign Privacy Preferences Project (P3P) Client BHO also exhibits spyware-like behavior. P3P is emerging as an industry standard for providing a simple and automated way for users to control the use of their personal information on web sites they visit. To this end, the P3P Client has to check the P3P settings of every web page that is visited. More precisely, whenever the user visits a web site, the BHO connects to that site and tries to retrieve its P3P-specific privacy policy. This is implemented by opening a connection via the Windows API function InternetConnectA in response to the event that indicates that a document has been loaded.

Two other false positives are Spybot and the T-Online toolbar. In both cases, the static analysis results indicate that a suspicious WriteFile call could be invoked in response to some events. This would allow the browser extensions to write event-specific information into a file for later retrieval. Although writes to a file are generally suspicious in response to events, there are also cases in which such an action is legitimate. For example, we discovered that the T-Online toolbar, a benign application that allows users to send SMS messages, uses a caching mechanism to store images in files. Spybot, a benign anti-spyware application, uses black lists to block web access to spyware distribution sites and keeps a cache to track cookies. The fifth false positive is Microgarden, a BHO that extends Internet Explorer with the ability to open multiple tabs in a single browser window. Although no suspicious API calls are invoked directly in response to events, this BHO makes use of timers. As a result, we have to conservatively consider all Windows API calls that this sample can possibly call (among which, a number of suspicious functions are found). The last three false positive examples suggest that the static analysis of Windows API calls may not deliver optimal detection results. Instead, one should seek to combine the results of our static analysis with those of our dynamic analysis to lower the number of false positives.

Taking a step back, a simple dynamic technique to identify spyware (Strategy 3) is to classify all BHO and toolbar components as malicious if they register as event sinks. As expected, all of the spyware samples receive browser events from Internet Explorer to monitor user behavior. In comparison, only eight of the 18 benign samples registered as event sinks. This observation suggests that many benign applications use BHO and toolbar extensions to improve Internet Explorer, but do not need to listen to events to implement their functionality. On the other hand, nearly half of the benign samples also use event information, for example, to display or modify the source of visited pages or to block pop-up windows.

**Figure 3:** *A priori* assembled list of suspicious COM browser functions.
$\scalebox{0.4}{\includegraphics{listing2.eps}}$

For Strategy 4, the dynamic analysis is extended to monitor the interaction of the BHO or toolbar with the web browser. As mentioned previously, this is realized by recording the invocation of COM functions provided by the IWebBrowser2 interface. To compile the list of suspicious COM functions, we analyzed this interface for functions that allow a browser extension to obtain information about the page or the location that a user is visiting. The complete list is shown in Figure 3. Of particular interest is the get_Document() method, which provides an IHTMLDocument2 pointer to the Document Object Model (DOM) object of the web page that is being displayed. Using this pointer, a BHO or toolbar can modify a page or extract information from its source.

Using the list of suspicious COM functions, dynamic analysis classifies a sample as spyware when at least one invocation of a suspicious function is observed in response to events. Unfortunately, this also results in more false positives than necessary. The reason is that several browser extensions interact with the browser in response to events. For example, the Lost Goggles toolbar requests a pointer to the DOM object of a loaded page to integrate thumbnails into search results returned by Google.

In our characterization of spyware, we claim that a malicious component both monitors user behavior and leaks this information to the environment. Thus, we expect the lowest number of false positives when employing a combination of dynamic and static analysis techniques. This is indeed confirmed by the detection results shown in Table 1 for Strategy 5. Compared to the results delivered by static analysis only, the misclassification of the benign Spybot and T-Online samples is avoided. The reason for this is that although these browser extensions might invoke a WriteFile API call in response to an event, the dynamic analysis confirms that they are not monitoring user behavior by calling any of the suspicious COM functions. Microgarden is also correctly classified as benign. Even though this toolbar uses timers, it does not access any relevant information in response to events. Airoboform and P3P Client, on the other hand, are still classified as spyware. The reason is that in addition to suspicious API calls, they also request the location of loaded pages via the get_LocationURL() function. However, as discussed previously, this is no surprise as these BHOs do indeed monitor surfing behavior and store (possibly sensitive) user information in files.

Table 2: Performance for different analysis steps.

Analysis Step	Execution Time
	Mean	Standard Deviation
1. Dynamic Analysis	30.97s	21.61s
2. Static Analysis (disassembly and CFG generation)	64.86s	137.94s
3. Static Analysis (per event CFG analysis)	80.01s	100.01s

Table 2 shows the various execution times for each step in the analysis on a 1.7 GHz Pentium M processor with 1 GB of RAM. The execution time for dynamic analysis may be slower than one might expect. This is due to the fact that this analysis must be done in a virtual environment because we must execute the possibly malicious code. Furthermore, once this code is invoked the performance of the machine tends to degrade significantly. The execution time for static analysis, on the other hand, is split in two. This is because the running time for static analysis is highly dependent on how many events a sample is listening for. Thus, we give the execution time for disassembly and CFG creation along with a separate measure for the execution time to analyze a single event. We consider these performance measures to be acceptable for a prototype analysis tool and note that the running times could be significantly improved with optimization.

9.1 API Call Blacklist Derivation

Until now, we have been using lists of suspicious Windows API calls and suspicious COM functions that were generated a priori. An alternate method, as discussed in Section 4, is to generate these lists automatically. More precisely, by applying our approach to both a set of known benign samples and a set of known malicious samples, one can cross-reference the two resulting sets of calls made in response to browser events (from both static and dynamic analysis), to identify calls that are frequently observed for spyware, but never observed for benign BHOs or toolbars.

**Figure 4:** Excerpts of extracted calls that (a) also appear in the *a priori* list and (b) are unique to the automatically derived list.
$\scalebox{0.4}{\includegraphics{generatedlisting.eps}}$

The major benefit of the automatic list generation approach is that it obviates the need to generate a list of suspicious calls a priori. Also, over time, as more samples are collected and analyzed, the list will become more refined, eliminating those calls that show up only in malicious samples by chance, and revealing new functions that were not considered before. These results are useful even in the case where one uses a list of calls generated a priori as the basis for detection, because there are a plethora of Windows API calls to consider, and the analysis can be used to update the ``suspicious function'' list with new calls as they begin to be utilized by spyware.

In the following, we briefly discuss our experience when automatically generating the list of suspicious Windows API functions. We refrained from applying automatic generation to the list of suspicious browser COM functions. The reason is that the IWebBrowser2 COM interface contains considerably less functions than the Windows API and these functions are well-documented, making this list more suitable for a priori compilation. Figure 4 shows an excerpt of where the Windows API list we generated a priori and the list we generated automatically converged (a), as well as some additional malicious API calls that were discovered (b). These lists do indeed match up well with our initial intuition. Interestingly, new calls that we did not originally consider, such as CreateToolHelp32Snapshot, which takes a snapshot of the processes currently running on a system and should probably not be called in response to browser events, can be added to the list of possibly malicious calls. The results indicate that our static list does a good job of detecting spyware, while our generated list can be used to further improve detection results as spyware authors try to adapt in order to evade detection.

Automated list generation, however, is not without its drawbacks. The reason is that we will likely be removing certain calls that do represent possible malicious intent. For example, when applied to our evaluation set, one of these calls would be the Windows API function WriteFile. Because WriteFile appears in both our benign and malicious sets of samples, we would disregard it as a common call that should not be taken into account when analyzing new and possibly malicious samples. This should reduce the number of false positives, but at the same time, it could result in an increase in the number of false negatives.

10 Conclusions and Future Work

Spyware is becoming a substantial threat to networks both in terms of resource consumption and user privacy violations. Current anti-spyware tools predominately use signature-based techniques, which can easily be evaded through obfuscation transformations.

In this paper, we present a novel characterization for a popular class of spyware, namely those components based on Browser Helper Objects (BHOs) or toolbars developed for Microsoft's Internet Explorer. This characterization is based on the observation that a spyware component first obtains sensitive information from the browser and then leaks the collected data to the outside environment. We developed a prototype detection tool based on our characterization that uses a composition of dynamic and static analysis to identify the browser COM functions and the Windows API calls that are invoked in response to browsing events. Based on this information, we are able to identify an entire class of spyware, thus making our approach more powerful than standard signature based techniques. In addition, our technique will provide a forensic analyst with detailed information about the behavior of unknown browser helper objects and toolbars.

Our approach was evaluated on a large test set of spyware and benign browser extensions. The results demonstrate that the approach is able to effectively identify the behavior of spyware programs without any a priori knowledge of the programs' binary structure, significantly raising the bar for malware authors who want to evade detection.

Future work will focus on extending our approach to spyware programs that do not rely on the Browser Helper Object or toolbar interfaces to monitor the user's behavior. We also plan to extend our characterization with more sophisticated data-flow analysis that would allow one to characterize the type of information accessed (and leaked) by the spyware program. This type of characterization would enable a tool to provide an assessment of the level of ``maliciousness'' of a spyware program.

Acknowledgments

This research was supported by the Austrian Science Foundation (FWF), under grant No. P18157, the Secure Business Austria competence center, the U.S. Army Research Office, under agreement DAAD19-01-1-0484, and by the National Science Foundation, under grants CCR-0238492 and CCR-0524853.

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About this document ...

Behavior-based Spyware Detection

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Footnotes

... recipient. ¹: Note that we are not taking into account the possibility of using covert channels to leak the information from the BHO or toolbar to the outside environment. Typically, these channels are either bandwidth-limited or generate distinctive patterns of execution.
... Windows API ²: The Windows API provides a large number of procedures that can be invoked to access the complete functionality of the Windows operating system (this includes kernel services, the graphical device interfaces, and other user interfaces).

Last changed: 21 June 2006 jel