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Technical Trends Affecting Software Security

Complex systems, by their very nature, introduce multiple risks. And almost all systems that involve software are complex. One risk is that malicious functionality can be added to a system (either during creation or afterward) that extends it past its primary, intended design. As an unfortunate side effect, inherent complexity lets malicious and flawed subsystems remain invisible to unsuspecting users until it is too late. This is one of the root causes of the malicious code problem. Another risk more relevant to our purposes is that the complexity of a system makes it hard to understand, hard to analyze, and hard to secure. Security is difficult to get right even in simple systems; complex systems serve only to make security harder. Security risks can remain hidden in the jungle of complexity, not coming to light until it is too late.

Extensible systems, including computers, are particularly susceptible to complexity-driven hidden risk and malicious functionality problems. When extending a system is as easy as writing and installing a program, the risk of intentional introduction of malicious behavior increases drastically—as does the risk of introducing unintentional vulnerabilities.

Any computing system is susceptible to hidden risk. Rogue programmers can modify systems software that is initially installed on the machine. Unwitting programmers may introduce a security vulnerability when adding important features to a network-based application. Users may incorrectly install a program that introduces unacceptable risk or, worse yet, accidentally propagate a virus by installing new programs or software updates. In a multiuser system, a hostile user may install a Trojan horse to collect other users' passwords. These attack classes have been well-known since the dawn of computing, so why is software security a bigger problem now than in the past? We believe that a small number of trends have a large amount of influence on the software security problem.

One significant problem is the fact that computer networks are becoming ubiquitous. The growing connectivity of computers through the Internet has increased both the number of attack vectors (avenues for attack) and the ease with which an attack can be made. More and more computers, ranging from home personal computers (PCs) to systems that control critical infrastructures (such as the power grid), are being connected to the Internet. Furthermore, people, businesses, and governments are increasingly dependent on network-enabled communication such as e-mail or Web pages provided by information systems. Unfortunately, because these systems are connected to the Internet, they become vulnerable to attacks from distant sources. Put simply, an attacker no longer needs physical access to a system to cause security problems.

Because access through a network does not require human intervention, launching automated attacks from the comfort of your living room is relatively easy. Indeed, the well-publicized denial-of-service attacks in February 2000 took advantage of a number of (previously compromised) hosts to flood popular e-commerce Web sites, including Yahoo!, with bogus requests automatically. The ubiquity of networking means that there are more systems to attack, more attacks, and greater risks from poor software security practice than ever before.

A second trend that has allowed software security vulnerabilities to flourish is the size and complexity of modern information systems and their corresponding programs. A desktop system running Windows/NT and associated applications depends on the proper functioning of the kernel as well as the applications to ensure that an attacker cannot corrupt the system. However, NT itself consists of approximately 35 million lines of code, and applications are becoming equally, if not more, complex. When systems become this large, bugs cannot be avoided.

Exacerbating this problem is the widespread use of low-level programming languages, such as C or C++, that do not protect against simple kinds of attacks (most notably, buffer overflows). However, even if the systems and applications codes were bug free, improper configuration by retailers, administrators, or users can open the door to attackers. In addition to providing more avenues for attack, complex systems make it easier to hide or to mask malicious code. In theory, we could analyze and prove that a small program was free of security problems, but this task is impossible for even the simplest of desktop systems today, much less the enterprise-wide systems used by businesses or governments.

A third trend exacerbating software security problems is the degree to which systems have become extensible. An extensible host accepts updates or extensions, sometimes referred to as mobile code, so that the system's functionality can be evolved in an incremental fashion. For example, the plug-in architecture of Web browsers makes it easy to install viewer extensions for new document types as needed.

Given a basic intuitive grasp of the architecture of a browser (a program that runs on top of an operating system and provides basic Web interface services), it is natural to assume that a browser may be used to enhance security. In reality, it is hard to tell where the boundaries of the browser are, where the operating system fits, and how a browser can protect itself. The two most popular browsers, Netscape Navigator and Microsoft Internet Explorer (MSIE), have very fuzzy boundaries and include many more hooks to external applications than most people realize.

On the most pervasive platform (Windows 95, 98, and Millennium Edition (ME)) there is really no way for a browser to protect itself or any secrets it may be trying to keep (like client-side certificates) at all. This means that if your design requires some security features inside the browser (like an intact Java Virtual Machine [ JVM] or a cryptographic secret), there is probably a real need for a more advanced operating system like Windows/NT or UNIX.

Without delving into the details of how a browser is constructed, it is worth showing a general-purpose abstract architectural diagram of each. Figures 1–2 and 1–3 show Netscape and MSIE respectively. From a high-level perspective, it is clear that there are many interacting components involved in each architecture. This makes securing a browser quite a monumental task. In addition, helper applications (such as AOL Instant Messenger for Netscape and ActiveX control functionality in MSIE) introduce large security risks.

Browsers are not the only extensible systems. Today's operating systems support extensibility through dynamically loadable device drivers and modules. Current applications, such as word processors, e-mail clients, spreadsheets, and (yes) Web browsers, support extensibility through scripting, controls, components, dynamically loadable libraries, and applets.

From an economic standpoint, extensible systems are attractive because they provide flexible interfaces that can be adapted through new components. In today's marketplace, it is crucial that software be deployed as rapidly as possible to gain market share. Yet the marketplace also demands that applications provide new features with each release. An extensible architecture makes it easy to satisfy both demands by letting companies ship the base application code early, and later ship feature extensions as needed.

Figure 1–2 An overview of the Netscape architecture.

Unfortunately, the very nature of extensible systems makes security harder. For one thing, it is hard to prevent malicious code from slipping in as an unwanted extension. Meaning, the features designed to add extensibility to a system (such as Java's class-loading mechanism) must be designed with security in mind. Furthermore, analyzing the security of an extensible system is much harder than analyzing a complete system that can't be changed. How can you take a look at code that has yet to arrive? Better yet, how can you even begin to anticipate every kind of mobile code that may arrive?

Figure 1–3 An overview of the Internet Explorer architecture.

Together, the three trends of ubiquitous networking, growing system complexity, and built-in extensibility make the software security problem more urgent than ever. There are other trends that have an impact as well, such as the lack of diversity in popular computing environments, and the tendency for people to use systems in unintended ways (for example, using Windows NT as an embedded operating system). For more on security trends, see Bruce Schneier's book Secrets and Lies [Schneier, 2000].

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