Software Security: Building Security In

A central activity in design-level risk analysis involves building up a consistent view of the target system at a reasonably high level. The idea is to see the forest and not get lost in the trees. The most appropriate level for this description is the typical whiteboard view of boxes and arrows describing the interaction of various critical components in a design. For one example, see the following box, .NET Security Model Overview.

Commonly, not enough of the many people often involved in a software project can answer the basic question, "What does the software do?" All too often, software people play happily in the weeds, hacking away at various and sundry functions while ignoring the big picture. Maybe, if you're lucky, one person knows how all the moving parts work; or maybe nobody knows. A one-page overview, or "forest-level" view, makes it much easier for everyone involved in the project to understand what's going on.

The actual form that this high-level description takes is unimportant. What is important is that an analyst can comprehend the big picture and use it as a jumping-off place for analysis. Some organizations like to use UML (the Unified Modeling Language) to describe their systems.[5] I believe UML is not very useful, mostly because I have seen it too often abused by the high priests of software obfuscation to hide their lack of clue. But UML may be useful for some. Other organizations might like a boxes-and-arrows picture of the sort described here. Formalists might insist on a formal model that can be passed into a theorem prover in a mathematical language like Z. Still others might resort to complex message-passing descriptionsa kind of model that is particularly useful in describing complex cryptosystems. In the end, the particular approach taken must result in a comprehensible high-level overview of the system that is as concise as possible.

[5] For more on UML, see <http://www.uml.org/>.

The nature of software systems leads many developers and analysts to assume (incorrectly) that code-level description of software is sufficient for spotting design problems. Though this may occasionally be true, it does not generally hold. eXtreme Programming's claim that "the code is the design" represents one radical end of this approach. Because the XP guys all started out as Smalltalk programmers they may be a bit confused about whether the code is the design. A quick look at the results of the obfuscated C contest <http://www.ioccc.org> should disavow them of this belief.[6]

[6] Incidentally, any language whose aficionados purposefully revel in its ability to be incomprehensible (even to the initiated) has serious issues. Perhaps experienced developers should require a license to use C. Newbies would not be permitted until properly licensed.

Without a whiteboard level of description, an architectural risk analysis is likely to overlook important risks related to flaws. Build a forest-level overview as the first thing you do in any architectural risk analysis.

.NET Security Model Overview

Figure 5-2 shows a one-page high-level architectural view of the .NET security model prepared while performing a .NET risk analysis. Before this diagram was created, the only high-level description of the .NET security architecture was a book-length description of its (way too many) parts. Putting all the parts together in one picture is an essential aspect of risk analysis.

Figure 5-2. A one-page overview of Microsoft's .NET security model. An architectural picture like this, though not in any sense detailed enough to perform a complete analysis, is extremely useful for thinking about components, modules, and possible attacks. Every one-page overview should list all components and show what is connected to what.

All risk analyses should begin by understanding and, if necessary, describing and documenting a high-level overview of the system to be analyzed. Sometimes the act of building this picture is a monumental undertaking. Sometimes a one-page overview already exists. In any case, making one is a great idea.

By referencing the picture in Figure 5-2, an analyst can hypothesize about possible attacks. This can be driven by a list of known attacks such as the attack patterns described in Chapter 8 (and fleshed out in vivid detail in Exploiting Software [Hoglund and McGraw 2004]), or it can be driven by deep technical understanding of the moving parts.

As an example of the latter approach, consider the flow of information in Figure 5-2. In this picture the Verifier feeds the just in time (JIT) compiler. As noted in Java Security, the Verifier exists to ensure that the bytecode (in this case, CLR code) coheres to various critical type-safety constraints [McGraw and Felten 1996]. Type safety is about objects having certain properties that can be guaranteed. If type-safety rules are not followed or the Virtual Machine becomes confused about type safety, very bad things happen.

Anyway, the Verifier does its thing and passes information on to the JIT compiler.

A JIT compiler transforms intermediate CLR code (or Java bytecode) into native code (usually x86 code) "just in time." This is done for reasons of speed. For the security model to retain its potency, the JIT compiler must carry out only transformations that preserve type safety. By thinking through scenarios in which the JIT compiler breaks type safety, we can anticipate attacks and identify future risks. Interestingly, several relevant security issues based on this line of reasoning about attacks and type safety led to the discovery of serious security problems in Java. (For a complete description of the Java attacks, see <http://www.securingjava.com>, where you can find a complete, free, online edition of my book Securing Java [McGraw and Felten 1999].)

Unless we built up a sufficient high-level understanding of the .NET security model (probably through the process of creating our one-page picture), we would not likely come across possible attacks like the one described here.

One funny story about forest-level views is worth mentioning. I was once asked to do a security review of an online day-trading application that was extremely complex. The system involved live online attachments to the ATM network and to the stock exchange. Security was pretty important. We had trouble estimating the amount of work to be involved since there was no design specification to go on.[7] We flew down to Texas and got started anyway. Turns out that only one person in the entire hundred-person company knew how the system actually worked and what all the moving parts were. The biggest risk was obvious! If that one person were hit by a bus, the entire enterprise would grind to a spectacular halt. We spent most of the first week of the work interviewing the architect and creating both a forest-level view and more detailed documentation.

[7] The dirty little trick of software development is that without a design spec your system can't be wrong, it can only be surprising! Don't let the lack of a spec go by without raising a ruckus. Get a spec.

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