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Why threat models are crucial for secure software development

Threat modeling is an important component of the secure software development process. Steve Lipner of SafeCode explains how threat models benefit software security.

Every year, hundreds of thousands of software developers join the workforce without a basic knowledge of security. Therefore, the burden of educating and training developers on software security is left to the development organizations that hire them.

While development organizations can and should train their employees on company-specific tools and processes, a basic understanding of the nature and importance of software security, and the sources of vulnerabilities, is as fundamental as other aspects of computing, such as data structures or performance. I'll examine one key aspect of software security -- threat modeling -- that is a fundamental practice that makes up a part of a secure development program.

Within a secure software development process, threat modeling is part of software design.

At one level, everyone builds threat models all the time, such as when you choose what clothes to wear based on the weather or take steps to protect your parked car against vandalism. Simply put, a threat model first illustrates all the components and subcomponents that make your system work, then considers the risks, along with the possible mitigations, and enables you to decide on an acceptable course of action.

Within a secure software development process, threat modeling is part of software design. Preparing threat models requires a good understanding of the system and, logically, the model needs to change to reflect any changes in the system.

Since the model needs to evolve, a repeatable approach to threat modeling is required to ensure consistency. In the example of car vandalism, the threat model would take into consideration potential adversarial events, such as vandals deliberately scratching car bodywork, stealing wing mirrors or smashing windows.

Building threat models

A threat model works by describing a software system, and then enumerating and evaluating potential events to assess their impacts. If necessary, the system design can be modified to prevent them or to mitigate their consequences. One threat modeling approach is to create an attack tree model that identifies the issues with the most risk first.

SafeCode threat model

The diagram above is a representation of a sample attack tree that illustrates the potential vulnerabilities that could result from a specific weakness within the application that is being threat-modeled.

Once threat modelers identify the potential vulnerabilities, they can devise mitigations that can minimize the risks associated with those weaknesses or eliminate them altogether. Risks that have both security and business impacts should receive the highest priority in a threat model. For example, a security breach that impacts the company's brand may also result in business losses.

Free resources are available to help organizations and developers understand the fundamentals of threat modeling. For example, SafeCode's tactical threat modeling white paper discusses how to examine a design, review its data flow diagram, create an attack tree and understand the attack tree's implications (Editor's note: The author is the executive director of SafeCode).

By giving software developers, and not just security specialists, a leg up on the fundamentals of threat modeling, organizations can help ensure their development teams strengthen their software security assurance practices. Over time, the development team will gain the skills to create and analyze many threat models on its own, helping to scale the secure development process and improve its efficiency.

About the author:
Steve Lipner is the executive director of SafeCode -- the Software Assurance Forum for Excellence in Code. He led Microsoft's Security Development Lifecycle team and was responsible for corporate supply chain security strategies and policies regarding government evaluation of Microsoft security products. He is also a member of the National Academy of Engineering, a Fellow of the International Information System Security Certification Consortium (ISC)² and a National Cyber Security Hall of Fame inductee. In addition, he co-authored the book, The Security Development Lifecycle.

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