While developing the "Criminal Mind" is crucial for uncovering vulnerabilities, there is another equally important skill to master: developing the "Engineer Mind." This involves building a robust mental image of code and architecture.

The "Engineer Mind" enables you to reverse engineer how applications are written and architected by observing behaviors, inspecting features, and testing their boundaries. Once you've built a strong mental model, you can often predict how systems will behave, where they might break, and what could go wrong.

Building a Mental Image of Code

When testing a web application, skilled pentesters and AppSec engineers are constantly reverse-engineering its behavior to construct a mental image of the underlying code. This mental process involves simulating the code in your mind or jotting down pseudo-code based on observed behaviors.

For example, consider a file upload feature. You upload an image and access it at /uploads/myimage.jpg. You can start mentally drafting the backend logic:

save(file, '/uploads/')

Next, you upload myimage.php and encounter an error. This reveals something about file type validation. Piece by piece, you build a mental representation of the logic:

if file is PHP
  return error

save(file, '/uploads/')

Then, you upload an image with the extension .php and encounter the same error. This reveals another layer about file type validation:

if file extension is PHP
  return error

save(file, '/uploads/')

The goal is to transition from black-box testing to a mental white-box testing approach by constructing a detailed internal image of the code.

Another way to build this mental image is by breaking down features into their possible implementation. For instance, when testing a login feature, consider how the application might be verifying credentials: Is it hashing passwords? Is there a session mechanism? What kind of database query might it run? These mental exercises sharpen your ability to anticipate the code’s behavior and design.

If you aren’t already doing this, start! It will transform how you approach testing and debugging.

Constructing a Mental Image of Architecture

Building a mental architecture involves visualizing how the components of a system interact and function together. As you test an application, details emerge that add to this mental model.

For example:

• If a file is uploaded and directly served from an S3 bucket, you can add S3 to your mental architecture image.
• If the application throws a MySQL error when an input is too long, you add MySQL to the architecture.

Little by little, you construct this mental architecture. The more detailed your mental image, the easier it becomes to identify where to look for vulnerabilities, inefficiencies, or potential failures.

Why This Matters

Building mental models of both code and architecture offers several key advantages:

• Predict Weak Points: Identify areas prone to vulnerabilities, even in complex systems.
• Think Like a Builder: Understand the constraints and assumptions behind design decisions.
• Navigate Complexity: Quickly assess large-scale, distributed systems with efficiency and accuracy.

How to Build a Mental Image of Code

Building a mental image of code requires deliberate practice and a structured approach to analyzing and understanding how features are implemented.

• Read Code: Dive into source code, focusing on how applications block attacks and remain vulnerable. Practice with real-world codebases, exploring common frameworks like Rails or Django.
• Do Gray-Box Testing: Blend black-box and white-box approaches. This lets you correlate observed behaviors with the underlying code.
• Break Down Features into Code: When encountering a feature, try to visualize or write out how it might be implemented. For example, think through the logic of a password reset flow: How is the token generated? Where is it stored? How is it validated?
• Build Applications: By creating small apps using frameworks, you’ll gain firsthand experience in understanding typical patterns, strengths, and pitfalls.

How to Build a Mental Image of an Architecture

To develop a strong mental image of an architecture, you need to combine hands-on experience with theoretical knowledge. The following tasks will guide you in understanding how systems are designed, deployed, and how their components interact, so you can build a clear and detailed picture of their architecture.

• Deploy Complex Applications:
  • Experiment with containerized systems (e.g., Kubernetes).
  • Set up CI/CD pipelines and monitoring tools.
  • Observe system interactions and error handling in real-world scenarios.
• Read Architecture Books:
  • Designing Data-Intensive Applications by Martin Kleppmann.
  • The Art of Scalability by Abbott and Fisher.
  • Clean Architecture by Robert C. Martin.
• Follow Engineering Blogs:
  • The Netflix TechBlog
  • AWS Architecture Blog
  • GitHub Engineering Blog
  • Stripe Engineering Blog
  • ...
• Break Down Systems in Your Mind:
  • Analyze how components communicate and respond to failures.
  • Practice deconstructing known applications, such as social media platforms or e-commerce systems.

Combining the "Engineer Mind" with the "Criminal Mind"

The most successful pentesters and AppSec engineers combine the creative, adversarial thinking of the "Criminal Mind" with the systematic, structural thinking of the "Engineer Mind." Together, these approaches enable you to:

• Understand systems deeply enough to uncover weaknesses.
• Predict how attackers might exploit vulnerabilities.
• Provide actionable recommendations to enhance security.

Conclusion

The "Engineer Mind" and the "Criminal Mind" are two sides of the same coin, working together to master the art of hacking and application security. While the "Criminal Mind" focuses on exploiting weaknesses and uncovering vulnerabilities, the "Engineer Mind" builds a deep understanding of how systems are designed and operate. Together, they form a balanced and powerful approach—like yin and yang—where creativity meets structure, and intuition meets technical precision.

By developing both mindsets, you can not only identify vulnerabilities but also understand their root causes and how to prevent them. Mastering the "Engineer Mind" through building applications, studying architecture, and analyzing systems will elevate your skills, making you a more effective pentester, AppSec engineer, and problem solver. With this dual perspective, you’ll be ready to tackle even the most complex security challenges.