How Engineers Can Contribute to API Security

I’m an engineer, not a security professional. I’ve worked at several startups over the years and have discovered ways to shift my engineering mindset to include a security focus and to incorporate security into my coding practice.

I’ve found that when you work with others, an engineering organization can significantly contribute to API security. But how we think about software makes it difficult to get started. There is an alternative perspective—a road not taken—that must be considered when it comes to securing endpoints from threat actors.

The Road Not Taken

Engineers create and optimize applications to solve problems for real users. Engineering time is often in short supply, so we optimize the speed at which we can write those applications. To speed up our processes, we simplify problems and find reusable patterns or techniques. Add a bit of glue, and you have software.

This perspective is constructive. Thoughts come first and software comes next. And when refactoring existing code for software, we need to think this way. But this utility comes at a cost: If the software works, we only see what it should do. The real story behind the scenes can be much more complex.

A few years ago, I worked with an engineer on new endpoints for an authorization API. We were working out an issue in the implementation where I could call the endpoints out of order and bypass a challenge. We worked through the example, and he said, “But they are supposed to be called in 1-2-3 order.”

Attackers don’t share this perspective. Instead, they want to manipulate the software to act in their own interests. They ask, “What is possible?” Instead of “What is it supposed to do?” and find ways in which the system fails. To an attacker, the glue is more interesting than the design.

While probing, an attacker may be thinking some of the following questions:

“The password reset endpoint is on v2? Is v1 still running?”

“What happens if I skip the verify endpoint?”

“What happens when I pass null in the challenge? Empty string? SQL injection?”

As creators, we’ve converged on this constructive perspective. To build secure software, we need to consider the road not taken and incorporate greater security into our applications.

Playing Defense

When you are defending software, your limited time is a disadvantage. There are many engineers—and some have been writing software for many years—which means that you have a lot of code to defend. Each application all on hundreds or thousands of open source projects, from libraries to deployment tools to server operating systems.

Security tools can help you think from both perspectives, especially those that automatically discover endpoints and track response statistics. You can use this data to understand how code maps to your public API surface.

In addition to tools, here are several suggestions for the ground techniques that you could try out.

Read the Code

Pick an application, find out where the authentication and authorization code is and read it. Even if you aren’t a strong coder, you can learn something interesting. Better yet, try and take the attacker’s perspective and look for ways you can exploit the code and make it work for you. For instance, you may see the following:

“SQL injection might be possible in this area. I might be able to extract information about other accounts.”

“Authorization is complex and confusing in this area. I might be able to use my standard user account to perform actions on other accounts.”

“Logging is very weak in this sensitive area. I might be able to hammer this endpoint without being noticed.”

“Code or comments indicate the developer assumes the application is secure for a particular reason. Can I break those assumptions with an external request?”

The knowledge gained in performing an exercise like this is priceless. You can use it to guide the rest of your API/AppSec work (pentesting, scan remediation, etc.). Further, you’ll often find logical vulnerabilities that scans couldn’t detect.

Read Code Reviews

Engineers make decisions in pull requests (PRs). Content in past PRs and review comments have tons of context and can be used to help inform your decision-making. Look through the history, then think through some security-focused review questions:

“If the code tries to enforce security constraints, can I think of a way around them?”

“Are there obvious security mechanisms that should be part of this change but are missing?”

“Would unusual attacker behavior appearing in logs?”

Examining these reviews and understanding the broader context can provide you with valuable information. You might find a sensitive action without any authorization logic, ways a user account can access data owned by other users, or a path through the password reset flow without a valid challenge token.

Work With Engineers

If your engineers are working on security-relevant code, ask for some time to talk. Find out what they are working on (and why). Help them think like an attacker, and explain some of the techniques an attacker might use to break their app. And remember to take the road not taken while you work to ensure the security of your applications.

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