OWASP Top 10 – A04: Insecure Design

While diving into software security, I came across the OWASP Top 10 list. As I reviewed the categories, one entry stood out: A04 – Insecure Design. Honestly, I had no idea what it really meant at first. So I decided to dig deeper and understand it thoroughly.

Below are my notes from this research process. I’m sharing them here in case they help someone else out there, just like they helped me organize my own understanding.

What Is Insecure Design?

“Insecure Design” refers to flaws in the architecture or design of a system that make it vulnerable — even before a single line of code is written. These issues arise not from programming mistakes, but from decisions made during planning.

In simple terms:

The code might be written correctly.
Configurations may seem fine.
But if the design leaves the system open to misuse or abuse, that’s what we call insecure design.

When Does It Happen?

When security requirements aren’t defined during planning.
If no threat modeling is done (i.e., no analysis of how the system could be attacked).
If critical flows lack extra validation layers.
If user flows are too flexible or easily abusable, and no one has considered “what if someone tries to break the rules?”

Why Is It Critical?

These aren’t coding bugs — so tools like scanners or linters won’t catch them.
Attackers exploit the logic of your system, not necessarily the code itself.
These flaws live in business logic, like payment systems, role management, or password reset flows.

In short: if security isn’t baked into the design, it won’t matter how clean the code is — the system will still be vulnerable.

Real-World Examples

1. Poorly Designed Password Reset Flow

User clicks “forgot password.” The system jumps straight to the new password screen without verifying the user’s identity (e.g., via email). Anyone could reset anyone else’s password.

2. Missing Rate Limiting

Login form exists, but there’s no limit to how many attempts someone can make. Attackers can brute-force credentials indefinitely.

3. Improper Role Control

Even though roles exist, the system doesn’t restrict backend access by role — only the frontend UI hides the links. Attackers can access admin routes directly via URL.

4. Abusable Logic Flow

In an e-commerce site, users can delete items from the cart, then proceed to the checkout page anyway — buying something for $0. This logic wasn’t safeguarded during design.

Insecure Design vs Insecure Implementation

This part really clicked for me: insecure design is about what you planned, insecure implementation is about how you built it.

Category	Insecure Design	Insecure Implementation
Stage	Architecture / Planning	Coding / Configuration
The Problem	Security wasn’t considered	Security was misapplied
Example	No rate limiting exists	Rate limiting exists but is misconfigured

How to Prevent It

Here are some key prevention techniques I found during my research:

1. Do Threat Modeling

Think like an attacker.
Ask: “How could someone abuse this feature?” not just “How should this work?”

2. Define Security Requirements Early

Don’t just plan for features — plan for how to protect them.
For example: “If a user changes their email, re-authentication is required.”

3. Add Extra Controls for Critical Actions

Password reset, payment, account updates — these should require additional verification (2FA, email confirmation, etc.).

4. Apply the Principle of Least Privilege

Users should only have the access they absolutely need — nothing more.

5. Conduct Design Reviews

Review architecture diagrams from a security perspective before any code is written.
Focus especially on logic flows and decision points.

Best Practices

Here are some habits I noted that can make a huge difference:

Test user flows from a malicious user’s perspective.
Use security checklists during design review meetings.
Make threat modeling a part of your dev lifecycle — not an afterthought.
Train developers on secure design principles.
Adopt “Security by Design” — security first, features second.

Role-Based Responsibilities

Developers:

Don’t just ask “how will this work?” — ask “how can this be misused?”

Architects:

Create security-aware system architectures.
Avoid leaving security decisions for “later.”

Project Managers:

Treat security requirements as seriously as business requirements.

How to Test for Insecure Design

Testing design is different from testing code. Here are some ideas I found useful:

Manual Business Logic Testing – Try using the system in unintended ways.
Threat Model Validation – Make sure each threat has a countermeasure.
Design Reviews – Use checklists when reviewing architecture and workflows.
Focused Security Pen Testing – Especially manual testing of business logic.