OWASP Security Lessons in a Minimal Blazor Server API

By Alexander Batenhorst
Aug 6
3 min read

Updated: 5 days ago

Project Overview

BlazorAuthDemoAlice is a compact Blazor Server app using a minimal API structure. It's built with OWASP best practices in mind and serves as a great starting point for developers looking to showcase secure coding skills — even in small proof-of-concepts. Whether you're learning or building a portfolio, this project demonstrates that security isn't just for enterprise apps.

What It Does

The app sets up a Blazor Server UI, exposes a simple API endpoint, and adds basic authentication. But the real value is in how it handles bad or unauthorized requests — clean failures, no stack traces, and no metadata leaks.

It highlights two key best practices:

Fail-safe error handling: problems don't reveal internal details
Deny-by-default access control: nothing is exposed unless explicitly allowed

Let’s break that down.

1. Graceful Error Handling

In development, stack traces are useful. In production, they’re risky. OWASP specifically warns against exposing internals like error dumps or version info (OWASP, 2021). BlazorAuthDemoAlice uses app.UseExceptionHandler("/Error") in Program.cs to route unhandled errors to a generic page.

This means users see a friendly message, not your codebase. It’s a simple adjustment that greatly reduces exposure.

2. Deny-by-Default Access Control

The app only exposes what it needs to. No extra routes and no accidental endpoints. Unknown routes return 404. Calls to /api/Auth/login with bad credentials return 401. If a user is authenticated but not allowed to access a resource, return 403. Failed logins do not reveal whether a username exists, so there’s no breadcrumb trail for attackers.

The login logic is straightforward but powerful. This one controller is where the magic happens:

This controller isn’t part of the default Blazor Server template — it’s a custom-built endpoint that plugs into a lightweight authentication service. That’s what makes it worth highlighting. Even a small API setup can demonstrate proper secure design. The controller responds with a clean 401 Unauthorized if login fails. It doesn't reveal whether the username exists or what went wrong. Whether you're protecting static endpoints or authenticating users, ASP.NET Core makes it easy to keep things locked down unless explicitly allowed.

To demonstrate this, here’s a basic forced browsing test:

Below, the user manually enters /admin in the browser bar:

The response?

The app doesn’t leak metadata, error codes, or even suggest that an admin panel might exist. It simply returns a neutral 404 or custom error message. That’s exactly how it should behave.

3. Smaller Attack Surface by Design

The app is intentionally minimal. No extra controllers, no unnecessary services. That keeps the attack surface small and the code easy to audit. It’s the security version of traveling light.

Suggestions to Go Further

This project does a lot right. Still, here’s how to make it even better:

• Token-Based AuthenticationSwitch to JWT or a similar token system for API calls. It’s stateless, scalable, and integrates with [Authorize] easily.

• Rate LimitingUse ASP.NET Core’s built-in rate limiter to slow down brute-force or bot activity (OWASP, 2021).

• Input Validation and Output EncodingSanitize incoming data and properly encode any output. This reduces the risk of XSS and injection attacks.

• Security LoggingTrack failed logins, blocked requests, and anything that smells off. Good logging helps detect issues early without exposing sensitive info.

Final Thoughts

BlazorAuthDemoAlice is tiny, but it gets the fundamentals right:

• Hide your internals

• Control access by default

• Keep it lean

If you're a newer developer or prepping for interviews, a project like this helps you stand out. It shows you understand how real-world apps get attacked and what it takes to keep them safe. That’s the kind of signal that recruiters notice.

Security isn't just a checklist. It's a mindset, and this demo gets that.

BlazorAuthDemoAlice repository link: https://github.com/iotalex/BlazorAuthDemoAlice