ASP.NET - Web Application Firewalls with ASP.NET

A Web Application Firewall (WAF) protects ASP.NET applications from malicious web traffic. It sits in front of the application and inspects incoming requests before they reach the server. Its main role is to block common attacks such as SQL injection, cross-site scripting and request flooding.

Why a WAF Is Important
Web applications are exposed to the internet and are frequent targets of automated attacks. A WAF acts as a security shield that filters harmful requests early. This reduces risk even if the application code has vulnerabilities and helps maintain availability during attack attempts.

How WAFs Work with ASP.NET
A WAF analyzes HTTP requests based on rules and patterns. If a request matches a known attack signature or suspicious behavior, it is blocked or challenged. ASP.NET apps do not need to change their code because the WAF operates at the network or reverse-proxy level.

Common WAF Deployment Options
WAFs are usually deployed in front of Kestrel using IIS, Nginx or cloud services. Examples include Azure Web Application Firewall, AWS WAF or ModSecurity with Nginx. These systems inspect traffic before forwarding safe requests to the ASP.NET application.

Built-in Protections vs WAF
ASP.NET includes security features such as input validation and request limits, but these work inside the application. A WAF adds an outer security layer that blocks threats before they consume application resources. Using both together provides stronger protection.

Why WAFs Improve Security
A WAF reduces attack surface, protects against zero-day exploits and prevents traffic spikes from overwhelming the app. It also provides logging and monitoring that help teams detect and respond to threats faster.

Example (ASP.NET Request Filtering + Reverse Proxy Concept)

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

// Simple app logic
app.MapPost("/login", (string username) =>
{
    return $"Welcome {username}";
});

app.Run();

Typical deployment flow

Internet Request
     ↓
WAF (Azure WAF / Nginx + ModSecurity)
     ↓
Kestrel (ASP.NET Core App)

What the WAF does

  • Blocks SQL injection patterns

  • Stops malicious payloads

  • Limits abusive traffic

  • Allows only safe requests to reach the app

This layered approach keeps ASP.NET applications secure and stable in production environments.