Security  

Top 10 Application Security Best Practices for .NET Developers

Introduction

Application security is one of the most critical aspects of modern software development. As a .NET developer , you are often building enterprise-grade applications that handle sensitive business and user data. A single security vulnerability can expose your application to SQL injection, XSS, CSRF, data leaks, or denial-of-service attacks .

In this article, we’ll explore the top 10 application security best practices every .NET developer should follow when building applications with ASP.NET Core MVC, Web API, and .NET Framework .

1. Enforce HTTPS Everywhere

  • Configure HSTS (HTTP Strict Transport Security) to prevent downgrade attacks.

  • Redirect all HTTP traffic to HTTPS.

  
    app.UseHttpsRedirection();
app.UseHsts();

Always use TLS certificates (Let’s Encrypt, Azure Key Vault, AWS ACM).

2. Validate and Sanitize All Inputs

  • Never trust user inputs (form data, query strings, JSON).

  • Use Data Annotations or FluentValidation .

  
    public class RegisterModel
{
    [Required]
    [EmailAddress]
    public string Email { get; set; }
}

Prevents injection attacks, XSS, and malformed payloads .

3. Prevent SQL Injection

  • Always use parameterized queries or Entity Framework LINQ .

  
    // Vulnerable
var users = db.Users.FromSqlRaw("SELECT * FROM Users WHERE Name = '" + name + "'");

// Safe
var users = db.Users.FromSqlInterpolated($"SELECT * FROM Users WHERE Name = {name}");

Never concatenate strings into SQL commands.

4. Secure Authentication and Authorization

  • Use ASP.NET Core Identity or external providers (Azure AD, IdentityServer).

  • Enforce role-based or policy-based authorization .

  
    [Authorize(Roles = "Admin")]
public IActionResult AdminDashboard()
{
    return View();
}

Implement Multi-Factor Authentication (MFA) wherever possible.

5. Protect Against Cross-Site Request Forgery (CSRF)

  • Enable CSRF protection in MVC forms:

  
    <form asp-action="Save">
    @Html.AntiForgeryToken()
</form>

  • For APIs, use JWT tokens instead of relying on cookies.

Prevents attackers from performing actions on behalf of authenticated users.

6. Implement Secure File Handling

  • Restrict file types and sizes.

  • Store files outside wwwroot .

  • Always rename files before saving.

  
    var safeFileName = Guid.NewGuid() + Path.GetExtension(file.FileName);

Prevents malware uploads, path traversal, and remote code execution .

7. Use Security Headers

Apply HTTP security headers using middleware:

  
    app.Use(async (context, next) =>
{
    context.Response.Headers.Add("X-Frame-Options", "DENY");
    context.Response.Headers.Add("X-Content-Type-Options", "nosniff");
    context.Response.Headers.Add("Referrer-Policy", "no-referrer");
    await next();
});

Helps mitigate clickjacking, XSS, and data leaks .

8. Rate Limiting and Throttling

  • Prevent brute-force attacks and DoS by applying rate limiting .

  • Use the AspNetCoreRateLimit NuGet package.

  
    "IpRateLimiting": {
  "EnableEndpointRateLimiting": true,
  "GeneralRules": [
    {
      "Endpoint": "*",
      "Period": "1m",
      "Limit": 100
    }
  ]
}

Protects APIs from abuse and overload attacks .

9. Secure Sensitive Data

  • Use the Data Protection API for encrypting cookies and tokens.

  • Store secrets in Azure Key Vault or AWS Secrets Manager , not appsettings.json .

  
    builder.Configuration.AddAzureKeyVault(new Uri(keyVaultUrl), new DefaultAzureCredential());

Never hardcode passwords, API keys, or connection strings.

10. Keep Dependencies and Frameworks Updated

  • Regularly update .NET runtime, NuGet packages, and third-party libraries .

  • Use GitHub Dependabot or OWASP Dependency-Check to identify vulnerabilities.

Many attacks exploit known vulnerabilities in outdated libraries.

Conclusion

Security is not a one-time setup —it’s a continuous process. By applying these 10 best practices , you can significantly reduce the risk of attacks in your ASP.NET Core MVC and Web API applications .

Always follow the OWASP Top 10 guidelines, perform regular security testing , and stay updated with the latest .NET security patches .

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