Fix BFLA (Broken Function Level Authorization) in ASP.NET Core

Executive Summary

BFLA (Broken Function Level Authorization) is a critical vulnerability where an application fails to properly verify if a user has the permissions to perform a specific administrative or sensitive action. Attackers exploit this by guessing or discovering hidden endpoints (e.g., /api/admin/delete-user) that lack server-side authorization checks. In ASP.NET Core, simply hiding a button in the UI is useless; if the API route isn't locked down with policies or roles, it's public domain for any malicious actor with a proxy.

The Vulnerable Pattern

VULNERABLE CODE

[ApiController]
[Route("api/users")]
public class UserController : ControllerBase
{
    private readonly DbContext _db;
// VULNERABLE: No authorization attribute.
// Any user can call this if they know the route.
[HttpDelete("{id}")]
public IActionResult DeleteUser(int id)
{
    var user = _db.Users.Find(id);
    _db.Users.Remove(user);
    _db.SaveChanges();
    return Ok();
}
}

The Secure Implementation

The fix involves shifting from 'Security through Obscurity' to 'Explicit Authorization'. The secure snippet implements Policy-Based Authorization. By decorating the sensitive action with [Authorize(Policy = 'RequireAdminRole')], the ASP.NET Core middleware intercepts the request and validates the user's claims (JWT or Cookie) against the defined policy before the controller logic executes. Always apply the Principle of Least Privilege: default to [Authorize] at the controller level and override with specific roles or policies for administrative functions.

SECURE CODE

[ApiController]
[Route("api/users")]
[Authorize] // Ensure user is at least authenticated
public class UserController : ControllerBase
{
    private readonly DbContext _db;
// SECURE: Explicitly requiring the 'Admin' policy
[HttpDelete("{id}")]
[Authorize(Policy = "RequireAdminRole")]
public IActionResult DeleteUser(int id)
{
    var user = _db.Users.Find(id);
    if (user == null) return NotFound();

    _db.Users.Remove(user);
    _db.SaveChanges();
    return Ok();
}
}
// In Program.cs / Startup.cs
builder.Services.AddAuthorization(options =>
{
options.AddPolicy(“RequireAdminRole”, policy => policy.RequireRole(“Admin”));
});

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How to fix BFLA (Broken Function Level Authorization)
in ASP.NET Core

Executive Summary

The Vulnerable Pattern

The Secure Implementation

Protect your ASP.NET Core API

Verified by Ghost Labs Security Team