Fix BOLA (Broken Object Level Authorization) in ASP.NET Core

Executive Summary

BOLA (IDOR) is the #1 threat in the API landscape. It occurs when an application exposes a resource identifier in an API endpoint and fails to validate if the authenticated user has the right to access that specific object. Attackers simply iterate IDs to scrape data. To kill BOLA in ASP.NET Core, you must stop trusting the client-provided ID as the sole source of truth and enforce ownership checks at the database query level.

The Vulnerable Pattern

VULNERABLE CODE

[HttpGet("api/orders/{id}")]
public async Task GetOrder(int id)
{
    // VULNERABLE: Only checks if the order exists, not who owns it.
    var order = await _context.Orders.FirstOrDefaultAsync(o => o.Id == id);
if (order == null) return NotFound();

return Ok(order);
}

The Secure Implementation

The fix shifts authorization from the application logic to the data access layer. By injecting the 'UserId' from the authenticated 'ClaimsPrincipal' directly into the EF Core 'Where' clause, you ensure that the database engine itself enforces the security boundary. Even if an attacker guesses a valid 'orderId', the query will return null because the 'UserId' filter won't match. Always prefer this 'Query Scoping' pattern over manual 'if (order.UserId != currentUserId)' checks to prevent race conditions and ensure consistent enforcement across your API.

SECURE CODE

[Authorize]
[HttpGet("api/orders/{id}")]
public async Task GetOrder(int id)
{
    // Get the User ID from the JWT/ClaimsPrincipal, not the request body/URL
    var userId = User.FindFirst(ClaimTypes.NameIdentifier)?.Value;
// SECURE: Scope the database query to both the Resource ID AND the Owner ID
var order = await _context.Orders
    .FirstOrDefaultAsync(o => o.Id == id && o.UserId == userId);

if (order == null) 
{
    // Return NotFound to prevent ID enumeration/leaking existence of records
    return NotFound();
}

return Ok(order);
}

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How to fix BOLA (Broken Object 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