How to 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
[HttpGet("api/orders/{id}")] public async TaskGetOrder(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.
[Authorize] [HttpGet("api/orders/{id}")] public async TaskGetOrder(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);
}
Your ASP.NET Core API
might be exposed to BOLA (Broken Object Level Authorization)
74% of ASP.NET Core apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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