How to fix Logic Flow Bypass
in ASP.NET Core
Executive Summary
Logic flow bypasses occur when an application assumes a user will follow a predefined sequence of steps without server-side enforcement. In ASP.NET Core, this usually manifests as skipping authorization checks, payment steps, or multi-factor authentication by hitting endpoints out of order. If you aren't strictly validating the state transition on the backend, you're leaving the door wide open for state-smuggling and workflow subversion.
The Vulnerable Pattern
[HttpPost("checkout/process-payment")] public async TaskProcessPayment([FromBody] PaymentRequest request) { // VULNERABILITY: The endpoint assumes the user has already passed // the 'validate-cart' and 'shipping-selection' steps. // An attacker can POST here directly to bypass business rules. var order = await _orderService.CreateOrder(request); await _paymentGateway.Charge(request.Amount); return Ok(new { Status = "Success", OrderId = order.Id });
}
The Secure Implementation
To fix logic flow bypass, you must treat the application workflow as a Finite State Machine (FSM). The server, not the client, must track the user's current progress using a secure, server-side store (like Distributed Cache or Database). Every sensitive endpoint must verify that the user is in the correct 'source' state before allowing a transition to the 'destination' state. Once a step is completed, the state should be updated or invalidated to prevent the user from re-entering the flow at an arbitrary point.
[HttpPost("checkout/process-payment")] [Authorize] public async TaskProcessPayment([FromBody] PaymentRequest request) { var userId = User.FindFirst(ClaimTypes.NameIdentifier)?.Value; var sessionState = await _cache.GetAsync ($"checkout:{userId}"); // SECURE: Strict enforcement of the Finite State Machine (FSM) if (sessionState == null || sessionState.CurrentStep != CheckoutSteps.ShippingSelected) { return BadRequest(new { Error = "Invalid workflow sequence. Complete shipping first." }); } var order = await _orderService.CreateOrder(request); var chargeResult = await _paymentGateway.Charge(request.Amount); if (chargeResult.Succeeded) { // Transition state and invalidate to prevent replay await _cache.RemoveAsync($"checkout:{userId}"); return Ok(new { Status = "Success", OrderId = order.Id }); } return StatusCode(500, "Payment Failed");
}
Your ASP.NET Core API
might be exposed to Logic Flow Bypass
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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