Fix Logic Flow Bypass in Revel

Revel's session management is client-side and cookie-based by default. Logic flow bypasses occur when developers assume a linear UI progression without enforcing state transitions on the backend. Attackers skip 'Step 1' (validation) and jump directly to 'Step 2' (execution) by hitting the endpoint directly. To kill this, you must implement a server-side state machine or use cryptographically signed session markers to verify the prerequisite actions were completed.

The Vulnerable Pattern

func (c App) ConfirmOrder() revel.Result {
    // VULNERABILITY: No check to see if the user actually passed the 'Payment' step.
    // An attacker can browse directly to /confirm without paying.
    orderID := c.Params.Get("orderId")
    return c.Render(orderID)
}

The Secure Implementation

The fix implements a mandatory state-check. By storing a 'payment_verified' token in Revel's signed session, we ensure the user cannot reach the confirmation logic without the server having set that flag in a previous step. For complex flows, use Revel Interceptors (c.Intercept) to centralize this logic, ensuring that sensitive actions are unreachable unless the internal state machine confirms the prerequisite steps were executed in the correct order.

func (c App) ConfirmOrder() revel.Result {
    // FIX: Verify the state transition marker in the session.
    val, ok := c.Session["payment_verified"]
    if !ok || val != "true" {
        revel.AppLog.Warn("Logic bypass attempt detected")
        return c.Redirect(App.PaymentStep)
    }
// Clear state after consumption to prevent replay
delete(c.Session, "payment_verified")

orderID := c.Params.Get("orderId")
return c.Render(orderID)
}
// Use an Interceptor for global flow control
func (c App) CheckFlow() revel.Result {
if c.Action == “App.ConfirmOrder” && c.Session[“payment_verified”] != “true” {
return c.Forbidden(“Sequence Error”)
}
return nil
}