How to fix Logic Flow Bypass
in NancyFX
Executive Summary
NancyFX's lightweight routing is a double-edged sword. Logic flow bypasses typically manifest when developers assume a linear execution path in multi-step processes (like checkouts or password resets). Attackers skip intermediate validation steps by directly hitting the terminal URI. If your module doesn't enforce state-machine integrity via pipeline hooks, your business logic is effectively optional.
The Vulnerable Pattern
public class OrderModule : NancyModule { public OrderModule() { Post("/order/pay", _ => { // Logic to process payment return Response.AsRedirect("/order/confirm"); });Get("/order/confirm", _ => { // VULNERABLE: Direct access allows skipping /pay return View["confirmed.html"]; }); }
}
The Secure Implementation
The fix utilizes Nancy's 'Before' interceptor to enforce a mandatory state check. By validating a server-side session variable ('OrderPaid') before allowing access to the '/confirm' route, we prevent 'forced browsing' attacks. The session state is cleared immediately after the successful rendering of the final step to prevent replay attacks. For complex flows, implement a cryptographically signed state token or a server-side state machine to track progress through the application lifecycle.
public class OrderModule : NancyModule { public OrderModule() { Before += ctx => { if (ctx.Request.Path.EndsWith("/confirm")) { var isPaid = ctx.Session["OrderPaid"] as bool?; if (isPaid != true) return Response.AsRedirect("/order/pay"); } return null; };Post("/order/pay", _ => { // Process payment logic Session["OrderPaid"] = true; return Response.AsRedirect("/order/confirm"); }); Get("/order/confirm", _ => { Session["OrderPaid"] = null; // Burn the token after use return View["confirmed.html"]; }); }
}
Your NancyFX API
might be exposed to Logic Flow Bypass
74% of NancyFX apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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