How to fix Improper Error Handling
in NancyFX
Executive Summary
NancyFX's 'Super Simple' philosophy is a double-edged sword. Out of the box, unhandled exceptions can leak stack traces, local file paths, and internal logic—providing attackers with a roadmap for further exploitation. To harden a Nancy application, you must intercept the pipeline and sanitize all error responses at the global level.
The Vulnerable Pattern
public class UserModule : NancyModule {
public UserModule(IUserRepository repo) {
Get("/users/{id}", parameters => {
// If repo throws an exception (e.g., DB connection fail or null ref),
// Nancy default behavior may leak the full stack trace to the client.
return repo.GetById(parameters.id);
});
}
}The Secure Implementation
The vulnerability stems from relying on Nancy's default exception handling, which is often too verbose for production environments. The fix implements a global error handler via the `IPipelines.OnError` hook in the `Bootstrapper`. This ensures that regardless of where an exception occurs in the module logic, the client only receives a generic 500 status code and a sanitized JSON message. This prevents 'Error-Based Information Disclosure' while allowing developers to log the actual stack trace to a secure, internal logging sink.
public class SecureBootstrapper : DefaultNancyBootstrapper { protected override void ApplicationStartup(TinyIoCContainer container, IPipelines pipelines) { pipelines.OnError += (ctx, ex) => { // 1. Log the raw exception internally for debugging Console.WriteLine($"[!] Internal Error: {ex.Message}");// 2. Return a generic, sanitized response to the user var response = (Response)"{\"error\": \"An unexpected error occurred.\"}"; response.ContentType = "application/json"; response.StatusCode = HttpStatusCode.InternalServerError; return response; }; }
}
Your NancyFX API
might be exposed to Improper Error Handling
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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