In this lesson, you will learn how to harden your SignalR infrastructure by enforcing strict header validation and path restrictions during the initial WebSocket handshake.
While SignalR handles the bulk of the connection process, the underlying WebSocket upgrade request remains a standard HTTP request. If you don't restrict this request, malicious clients might attempt to bypass your security logic or probe your infrastructure by sending arbitrary upgrade requests to your Hub paths.
Enforcing Strict Path Restrictions
The first layer of defense is ensuring that only authorized traffic can even trigger the WebSocket handshake. By using middleware, you can inspect the HttpContext before the SignalR middleware executes. This prevents "path-traversal" or unauthorized discovery attempts by ensuring only exact, expected Hub endpoints are processed.
app.Use(async (context, next) =>
{
// Restrict handshake to specific paths only
if (context.Request.Path.StartsWithSegments("/hubs/chat"))
{
// Add custom header validation for the upgrade request
if (context.Request.Headers["X-Custom-SignalR-Key"] != "Expected-Secret-Token")
{
context.Response.StatusCode = StatusCodes.Status403Forbidden;
return;
}
}
await next();
});
Validating Headers
Standard WebSocket handshakes include headers like Upgrade: websocket and Sec-WebSocket-Key. However, you can enforce additional custom security headers to ensure that the client connecting is not only authenticated but also "aware" of your application's security context.
By checking for a Sec-WebSocket-Protocol header or a custom X-Custom-SignalR-Key (as shown above), you create a "pre-flight" check that isn't native to the standard WebSocket spec. Since SignalR relies on the MapHub<T> endpoint, placing this logic in your pipeline ensures that any request reaching the Hub has already satisfied your custom integrity requirements.
If these headers are missing or invalid, reject the request immediately. This prevents the server from allocating resources to a persistent connection that shouldn't exist in the first place, effectively mitigating potential resource exhaustion attacks during the handshake phase.
Key Takeaway: Securing WebSocket upgrades requires validating custom headers and restricting endpoints at the middleware level before the connection is ever upgraded from HTTP to persistent binary transport.