If you’ve ever poked around the diagnostics blade in Azure App Service and spotted a bunch of 500 errors blamed on something called WebSocketMonitoringModule, you’re probably thinking: “Wait… what? I didn’t even turn on WebSockets!”
And you’d be right. You scroll back through your app settings, double-check the config (yep, WebSockets: Off), and yet there it is — a mysterious platform module acting up like it runs the place.
At first, it feels like a ghost in the machine. Or maybe some rogue client is doing something weird? You start wondering if you’re missing a setting, a bug, or possibly a cosmic joke being played by Azure’s diagnostics engine.
But don’t worry — you’re not alone, and you’re definitely not going crazy. This is one of those quirky little App Service mysteries that’s more common than you might think. So let’s peel back the curtain on this WebSocketMonitoringModule situation, find out what’s actually going on, and help you decide whether to take action — or just shrug, sip your coffee, and move on.
What Is WebSocketMonitoringModule?
WebSocketMonitoringModule is a behind-the-scenes component built into the Azure App Service platform. It’s not part of your codebase, and you won’t find it in your NuGet packages — but it plays a key role in how the platform handles incoming HTTP requests that might turn into WebSocket connections.
At its core, this module’s job is to listen for and respond to WebSocket handshake attempts. It keeps tabs on whether a client is trying to initiate a WebSocket connection (Connection: Upgrade, anyone?), whether the upgrade succeeds or fails, and whether anything weird happens during that process — like timeouts, dropped connections, or unhandled exceptions.
But here’s the twist: this module is always on, whether you like it or not.
Even if you’ve gone into your App Service settings and flipped the WebSockets switch to “Off”, the platform’s pipeline doesn’t skip over this module. It still evaluates upgrade requests, checks headers, and tries to keep things neat and tidy — like a bouncer checking everyone’s ID even when the club is technically closed.
That means any client attempting a WebSocket connection — intentionally or not — will go through this module. And if something fails along the way (say, your app returns a 500, or doesn’t know how to handle the upgrade request), the platform logs it under WebSocketMonitoringModule, even though your app has zero interest in WebSockets.
It’s like having a WebSocket gatekeeper on duty 24/7, even when your app is politely saying, “No thanks, we’re a REST-only kind of establishment.”
Why Are You Seeing 500 Errors From This Module?
Even if you’ve turned off WebSocket support in the App Service configuration, you might still see HTTP 500 errors logged by WebSocketMonitoringModule. Here’s why:
The Platform Always Runs the Module
One of the key things to understand about Azure App Service is that your app doesn’t live in isolation — it runs inside a managed hosting environment, with a standardized HTTP request pipeline that includes several built-in modules. These modules handle everything from routing and authentication to diagnostics and, yes, WebSocket monitoring.
So even if your app isn’t using WebSockets — and even if you’ve explicitly disabled them in the settings — the WebSocketMonitoringModule is still part of that pipeline. Think of it like airport security: even if you’re just grabbing a coffee at the terminal and not flying, you still go through the checkpoint.
If any inbound request includes headers that suggest a client wants to upgrade the connection to a WebSocket (like Connection: Upgrade and Upgrade: websocket), this module steps in to evaluate the handshake attempt. Whether the connection is accepted, denied, or falls apart in some strange and dramatic fashion — it’s the WebSocketMonitoringModule that logs the final word.
This means that even failed or rejected attempts will be caught and attributed to this module. And because it sits so early in the pipeline — often before your app code even gets a say — the error might be logged before your app has had a chance to respond, or even before it’s fully loaded.
The takeaway? Just because your app isn’t participating in a WebSocket conversation doesn’t mean other clients aren’t trying to start one — and it doesn’t stop the platform from trying to handle that exchange on your behalf.
Clients Still Try to Upgrade — Even If You Didn’t Ask Them To
Here’s where things get interesting: you might not be using WebSockets, but that doesn’t mean your clients got the memo.
Modern web libraries love to negotiate for WebSockets. Frameworks like SignalR, chat SDKs, real-time dashboards, IoT apps, or even some browser extensions will happily try to initiate a WebSocket connection as a first-class citizen — it’s fast, persistent, and shiny, after all.
The problem? These clients may automatically attempt an upgrade to WebSockets on any endpoint that looks like it might support it. Even if you’ve turned off WebSockets in your app, these requests still go through the App Service pipeline, and the platform still evaluates the handshake before deciding, “Nope, this place doesn’t support that.”
That handshake attempt might fail quietly — or it might explode dramatically with an HTTP 500 if something goes wrong during the negotiation (like your app returning a 500 due to missing logic or a bad header). Either way, WebSocketMonitoringModule logs the event, because it’s the one that intercepted the attempt.
So unless you’ve explicitly locked things down or blocked the headers at the edge, there’s a decent chance you’ve got some overeager client out there whispering “Upgrade?” every time they make a request.
Failed Upgrade Handshakes = Blame the WebSocket Bouncer
Let’s say a WebSocket upgrade attempt comes in. Your app doesn’t support it, maybe doesn’t even recognize the headers, and somewhere early in the request lifecycle — boom — an exception is thrown, or the response is malformed.
The platform’s response? “This one’s on WebSocketMonitoringModule.”
Because it was the module standing at the gate when things fell apart, it gets tagged as the origin of the failure — even though your app never had any intention of chatting via WebSockets in the first place.
It’s a little like blaming the receptionist for a meeting that went badly just because they opened the door.
This is why you’ll see 500 errors show up from WebSocketMonitoringModule, even when there’s no WebSocket logic in your codebase. The request never even made it to your app logic — it tripped over the platform’s front door.
Proxies and Load Balancers Could Be Misbehaving
Ah yes — the unsung middlemen of modern cloud architecture: proxies, gateways, and load balancers. These helpful traffic directors keep your app afloat by managing routing, health checks, SSL termination, and more. But sometimes? They try to be too helpful.
In some cases, intermediate services like Azure Front Door, Application Gateway, NGINX, or even a CDN might decide on their own that WebSocket upgrades sound like a great idea. They slap on headers like Connection: Upgrade and Upgrade: websocket without asking your app whether it’s interested in long-term commitment.
The result? Your app — or more accurately, the App Service pipeline — gets handed a request that smells like a WebSocket upgrade. The WebSocketMonitoringModule steps in, reviews the request, sees that your app wants nothing to do with it, and then logs the rejection as a 500 error.
But wait, there’s more! During events like:
- App restarts or swaps
- Scale-up/down events
- Cold starts
- Platform upgrades or instance reallocations
…the timing gets weird. A load balancer might retry the request or resend headers in a slightly off way, and suddenly you’re seeing mysterious 500s appear — all tied to WebSocketMonitoringModule, which is just trying its best to stay professional during the chaos.
And since these errors happen before your application logic gets to speak, they can feel totally random. “But I didn’t change anything,” you think. And you’re right — but your infrastructure buddies might’ve thrown a surprise party your app wasn’t ready for.
How To Investigate Further
If you’re not intentionally using WebSockets, but still see 500 errors tied to this module, here are a few steps to dig deeper:
Confirm WebSocket Settings
Head to:
Azure Portal → App Service → Configuration → General Settings
Make sure WebSockets are disabled.
Log Incoming Request Headers
Add middleware or logging to inspect headers on incoming requests. You’re looking for:
Connection: Upgrade
Upgrade: websocket
If these headers are present, something (client or proxy) is trying to initiate a WebSocket connection.
Use Application Insights (KQL)
Filter failed WebSocket-related requests in Kusto:
requests
| where resultCode == "500"
| where url has "ws" or url has "socket" or url contains "/signalr"
| project timestamp, name, url, resultCode, operation_NameThis can help pinpoint what routes are involved in the failure.
Optional ASP.NET Middleware for Detection
In .NET Core apps, add middleware to detect upgrade attempts:
app.Use(async (context, next) =>
{
if (context.Request.Headers["Connection"] == "Upgrade" &&
context.Request.Headers["Upgrade"] == "websocket")
{
// Log or handle as needed
Console.WriteLine("WebSocket Upgrade Attempt Detected");
}
await next();
});
Final Thoughts
Don’t Panic: Your App (Probably) Isn’t Broken
Seeing WebSocketMonitoringModule pop up in your error logs can feel like a red alert — especially when it’s tied to a bunch of ominous-looking 500 Internal Server Errors. But take a breath — and maybe a sip of coffee — because this doesn’t automatically mean your app is broken, misconfigured, or moonlighting as a chat server.
In fact, most of the time, these errors have nothing to do with your application logic. What you’re seeing is simply the App Service platform throwing up a flare to say, “Hey, someone tried to do a WebSocket thing, and it didn’t go well.”
It’s often the result of:
- Curious clients probing your endpoints
- Load balancers adding headers just for fun
- SignalR or other libraries testing available transports
- External systems that assume everyone wants a WebSocket relationship
In short: external behavior, not internal failure.
So What Should You Do About It?
If these errors are:
- Infrequent
- Not impacting real users
- Not triggering alerts or breaking dashboards
Then honestly? You can ignore them. They’re the equivalent of someone ringing your doorbell, realizing they have the wrong house, and walking away — slightly awkward, but harmless.
However, if:
- You’re seeing a large volume of these errors,
- They correlate with latency spikes or availability issues,
- Or your alerting system thinks the sky is falling every time they show up…
Then it’s worth doing a little detective work. Trace the requests, identify where those upgrade attempts are coming from, and consider:
- Blocking or filtering the headers upstream (e.g., at the gateway or CDN)
- Redirecting or gracefully rejecting WebSocket traffic in middleware
- Educating your clients (or vendors) that your app isn’t in the WebSocket business
You don’t need to build a full firewall — just politely but firmly close the door on any unwanted upgrade attempts.
Azure Advice 