[kdunglas]

Hi, Mercure author here.

WebSockets are hard to secure (they totally bypass CORS as well as other browser built-in protections), don't work (yet) with HTTP/3 and for most use cases require to implement many features by yourself: reconnection in case of network failure, refetch of lost messages, authorization, topic mechanism…

Mercure, which is built on top of SSE (it's more an extension to SSE than an alternative to it) fix these issues.

However, SSE (as well as WebSockets) can be hard to use with stacks not designed to handle persistent connections such as PHP, serverless, most web servers proxying Ruby, Python etc apps. Even for languages designed to handle persistent connections, it's often more efficient and easier to manage persistent connections with ah-hoc software running on dedicated hardware.

That's what Mercure allows. Mercure provides a "hub", a server that will maintain the persistent connections with the browsers, store events, and re-send them in case of network issues (or if asked for old messages by a client). To broadcast a message to all connected users, the server app (or even a client) can just send a single POST request to the hub. The hub will also check that clients are authorized to subscribe or publish to a given topic (that's why JWT is used).

The reference implementation is written in Go, as a module for the Caddy web server, and his very efficient/optimized (it can handle thousands of persistent connections on very small machines).

Install a Mercure hub and you have all these features available without having to write any code. Client-side, no SDK is required, you can embrace the built-in EventSource JavaScript class.

[Thorrez]

> they totally bypass CORS

You can reimplement the Same Origin Policy serverside by checking that the Origin header equals the Host header. Even more secure would be to check both against an allowlist (this protects against DNS rebinding, which the Same Origin Policy doesn't protect against).

>as well as other browser built-in protections

I'm curious what those are.

>for most use cases require to implement many features by yourself: [...] authorization

Isn't auth of websockets generally the same as auth of any Javascript-initiated HTTP request (e.g. fetch())? Check that the cookie looks good? Now, in the cause of OAuth tokens, websockets are more difficult than fetch(), because you cannot attach an Authorization: Bearer header to a websocket. But OAuth is less common than cookies for websites.

[kdunglas]

Once the connection is upgraded, you loose all metadata included in the HTTP headers (because it’s not HTTP) and all protections relying on it. Also CORS and SOP can be bypassed: https://dev.to/pssingh21/websockets-bypassing-sop-cors-5ajm

Even very experienced teams make mistake with this kind of things. See for instance, this Kubernetes vulnerability: https://goteleport.com/blog/kubernetes-websocket-upgrade-sec...

Of course you can reimplement everything by hand (and you must if you use WebSockets), but with SSE/Mercure you don't have to because it's plain old HTTP.

[whilenot-dev]

> Once the connection is upgraded, you loose all metadata included in the HTTP headers (because it’s not HTTP) and all protections relying on it.

The Upgrade request is HTTP and you can extract all needed metadata from there and store it server side as needed. Those metadata wouldn't change during an active WebSocket session anyway, would they?

[matharmin]

With your own native client: Yes, you can send arbitrary headers in the Upgrade request.

In a browser however, you can't. It typically sets very little headers itself, and doesn't allow you to add custom headers.

[whilenot-dev]

The auth headers (Authorization, Cookie) are all passed along, and that's what I want to establish a secure connection from the browser.

For more customized wishes there's always this "ticket"-based flow[0][1] that shouldn't be hard to implement. I might be a bit naive, but what needed metadata and custom headers are we talking about?

[0]: https://devcenter.heroku.com/articles/websocket-security#aut...

[1]: https://lucumr.pocoo.org/2012/9/24/websockets-101/#authoriza...

[Thorrez]

>Also CORS and SOP can be bypassed: https://dev.to/pssingh21/websockets-bypassing-sop-cors-5ajm

Like I said, you can reimplement SOP by checking that the Origin header equals the Host header. Or alternatively check them against an allowlist. It's just a couple lines of code, and then it's as secure as normal HTTP.

https://dev.to/pssingh21/websockets-bypassing-sop-cors-5ajm

That article contains a confusing quote:

>The initial handshake occurs via HTTP Upgrade request, the response body is disregarded and the HTTP/HTTPS protocol is upgraded to WS/WSS protocol.

But the response body isn't disregarded. What could best be described as the response body is the server->client websocket messages. Those aren't disregarded.

>Even very experienced teams make mistake with this kind of things. See for instance, this Kubernetes vulnerability: https://goteleport.com/blog/kubernetes-websocket-upgrade-sec...

Thanks, that's an interesting vulnerability. I'm not so sure it's websocket-specific though. It looks like the attack could be done without websockets at all. The end request that does the malicious action is plain HTTP. The initial request that triggers the desync uses a Connection: Upgrade header, which is used in websockets, but isn't actually websocket-specific. Connection: Upgrade was created in RFC 2616 from 1999, which was before the websocket RFC 6455 in 2011.

The vulnerability was there was a proxy, and the proxy didn't properly handle the Connection header as specified in RFC 2616. RFC 2616 says that a proxy must remove the Connection header if it's not handling it itself, and the proxy in this case wasn't handling it itself (not checking for 101 response).

I would say this vulnerability is an HTTP request smuggling vulnerability. That's a vulnerability where there's a proxy that implements some type of security, and the proxy can be tricked into sending a request to the backend that the proxy didn't actually parse. HTTP request smuggling vulnerabilities impact regular HTTP, not just websockets. This has nothing to do with the Same Origin Policy.

https://portswigger.net/web-security/request-smuggling

https://portswigger.net/research/http-desync-attacks-request...

[jand]

> ... because you cannot attach an Authorization: Bearer header to a websocket.

Well, not properly. You can abuse the Sec-Websocket-Protocol header to pass an initial token to the server.

[Thorrez]

:) I have in fact done that.

[coolhand2120]

> WebSockets are hard to secure (they totally bypass CORS as well as other browser built-in protections), don't work (yet) with HTTP/3 and for most use cases require to implement many features by yourself: reconnection in case of network failure, refetch of lost messages, authorization, topic mechanism…

Having written WebSocket CORS with Authentication (Cognito) I know this isn't quite true. The initial connection is a standard HTTP request that returns a 100 series, with preflights and everything. That initial request has all the headers you might want to sent to a server for auth. It's a bit odd IMO but the auth string is sent in the web socket constructor, second argument, really easy to miss. Happy to provide both server and client code examples.

https://developer.mozilla.org/en-US/docs/Web/API/WebSocket/W...

[panarky]

This is helpful, thank you.

You might consider updating the readme to make it clear what Mercure/SSE does that WebSockets doesn't.

If it was me, I'd also put in the readme what WebSockets does that Mercure/SSE does not, such as bidirectional communication, low latency client-to-server messages, binary data transfer, etc.

[gregors]

Exactly this, the landing page seem much more about the reference implementation than the actual protocol. I will say the docs at https://mercure.rocks/spec are nice.

[archerx]

I have been using Server Sent Events with PHP since 2017 and it works very well. All you need is the right headers and a loop that breaks on user disconnect. Honestly it was super easy and had no issues setting it up and getting it to work.

[whilenot-dev]

It might be worth mentioning to the parent poster that SSEs (and Mercure) are unidirectional only (server -> client), whereas WebSockets are bidirectional.

[the_mitsuhiko]

Nowadays that's hardly an issue because SSE is multiplexed over HTTP/2 and HTTP/3 alongside regular requests. In some ways, SSE is today more efficient than websockets too because if you use it with HTTP/3 then you're avoiding head of line blocking which is still an issue with websockets.

(Also in practice many places websockets are not even supporting HTTP/2 because on the server they were implemented on top of raw sockets which is no longer possible with HTTP/2)

[chris_pie]

Head-of-line still exists in HTTP/3, but on another level. A stream can still get blocked. With websockets the whole connection gets blocked (HTTP/1), but that connection isn't shared with anything else, right?

Worth noting that HTTP/3 seems to recover faster than TCP-based protocols.