That's too little. All access from a different origin should be blocked by default, not only to local nets.

How shall origin be defined? I can envision the likes of Microsoft which have many, many second-level domains making calls between them.

We can’t allow the site itself to grant access. How would this be managed, other than “please stop and think what a domain name is supposed to be before spraying your product across twelve of them?”

(Which is in turn made harder by IPv6 public addressing, where you can’t just block the private IP range because you might not be behind a NAT in the first place, instead only behind a firewall. So your address A::B, can route to your Intranet peer A::C, which isn’t public-routable, but is a public address. But there’s nothing, other than the firewall, that says that that’s not a public address. It’s a hard problem!)

evil.com can set a short DNS TTL, and after you access it, it can rebind its address to 127.0.0.1. Then subsequent requests to evil.com go to localhost (e.g. fetch("evil.com", ...) on evil.com will go to 127.0.0.1 if the DNS rebound successfully).

Caching a website's IP on first use doesn't help, either, because it breaks long sessions on websites that use DNS rebinding for legitimate purposes (load balancing, fallover).

The only real way to fix this is for the local webserver to check the Host header on the HTTP request... or look at IP addresses. But building a global registry of IP addresses is hard, so we're stuck with trusting application developers (and malware writers) who run servers on localhost to use good security practices.

The process is different enough from cookies to warrant another large discussion about how to do it, with plenty of trial and errors. But the stakes are much lower, as in the worst case the user will get a dialog, instead of a site being broken.

Still, doesn’t mitigate attacks against non-HTTP speakers.

Forgoing CORS and making all inter domain requests user opt-in would make the web experience a lot worse, IMO. Making all intranet or localhost requests user opt-in seems less disruptive.

In the case of scanning, a CORS denial can still reveal information about the user's internal network, as a CORS denial is a different result than a network timeout or a TCP RST.

That a script is able to gather information about an origin that did not it in seems like a serious bug to me.

I don't think you need to overdo it in terms of making the warning red, etc. Just a popup will really discourage people from trying to use this for fingerprinting.

BTW the site says:

"Port scanning is malicious." I don't agree. There are many many things that can look like a port scan but are not malicious, most notably NAT traversal attempts by WebRTC, games, chat apps, and so on.

It's not against the law. It might be _shady_ but it's not illegal. When I'm teaching cyber intro classes, I let folks know portscanning is NOT illegal but shady. It's like going to a business after hours. It's not illegal to rattle doors and windows to see if they're locked. The police might have a different take on it, but it's not illegal.

While it can be used to get information to bad things, it itself can be used for good things too.

I’d be all for a user notification that says “fnord.com wants to access 192.168.0.10 on tcp/443, which seems to be a web server on your home/work network. Are you sure you want to allow this?” I’d want to see this for each new access request, such that port scanning would not be a use case that was supported.

Sure, have an about:config toggle to shut this off, with appropriate warnings.

I find it very surprising that now any random website can access it with no oversight. Why worry about spectre and meltdown when such blatant backdoors are implemented in browsers?!

For instance, if I'm running a local chat application and need it to communicate with the web version, why does the website need to be able to port scan to accomplish this? I can think of other ways to accomplish this that are a lot more secure.

But I think if same-origin were enforced more strictly, they could have found another way.

Here's an interesting tangential article about how they get around obstacles with SSL certs for localhost: https://letsencrypt.org/docs/certificates-for-localhost/

Your bot is running a redis server locally, it allows local connections, because it's just a bot, boom.

Taking it a bit further, if we have really smart people involved: the timing of the attempted connections/rejections tell you something about the system that you can use to detect bots/scrapers.

Another example of this being used in the past is to scan for chrome extensions that scrape site content as well. I believe LinkedIn might have gotten hit hard for trying something similar but they were using extension URLs not localhost. Some extensions do spin up localhost services though.

Such as?

A port scanner running on a webpage without the users knowlege is never legitimate.

So the question is what legitimate reason is there for a port scanner running in a web browser with the users knowledge?

I don't get upset if someone opens and closes a socket to my VPS to see if something's there. My VPS is exposed to the internet. If a socket opens, it should be secure anyways. There's the chance nginx has an unknown zero day, but if I wanted to avoid that, I'd firewall it.

Things are a little less nice if you open a socket and start sending data to see what's there, assuming the server doesn't respond with a banner.

That might be a reason for you to port scan your network.

It is not a reason for your website to port scan my network. Especially since your website running inside my browser is inside my firewall.

> and want to run a security audit

Then you use tools designed to run security audits. You don't open a huge security hole in everybody's browser just so you can use a browser to run a security audit.

There's really no legitimate reason for eBay, or any other website, to portscan your computer. There's nothing there needed for browsing their website.

That's not what's happening here.

My laptop is not exposed to the public internet because it's behind a firewall / NAT. This is like going to my house, plugging a device into an Ethernet port on my router, and scanning my internal network from inside my network.

Except instead of them planting a device, all they have to do is get you to navigate to their webpage. They're getting your laptop to do the port scanning for them, and in so doing, they get access to your internal network. The problem isn't port scanning, the problem is NAT busting.

It shouldn't matter what malware is on a client device as long as the client has authenticated; the server/company/ebay should be protecting their API from abuse at the API layer, not the client layer.

Know your network.