It's not just CORS that's hard to understand. Many (most?) developers don't really understand the threat model. And even when it's explained it hard to see why it's a big deal. Part of this is that backend developers usually have to configure CORS and it's not an access privilege protection. From the point of view of the backend it doesn't seem to matter. Bad guys can't get it. From the point of view of the front-end it's often seen as a nuisance.

The article does a nice job giving a concrete example.

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yaur 2 hours ago | parent | next [-]

It’s not that hard to understand… in the cors threat model an attacker gets one your users to take an action on your site by visiting their site.

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hn_throwaway_99 an hour ago | parent | next [-]

> in the cors threat model an attacker gets one your users to take an action on your site by visiting their site

This is really oversimplifying things, incorrectly IMO, and that sentence makes it sound like you're confusing a CSRF vulnerability with CORS protections. Normally when you write a backend server you implement some sort of authentication and access control, and in that scenario the threat model that lets "an attacker gets one your users to take an action on your site by visiting their site" is a CSRF vulnerability, unrelated to CORS.

The scenario presented in TFA is actually a very special case, because the bug is with a webserver running on localhost that doesn't (apparently) implement access control - not something most web apps entail.

In fact, one of the parts that confuses a lot of people is that CORS rules only prevent the JavaScript web client from reading the response from a remote endpoint - if the endpoint is available on the public Internet then anyone can still make a request to it.

The other thing that is confusing about CORS is that browsers already let you load lots of resources from cross origin servers - you can load images (as TFA points out that Zoom did as a workaround), scripts, stylesheets, form submissions, etc. The one thing you can't do, unless the server implements the appropriate CORS headers, is make a cross origin fetch request from JavaScript.

	▲	ctidd an hour ago \| parent [-]
		All CORS does is allow for selective loosening of anti-CSRF controls. CORS is a mechanism for a service to tell a client “I’m CSRF-resistant” so that that the client doesn’t need to protect its user as tightly when interacting with that service.

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harrall 2 hours ago | parent | prev [-]

It’s easy to understand but most people don’t naturally think of ways people try to break in. Without thinking about that, the importance of security is low.

It isn’t a knowledge thing (though it could be), or a capability thing, or intelligence. It’s pure mindset.

Ask yourself: is the average person noticing holes in fences and trying random doorknobs… probably not.

But on the other hand, most security people don’t think of product or UX (but some might) so that’s why you have roles.

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cookiengineer 2 hours ago | parent | prev [-]

On top of that, it's a threat model that doesn't make really sense from an attacker vs defender perspective. Because it's optional, and all kinds of other libraries and tools can just blatantly ignore it anyways.

CORS literally exists only against XSS and CSRF for actively logged in human users. Anything else in CORS is absolutely pointless because every other attack scenario uses scripts or programs that fake HTTP headers anyways. It's just as useless as the Sec-CH (client hint) headers because some Browser made by a company that starts with Micro and ends with Slop decided that the User Agent always needs to be Windows 10 for compatibility reasons.

That is why you see everyone just enabling every CORS option anyways, even though that is literally the worst case that allows XSS and CSRF. And a lot of websites have user edited content at some place, at the very least in images that aren't filtered for embedded scripts (PNGs, anyone?).

	▲	wonnage 2 hours ago \| parent [-]
		What else is there in CORS? It’s all basically a way for an origin to communicate to the browser which other origins it can share data with. Of course if there’s no browser involved then there’s no need for it. Client hints are useful for all the shitty “responsive” websites that don’t know how to use media queries. And for ad tracking. Mostly the latter