Hacker News Reader

Comment by anaphor | Hacker News Reader

> I'm pretty sure you're incorrect - requests to sites different from the one you are currently on will not include auth data by default because of the same-origin policy

Not necessarily true. You can have Javascript running on evil.com that submits a form to banking.com and it will send your session cookies for banking.com along with the request. It's just that evil.com can't read the response content.

View on HN

I thought the browser at evil.com would first send a pre-fetch request without the cookies, but not send the full request when it doesn't get the correct value from Access-Control-Allow-Origin in the response from banking.com.

I'll admit I may be one of the developers that doesn't understand CORS...

I think you could implement that with CORS + CSRF tokens (blocking all types of Cross-Origin requests) but the default behaviour is to allow Cross-Origin writes (e.g. with form submissions).

See https://developer.mozilla.org/en-US/docs/Web/Security/Same-o...

This is why we have CSRF tokens. So that you can effectively block Cross-Origin writes.

I think pixelperfect is talking about preflights.

That makes more sense then. Yes, it will make sure not to send authentication data in that case.

I see what you mean, evil.com could still make a request that includes cookies, which is why we need CSRF tokens. But from my understanding, it wouldn't be able to do that in a XMLHttpRequest hidden on the page. It would have to be a request from something like submitting a form which would navigate the user off the page. Is that correct? Of course it doesn't make much difference from a security perspective.

You can work around that by submitting it within an invisible iframe element, e.g. https://stackoverflow.com/a/17953761/903589

But yeah, you can't just make arbitrary requests like this with XHR

If you use fetch or XHR, yes.

If you just create an HTML form with the bank as action, fill in the inputs and submit via JS, no, because that's _navigating away_. This is also why GET requests that perform actions are dangerous, you can just embed them as an image to provoke a request. Which is exactly how zoom built their API. (they also then measured the size of the image to determine the response code, which is.... inventive)

CORS doesn't apply to navigations by default, so anaphor is correct: evil.com can just submit a form to banking.com and it will send the banking.com cookies. This is the whole field of CSRF (cross-site request forgery) mitigation.

CORS was introduced as a way to allow requests that browsers used to not allow at all: things like cross-site XHR in the first instance. Then it was expanded so that requests that are not normally subject to CORS checks (image loads, script loads, stylesheet loads) could opt-in to being subject to them, for various reasons. The default for those loads is still "no CORS". And there still isn't a way to do a navigation subject to a CORS check, even with opt-in.

Disclaimer: I work on Gecko and I've reviewed/implemented parts of the CORS spec.

Is my take on this also generally correct? That it wouldn't have been a problem had cookies and such been designed to take into account the origin properly (and hence why it's unintuitive and catches people off-guard)?

If cookies were scoped to the (source, target) pair, then that would remove one of the main motivations for CORS, yes: evil.com would not be able to get any information from bank.com by making your browser make the request that they could not get by doing the request server-side.

There's a second problem CORS kinda tries to solve, which is the ambient authority problem: services that run behind firewalls and assume that if someone can reach them the someone should have access. If someone runs a browser behind the firewall and opens a page on unsafe side of the firewall, that page can then issue network requests from the browser and thus end up access things on the "safe" side of the firewall. This is a large part of why CORS has the whole preflight complication and the rules around when preflights happen: the idea is that in this situation just making the request, not even receiving a response, is potentially damaging. There are the carve-outs for requests that could be generated without things like XHR that are subject to CORS (e.g. by doing a form submission or <img> load or whatnot); if your ambient-authority-using server responds in interesting ways to those, CORS is not going to help you... The _right_ fix for this stuff, of course, is for services to stop using ambient authority and/or for browsers to block requests from public sites to private IPs. Unfortunately in practice detecting "private IPs" reliably is not trivial, because fundamentally it depends on the routing and firewall topology, which the browser doesn't really know about.

Thank you for the reply! I didn't realize this actually gets to another question I've had in another context, which is why can't a browser at least assume 192.168.0.0/16 etc. are private networks and block requests from nominally-public addresses from being sent to those? (Or do they do that already?) This should be possible without needing to detect anything at all, right?

Pretty sure some tools like NoScript already do that, but to protect against DNS rebinding (which can subvert the SOP), not as a protection against cross-origin requests coming from a public site to an internal one.

https://en.wikipedia.org/wiki/DNS_rebinding

> The NoScript extension for Firefox includes ABE, a firewall-like feature inside the browser which in its default configuration prevents attacks on the local network by preventing external webpages from accessing local IP addresses.

Yeah but why shouldn't the browser do this itself to provide the cross-origin protection?

I think just because they're afraid of breaking things? I'm honestly not sure what the rationale was for not implementing this. They do DNS pinning which helps mitigate DNS rebinding attacks, but I don't think they do anything specifically to restrict access to internally routable IPs.

They do block certain ports that are known to be problematic (25, 6667, 5222, etc)

It's so stupid. Makes me want to just fork the browsers and add blatantly obvious protections like this if I can find the time...

If you do that, I would recommend looking at writing an extension first to see if there's any way to do it without a fork (maybe using the same technique that things like ublock origin have)

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...

Yeah you can almost certainly do it with an extension, but half the point of a fork would be to get the message across that they need to get their act together. (I almost certainly won't get around to it though, so this is just daydreaming.)

There turn out to be a bunch of complications when one tries to do that, if the goal is to not break existing legitimate uses.

I haven't been following this closely, but for Firefox https://bugzilla.mozilla.org/show_bug.cgi?id=354493 is the relevant bug, with some (failed) attempts to do that.

Interesting. It seems the "legitimate" uses are corporate? Seems like providing a group policy or config option to disable protection against this would be the sensible way to go, rather than increasing the attack surface of home users just because some corporate users do weird things. Although honestly major software vendors are already happy breaking so many things in the name of security that this one could just be another one on top...

Reply on news.ycombinator.com