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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. T

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.

mehrdadn 7y ago
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)?
- bzbarsky 7y ago
  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.
  - mehrdadn 7y ago
    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?
    - anaphor 7y ago
      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 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.
      - mehrdadn 7y ago
        Yeah but why shouldn't the browser do this itself to provide the cross-origin protection?
        anaphor 7y ago
        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)
        mehrdadn 7y ago
        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...

mehrdadn 7y ago

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)?

bzbarsky 7y ago
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.
- mehrdadn 7y ago
  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?
  - anaphor 7y ago
    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 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.
    - mehrdadn 7y ago
      Yeah but why shouldn't the browser do this itself to provide the cross-origin protection?
      - anaphor 7y ago
        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)
        mehrdadn 7y ago
        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...