8 ms·

Doesn't CORS just restrict whether the webpage JS context gets to see the response of the target request? The request itself happens anyway, right? So the atta

• tombakt 1y ago
  No, a preflight (OPTIONS) request is sent by the browser first prior to sending the request initiated by the application. I would be surprised if it is possible for the client browser to control this OPTIONS request more than just the URL. I am curious if anyone else has any input on this topic though. Maybe there is some side-channel timing that can be used to determine the existence of a device, but not so sure about actually crafting and delivering a malicious payload.
  • rafram 1y ago
    You’re forgetting { mode: 'no-cors' }, which makes the response opaque (no way to read the data) but completely bypasses the CORS preflight request and header checks.
    • chuckadams 1y ago
      Thankfully no-cors also restricts most headers, including setting content-type to anything but the built-in form types. So while CSRF doesn't even need a click because of no-cors, it's still not possible to do csrf with a json-only api. Just be sure the server is actually set up to restrict the content type -- most frameworks will "helpfully" accept and convert form-data by default.
    • jonchurch_ 1y ago
      This is missing important context. You are correct that preflight will be skipped, but there are further restrictions when operating in this mode. They don't guarantee your server is safe, but it does force operation under a “safer” subset of verbs and header fields. The browser will restrict the headers and methods of requests that can be sent in no-cors mode. (silent censoring in the case of headers, more specifically) Anything besides GET, HEAD, POST will result in an error in browser, and not be sent. All headers will be dropped besides the CORS safelisted headers [0] And Content-Type must be one of urlencoded, form-data, or text-plain. Attempting to use anything else will see the header replaced by text-plain. [0] https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header https://developer.mozilla.org/en-US/docs/Glossary/CORS-safel...
      • rafram 1y ago
        That’s just not that big of a restriction. Anecdotally, very few JSON APIs I’ve worked with have bothered to check the request Content-Type. (“Minimal” web frameworks without built-in security middleware have been very harmful in this respect.) People don’t know about this attack vector and don’t design their backends to prevent it.
        • jonchurch_ 1y ago
          I agree that it is not a robust safety net. But in the instance you’re citing, thats a misconfigured server. What framework allows you to setup a misconfigured parser out of the box? I dont mean that as a challenge, but as a server framework maintainer Im genuinely curious. In express we would definitely allow people to opt into this, but you have to explicitly make the choice to go and configure body-parser.json to accept all content types via a noop function for type checking. Meaning, its hard to get into this state! Edit to add: there are myriad ways to misconfigure a webserver to make it insecure without realizing. But IMO that is the point of using a server framework! To make it less likely devs will footgun via sane defaults that prevent these scenarios unless someone really wants to make a different choice.
          • rafram 1y ago
  • deleted 1y ago
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  • LegionMammal978 1y ago
    The idea is, the malicious actor would use a 'simple request' that doesn't need a preflight (basically, a GET or POST request with form data or plain text), and manage to construct a payload that exploits the target device. But I have yet to see a realistic example of such a payload (the paper I read about the idea only vaguely pointed at the existence of polyglot payloads).
    • kbolino 1y ago
      Here's a formal definition of such simple requests, which may be more expansive than one might expect: https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CORS#simple_requests https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/COR...
    • MajesticHobo2 1y ago
      There doesn't need to be any kind of "polyglot payload". Local network services and devices that accept only simple HTTP requests are extremely common. The request will go through and alter state, etc.; you just won't be able to read the response from the browser.
      • EGreg 1y ago
        Exactly. People who are answering must not have been aware of “simple” requests not requiring preflight.
    • mirashii 1y ago
      You don't even need to be exploiting the target device, you might just be leaking data over that connection. https://news.ycombinator.com/item?id=44169115 https://news.ycombinator.com/item?id=44169115
      • notpushkin 1y ago
        Yeah, I think this is the reason this proposal is getting more traction again.
    • 1y ago
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  • layer8 1y ago
    There is a limited, but potentially effective, attack surface via URL parameters.
  • rerdavies 1y ago
    I can confirm that local websites that don't implement CORS via the OPTIONS request cannot be browsed with mainstream browsers. Does nothing to prevent non-browser applications running on the local network from accessing your website. As far as I can tell, the only thing this proposal does that CORS does not already do is provide some level of enterprise configuration control to guard against the scenario where your users are using compromised internet sites that can ping around your internal network for agents running on compromised desktops. Maybe? I don't get it. If somebody would fix the "no https for local connections" issue, then IoT websites could use authenticated logins to fix both problems. Non-https websites also have no access to browser crypto APIs so roll-your-own auth (the horror) isn't an option either. Fustrating!
    • dgoldstein0 1y ago
      I don't believe this is true? As others have pointed out, preflight options requests only happen for non simple requests. Cors response headers are still required to read a cross domain response, but that is still a huge window for a malicious site to try to send side effectful requests to your local network devices that have some badly implemented web server running.
    • rerdavies 1y ago
      [edit]: I was wrong. Just tested that a moment ago. It turns out NOT to be true. My web server during normal operation is current NOT getting OPTIONS requests at all. Wondering whether I triggered CORS requests when I was struggling with IPv6 problems. Or maybe it triggers when I redirect index.html requests from IPv6 to IPv4 addresses. Or maybe I got caught by the earlier roll out of version one of this propposal? There was definitely a time while I was developing pipedal when none of my images displayed because my web server wasn't doing CORS. But. Whatever my excuse might be, I was wrong. :-/
  • varenc 1y ago
    This tag: <img src="http://192.168.1.1/router?reboot=1"> triggers a local network GET request without any CORS involvement.
    • lyu07282 1y ago
      Exactly you can also trigger forms for POST or DELETE etc. this is called CSRF if the endpoint doesn't validate some token in the request. CORS only protects against unauthorized xhr requests. All decades old OWASP basics really.
      • formerly_proven 1y ago
        That highly ranked comments on HN (an audience with way above average-engineer interest in software and security) get this wrong kinda explains why these things keep being an issue.
        • lyu07282 1y ago
          I don't know why you are getting downvoted, you do have a point. Some of the comments appear knowing what CORS headers are, but neither their purpose nor how it relates to CSRF it seems, which is worrying. It's not meant as disparaging. My university thought a course on OWASP thankfully, otherwise I'll probably also be oblivious.
          • _cenw 1y ago
            If you're going cross-domain with XHR, I'd hope you're mostly sending json request bodies and not forms. Though to be fair, a lot of web frameworks have methods to bind named inputs that allow either.
            • bawolff 1y ago
              This misses the point a bit. CSRF usually applies to people who want only same domain requests and dont realize that cross domain is an option for the attacker. In the modern web its much less of an issue due to samesite cookies being default .
  • thayne 1y ago
    It depends. GET requests are assumed not to have side effects, so often don't have a preflight request (although there are cases where it does). But of course, not all sites follow those semantics, and it wouldn't surprise me if printer or router firmware used GETs to do something dangerous. Also, form submission famously doesn't require CORS.
  • bawolff 1y ago
    > No, a preflight (OPTIONS) request is sent by the browser first prior to sending the request initiated by the application. Note: preflight is not required for any type of request that browser js was capable of making prior to CORS being introduced. (Except for local network) So a simple GET or POST does not require OPTIONS, but if you set a header it might require OPTIONS (unless its a header you could set in the pre-cors world)
• nbadg 1y ago
  Or simply perform a timing attack as a way of exploring the local network, though I'm not sure if the browser implementation immediately returns after the request is made (ex fetch API is called) but before the response is received. Presumably it doesn't, which would expose it to timing attacks as a way of exploring the network.
  • dgoldstein0 1y ago
    Almost every js API for making requests is asynchronous so they do return after the request is made. The exception though is synchronous XHR calls, but I'm not sure if those are still supported ... Anyhow I think it doesn't matter because you can listen for the error/failure of most async requests. Cors errors are equivalent to network errors - the browser tells the js it got stays v code 0 with no further information - but the timing of that could lead to some sort of interference? Hard to say what that would be though. Maybe if you knew the target webserver was slow but would respond to certain requests, a slower failed local request could mean it actually reached a target device. That said, why not just fire off simple http requests with your intended payload? Abusing the csrf vulnerabilities of local network devices seems far easier than trying to make something out of a timing attack here.
  • baobun 1y ago
    eBay, for one, has been (was?) fingerprinting users like this for years. https://security.stackexchange.com/questions/232345/ebay-web-site-tries-to-connect-to-wss-localhostxxxxx-is-this-legit-or-they https://security.stackexchange.com/questions/232345/ebay-web...
• jonchurch_ 1y ago
  I think CORS is so hard for us to hold in our heads in large part due to how much is stuffed into the algorithm. It may send an OPTIONS request, or not. It may block a request being sent (in response to OPTIONS) or block a response from being read. It may restrict which headers can be set, or read. It may downgrade the request you were sending silently, or consider your request valid but the response off limits. It is a matrix of independent gates essentially. Even the language we use is imprecise, CORS itself is not really doing any of this or blocking things. As others pointed out it’s the Single Origin Policy that is the strict one, and CORS is really an exception engine to allow us to punch through that security layer.