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To summarize:

  • CSRF is an attack where a page in a different window/tab of the browser sends nonconsensual request to an authenticated web app, that can typically be prevented from server-side by checking the Referer, Origin header of the request or including anti-CSRF token in request header or body.

  • CORS is variously defined in different sources, that might roughly be summarized as: a mechanism that host-of-origin-B indicates to the browser how or whether a host-of-origin-A content should access its resources.

Cross-origin-related attacks and party responsible for defence

  1. Nonconsensual "state-changing" requests: The server.

Be it a treacherous form submission, XHR/fetch request from a "second-origin" page, the server need to recognize and reject them.

  1. Cross-origin data (credential, privacy) leaking: The browser.

Be it a media tag (img, video, audio), XHR/fetch request, the browser must isolate those data from the access of "second-origin" pages.

  1. Q1: anythine else?

Purpose of CORS in cross-origin settings

To let browsers make informed decisions on how:

  1. To make concensual "state-changing" possible cross-origin.

Q2: Is there an real-world example of concensual cross-origin "state-changing" user action?

  1. To allow "creative" ways of collecting and using of digital assets hosted at "second-origin" host(s) by the script.

Misc.

Q3: CORS doesn't help browser protect users from malicious digital asset hosts determined to steal data from users? But honest origin servers can prevent them from submitting CSRF requests?

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Bit of a rambling question, so bit of a rambling answer also.

CSRF can also be defended by including a custom header on XHR requests, and with same site cookies.

CORS is not variously defined; it is a W3C standard. What sometimes causes confusion is that CORS is not really a security mechanism.

Cross-origin data leaking is an area of active research and requires defences both built-in to the browser, and in both server-side and client-side application code.

Q1) The main other cross-origin attack is cross-site scripting. Defences needed in both server-side and client-side application code. Clickjacking is another one. There are probably some more although I can't think just now.

Q2) I've found most uses of CORS are for reads. Some REST APIs have permissive CORS settings but require a security token in requests. This lets third-party domains access the API, once the user has authorized them, e.g using OAuth.

Q3) That's right, CORS is not a security mechanism and doesn't in itself defend against anything. It does sometimes stop people using insecure alternatives like JSONP so it can help security.

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