Isn't CSRF a browser security issue?

Question

Regarding cross-site request forgery (CSRF) attacks, if cookies are most used authentication method, why do web browsers allow sending cookies of some domain (and to that domain) from a page generated from another domain?

Isn't CSRF easily preventable in browser by disallowing such behavior?

As far as I know, this kind of security check isn't implemented in web browsers, but I don't understand why. Did I get something wrong?

About CSRF:

• On wikipedia
• On coding horror

Edit: I think that cookies should not be sent on http POST in the above case. That's the browser behavior that surprises me.

Answer 1

Why wouldn't the browser send cookies?

Site A (http://www.sitea.com) sets a cookie for the user.

User navigates to site B (http://www.siteb.com). Site B features integration with site A - click here to do something on site A! The users clicks "here".

As far as the browser can tell, the user is making a conscious decision to make a request to site A, so it handles it the same way it would handle any request to site A, and that includes sending site A cookies in the request to site A.

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Edit: I think the main issue here is that you think there is a distinction between authentication cookies and other cookies. Cookies can be used to store anything - user preferences, your last high score, or a session token. The browser has no idea what each cookie is used for. I want my cookies to always be available to the site that set them, and I want the site to make sure that it takes the necessary precautions.

Or are you saying that if you search yahoo for "gmail", and then click on the link that takes you to http://mail.google.com, you shouldn't be logged in, even if you told gmail to keep you logged in, because you clicked on the link from another site?

Answer 2

It isn't that a browser is sending the cookie to or from an outside domain, it's the fact that you're authenticated and the site isn't validating the source of the request, so it treats it as if the request came from the site.

As far as whether a browser should disallow that... what about the many situations where cross-site requests are desirable?

Edit: to be clear, your cookie is not sent across domains.