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In this post, there is a paragraph that mentions a scenario where there is no need to validate anti-forgery token in login page:

When is it OK to leave off the anti-forgery token? In general, if the target is a URL, and accessing that URL has no side effects, then you don't need to include anti-forgery token in that URL.

I understand how a CSRF attack works but I am quite lost at this paragraph unfortunately. It says:

  • If the target is a URL
  • If accessing that URL has no side effects

Is there a target that is not URL? and what is an example for a "side effect"?

I have an ASP.NET MVC application that is hosted at a URL such as subdomain.domain.com. My users receive these errors:

“The provided anti-forgery token was meant for a different claims-based user than the current user.”

“The anti-forgery cookie token and form field token do not match.”

“The provided anti-forgery token was meant for user "", but the current user is > "XYZ".”

In other words, my question is that what are the scenarios in which it is okay not to use anti-forgery token in login page?

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    That's two separate questions: (1) what is a side effect, and (2) when is it OK to not use an anti-forgery token on a login page? Generally when you have two separate questions it is best to ask them separately. – D.W. Feb 21 at 22:15
  • I suppose "non-URL target" refers to anchor (<a>) or button/input with a JS listener that will stop the click propagation: user clicks on a link, but the browser doesn't actually follow the link to a web document (instead, JS listener stops the event and do treatments in the current page) – Xenos yesterday
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Visiting a URL can cause a side effect. For instance, if I'm logged into Amazon, visiting a URL may cause an item to be purchased and shipped to me (one-click buy). That's a side effect. Or, it might log me out, or log me in, or update some settings. Those are side effects, too.

In general, a operation has a side effect if it changes some (persistent) state on the server, or has some persistent observable effect on the world.

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CSRF protection is not required when the action performed by the cross site request itself is trivial. But it is ideal to have such a protection for any action which has a persistent aftereffect in the applications context.

For example:

A file upload for configuration files would require an anti-CSRF token where as a simple search query which will only display the information would not (if your application's CORS settings are set right. )

In case of login pages, it wouldn't be required in general unless a failed login action itself would create a persistent change in application's context.

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    You're neglecting the possibility of a successful login action, using the attacker's credentials (ie login CSRF). – AndrolGenhald Mar 24 at 0:07
  • @AndrolGenhald Even in that case, nothing other than the login action itself would be possible if the application itself is protected against CSRF in post-login actions. – hax Mar 24 at 0:44
  • Doesn't matter, it's still a vulnerability. I go to definitelynotmalicious.com and it uses login csrf to log me in to victimstore.com with an attacker's account; later I go to victimstore.com and buy something using my credit card, and since I expect to buy more stuff in the future I have it save my shipping address and credit card to "my" account. Attacker now has my credit card number and shipping address. – AndrolGenhald Mar 24 at 0:54
  • A bit far fetched but a valid case I agree. If the application you test is if that kind, then it makes sense. – hax Mar 24 at 1:08

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