Cross-site request forgery [LWN.net]

Cross-site request forgery

Posted Oct 18, 2007 2:56 UTC (Thu) by elanthis (guest, #6227)
In reply to: Cross-site request forgery by jwb
Parent article: Cross-site request forgery

Even if the app only modifies data in response to a POST, it is still very trivial for a site
to include a hidden form with automatic submission thanks to JavaScript.

The hidden magic numbers in form submissions is one approach to solving this I never thought
of.

I've just relied on the HTTP referer header: sans a bug in the user's browser, sites and
scripts cannot forge this header.  Someone could trivially write their own utility or program
which performs HTTP requests with a forged referer (curl and wget both support setting this to
arbitrary values), but then they wouldn't have the proper authentication cookie.

Not the most robust solution nor all that ideal in theory, but it's Good Enough(tm) in
practice.

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Cross-site request forgery

Posted Oct 18, 2007 3:01 UTC (Thu) by jwb (guest, #15467) [Link]

I agree that you need further measures to protect against malicious webmasters, but they are a
small part of the problem.  The larger part of the problem is that anybody can put <img
src="whatever"> into a forum post (including this one) which causes a simple GET request.  If
your application has side effects on GET requests, then it is vulnerable to this very simple
attack.

Cross-site request forgery

Posted Oct 18, 2007 3:04 UTC (Thu) by jwb (guest, #15467) [Link]

It should be noted that certain forum software is rather more permissive about the sorts of things which can be posted by anonymous users, while other software can be very restrictive.

Cross-site request forgery

Posted Oct 18, 2007 3:19 UTC (Thu) by tetromino (subscriber, #33846) [Link]

You forgot to add a <marquee>.

Cross-site request forgery

Posted Oct 18, 2007 5:27 UTC (Thu) by mitchskin (subscriber, #32405) [Link]

lol, the medium is indeed the message.

Cross-site request forgery

Posted Oct 18, 2007 7:32 UTC (Thu) by Los__D (guest, #15263) [Link]

MY EYES, MY EYES!!! :D

Cross-site request forgery

Posted Oct 25, 2007 18:07 UTC (Thu) by devinjones (guest, #11272) [Link]

Thank goodness for firebug.

Cross-site request forgery

Posted Oct 18, 2007 10:44 UTC (Thu) by epa (subscriber, #39769) [Link]

A hidden form that automatically sends POST requests to some other site without user
intervention?  Do you have an example?  I'm just surprised that this is allowed by
Javascript's security model (as tightened up by recent browsers).

Of course I agree strongly with the grandparent; don't use GET requests for stateful
operations.  That is just asking for trouble.  I once worked in a web development shop where
the policy was that clickable links were somehow cooler than form submit buttons, so the user
management page had a 'delete' link for each user.  A customer was using a web accelerator
program that prefetches links (a perfectly allowable thing to do, provided it just GETs) and
deleted all users from the site.

Hidden POST forms

Posted Oct 19, 2007 0:53 UTC (Fri) by jamesh (guest, #1159) [Link]

It is trivial to create such a page. An outline of such an attack is:

Include an iframe on your page, possibly hidden. Something like "<iframe name="foo" style="display: none"></iframe>
Add a form element on the page with target="foo" as an attribute. This will cause the post to be loaded into the hidden iframe.
Set the action attribute of the form to the site you want to attack, and the method to "post".
Include all the form data you want as hidden form fields.
Add an onload handler to the page that calls the submit() method on the form.

It worked pretty well in the browsers I tried, and doesn't give the user any indication that the form post occurred. So it isn't enough to just put all your unsafe operations into POST requests.

"Referer" header checking and nonces as hidden form fields will stop this attack dead in its tracks though, so they are worth investigating.

Cross-site request forgery

Posted Oct 26, 2007 15:12 UTC (Fri) by anton (guest, #25547) [Link]

[...] it is still very trivial for a site to include a hidden form with automatic submission thanks to JavaScript.

Not a problem for me, because I have disabled Javascript. Maybe just like some web designers show their lack of competence with messages like "Turn on JavaScript", other webmasters could show their security competence by welcoming users that have JavaScript enabled with "Turn off Javascript".