[CKAN-Security] Fwd: /util/redirect and referrer-based CSRF mitigation

Mon Nov 17 23:36:34 UTC 2014

Thanks, David. No, I'm not on the list. I guess that would be a good idea...

We're setting up referrer-based defences on our CKAN instances using the following Apache config:

    RewriteEngine On

    # block cross-domain posts except API
    RewriteCond %{REQUEST_METHOD} !(GET|HEAD|OPTIONS)
    RewriteCond %{REQUEST_URI} !/api\b [NC]
    RewriteCond %{HTTP_REFERER} !https://<server name>/ [NC]
    RewriteCond %{HTTP:Origin} !https://<server name>$ [NC]
    RewriteRule / - [F]

    # block cross-domain access to user pages except login page
    RewriteCond %{HTTP_REFERER} !https://<server name>/ [NC]
    RewriteCond %{HTTP:Origin} !https://<server name>$ [NC]
    RewriteCond %{REQUEST_URI} !/user/login$ [NC]
    RewriteRule user - [F]

The first block is a general CSRF protection; the second is meant to stop exfiltration of user details (like password reset keys) using JSONP. Which is a whole separate vulnerability...

My concern here is whether /util/redirect allows attackers to bypass the check for POST requests (which is what we're blocking), or whether the redirection only helps GET requests.

Checking for the specific page would work for Referrer, but we want to support Origin too, for those browsers that might send it.

Thrawn

------------------------------
On Mon, Nov 17, 2014 9:16 PM AEDT David Read wrote:

>Sorry, I didn't know if you are on the list and would get that email,
>so forwarded below.
>
>D
>
>---------- Forwarded message ----------
>From: David Read <david.read at hackneyworkshop.com>
>Date: 17 November 2014 10:15
>Subject: Re: [CKAN-Security] /util/redirect and referrer-based CSRF mitigation
>To: CKAN Security Alerts/Discussions <security at lists.okfn.org>
>
>
>Thrawn,
>
>Great that you're talking about adding in basic CSRF protection by
>checking the referrer. Can you simply disallow requests that are
>referred by the redirect page?
>
>David
>
>On 16 November 2014 23:56, Thrawn <shell_layer-github at yahoo.com.au> wrote:
>>
>> Hi, folks.
>>
>> As mentioned at https://github.com/ckan/ckan/issues/1419, the open redirect on /util/redirect is a concern because it allows attackers to bypass referrer checks. Which aren't a great defence against CSRF in the first place, of course, but they're the simplest to implement without touching the codebase.
>>
>> Can anyone clarify whether this would restrict attackers to GET requests, or whether it would also allow forgery of referrer on cross-site POSTs?
>>
>> Thrawn
>> _______________________________________________
>> CKAN security
>> https://lists.okfn.org/mailman/listinfo/security
>> https://lists.okfn.org/mailman/options/security/david.read%40hackneyworkshop.com
>>
>> Repo: https://github.com/ckan/ckan-security