On 03 Feb 2015, at 16:44, Jon Dufresne <jon.dufre...@gmail.com> wrote:
> However some URLs are accessed by a unique URL
> containing a nonce without a login. Login is not an option for these
> URLs. Sharing this URL is considered very bad and I would like to
> avoid it happening unintentionally.

I'm not following this: to prevent this case, you are actively
instructing all your users to disable referer headers in their browsers?
If not, how are you controlling what referrers your users send?

URLs without login, which contain a secret nonce, are indeed sensitive
to the nonce leaking through the referer. Dropbox ran into this a
while ago:
https://blog.dropbox.com/2014/05/web-vulnerability-affecting-shared-links/

This also affected Evernote for some time. The common resolution seems
to be not to disable referer headers, which is a client-side issue, but
to mask it by sending all external links through a specific URL first
without the nonce, which works as a simple redirector.

Far from ideal, especially when dealing with more complicated links like
when sharing office documents. But it seems to work for Dropbox and
Evernote. You'll notice for example that when viewing a PDF on Dropbox,
you're not using your in-browser PDF viewer but Dropbox' custom viewer,
which I imagine also modifies all external links.

Erik

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