Guy Podjarny wrote:
> Another similar option is to use a single-use random value (not
> encrypted), that gets invalidated after it's served back.
>
> You can save the random value on the (non persistent) session
> (server-side), and serve the PDF only if the correct random value is
> provided.
> Once a random value has been used, it's cleared (single-use).
> In any case where the wrong value is provided - recreate a random value,
> save it on the session, and redirect to the PDF with it (same behavior
> as when the token isn't provided at all).
>
>
Here's an attack against this scheme:

Attacker sends the user a link to http://www.attacker.site/script.cgi

When the user requests http://www.attacker.site/script.cgi, the
script.cgi requests file.pdf from vuln.site. It gets back a redirection
URL and a session cookie. Then, it creates a Flash object that requests
the URL with an injected Cookie header (with the session cookie) and
serves this to the victim client. Voila.

[ reply ]