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.
