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Re: The Dangers of Allowing Users to Post Images
From: "Dmitry Yu. Bolkhovityanov" <D.Yu.Bolkhovityanov () inp nsk su>
Date: Sun, 17 Jun 2001 13:30:31 +0700

On 15 Jun 01 at 12:52, zimage () upl cs wisc edu wrote:

When the request comes in, check if the incoming ticket matches the
one stored in this user's session.  If it does, this particular user
was given the offer by our server, and not by anyone else.  To spoof
this system, someone would have to guess or otherwise find out what
ticket value the victim was given by the server.

To make it harder to find the ticket value given to a user, you could give
the user many tickets, one for each possible action.  This solution would
require a "ticket pool" in the user's session.  I've implemented the
latter solution in both PHP and Java.  Let me know if you would like some
code.  (It's not at all hard to implement, of course.)

  My company  implemented this but went one more step.  They created a file
  that had (IP, ticket) pairs. The ticket was passed around in URLs, but
  wasn't valid unless it came from the specific IP.  To pretend to be
  someone else, one would have to spoof their IP and guess the value of
                                    ^^^^^^^^^^^^^^(2)  ^^^^^^^^^^^^^^^(3)
  their (10 hour life-cycle) ticket.  We did this, originally, because we
  wanted to support web browsers that didn't use cookies.  The file was,
  actually, more like (IP, ticket, cookie-type-options-and-settings).  It
  worked well for us.

    (2) should be extended with "or to use the same proxy".

    (3) becomes much more simple due to (1), especially because if you allow
inline images, the browser will supply the ticket in "Referer:" header when
requesting the image.

    So, in many cases this extra step gives more cons than pros.  But if
sending such tickets via POST instead of URLs, this step is a good thing.

       Dmitry Yu. Bolkhovityanov  |  Novosibirsk, RUSSIA
       phone (383-2)-39-49-56     |  The Budker Institute of Nuclear Physics
                                  |  Lab. 5-13

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