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Date: Fri, 05 Jan 2007 00:58:33 +0200
From: Amit Klein <>
To: Guy Podjarny <>
	Web Security <>
Subject: Re: [WEB SECURITY] Universal XSS with PDF files: highly dangerous

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

When the user requests, the 
script.cgi requests file.pdf from 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.

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