RE: [WEB SECURITY] Universal XSS with PDF files: highly dangerous

["Guy Podjarny" <gpodjarny () watchfire com>]

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).

So the flow is:

IF the URL Session[X] != null (we have a previously created random val)
AND request contains token_query AND token_query==Session[X]:

        Session[X] = null             -- > Clear the token (it's only
good for one use)
        serve the PDF resource as an in-line resource

ELSE:
        calculate X = new random token value
        Session["token"] = X   -- >   save X on session (works for that
user only)
        redirect to file.pdf?token_query=X
 
Time delays can be added for extra security, but since the session isn't
persistent, it'll usually not be required.


One more note about the redirect:
It seems that firefox retains the fragment portion when you redirect. 
So if you're browsing:
http://server/page.aspx#target And you redirect to
http://server/page.aspx, Firefox will keep the #target fragment!

To work around this, you can redirect to http://server/page.aspx#a - by
adding a fragment to your redirect, you make firefox get rid of the old
one. 

Cheers,
Guypo 

-----Original Message-----
From: Amit Klein [mailto:aksecurity () gmail com] 
Sent: Thursday, January 04, 2007 4:38 PM
To: bugtraq () securityfocus com; Web Security
Subject: Re: [WEB SECURITY] Universal XSS with PDF files: highly
dangerous

Updates:

1. In private communication, Tom Spector observed that the cookie
doesn't add any significant security. In retrospect, I could have
omitted it completely. It's there as a remnant of a previous idea I had.

In other words, I see nothing wrong with the following, simpler and more
elegant algorithm ("Look ma - no cookie"):

IF the URL doesn't contain token_query, then:
   calculate X=encrypt_with_key(server_time, client_IP_address)
   redirect to file.pdf?token_query=X
 
ELSE IF the URL contains token_query, and
decrypt(token_query).IP_address==client_IP_address and
decrypt(token_query).time>server_time-10sec
   serve the PDF resource as an in-line resource
 
ELSE
   serve the PDF resource as a "save to disk" resource via a proper
choice of the Content-Type header (and/or an attachment, via
Content-Disposition).


And big thanks to Tom who pointed this out.


2. While thinking more about this solution, I observed that if the
attacker can have an "agent" sharing the same IP address with the victim
(by agent I mean an entity that can communicate with the target web site
and read back its response data), then the algorithms I suggested will
not be effective. Note that an attacker can share IP address with the
victim when both share a forward proxy (e.g. some universities and
ISPs), or when the attacker and victim share the same machine
(multi-user environment). Still, that narrows down the attack surface
significantly.

Thanks,
-Amit



Amit Klein wrote:
> It seems that I forgot all about Flash when I wrote that (the 
> irony...). The solution I proposed is not secure enough as-is. It is 
> trivial to write a SWF object that will request 
> file.pdf?token_query=123 and add a "Cookie: token_cookie=123". This is

> discussed in yours truly's "Forging HTTP request headers with Flash" (

> http://www.securityfocus.com/archive/1/441014) and in Rapid7's "Rapid7

> Advisory R7-0026 - HTTP Header Injection Vulnerabilities in the Flash 
> Player Plugin" ( http://www.rapid7.com/advisories/R7-0026.jsp).
> Even adding cryptographic secret, time-based entropy or use counter 
> doesn't help - all this can be circumvented by a server script on the 
> attacker's site preparing the HTTP request and communicating it in 
> real-time to the SWF object at the victim's browser.
>  
> The solution I could come up with is to tie X to the IP address of the

> client. Yes, I know - it's ugly, and it doesn't work 100% of the 
> cases. But you stand nothing to lose if you simply fall back to the 
> "save to disk" option, suggested by an anonymous SlashDot submitter ( 
http://it.slashdot.org/comments.pl?sid=214868&threshold=1&commentsort=0&;
mode=thread&cid=17450834 
>
<http://it.slashdot.org/comments.pl?sid=214868&threshold=1&commentsort=0
&mode=thread&cid=17450834>).
>  
> So the more secure solution, as I see it, is as following:
>  
> Apply only for PDF resources:
>  
> IF the URL doesn't contain token_query, then:
>    calculate X=encrypt_with_key(server_time, client_IP_address)
>    redirect to file.pdf?token_query=X with Set-Cookie: token_cookie=X 
> to expire at server_time+10sec.
>  
> ELSE IF the URL contains token_query, and token_query==token_cookie 
> and decrypt(token_query).IP_address==client_IP_address and 
> decrypt(token_query).time>server_time-10sec
>    serve the PDF resource as an in-line resource
>  
> ELSE
>    serve the PDF resource as a "save to disk" resource via a proper 
> choice of the Content-Type header (and/or an attachment, via 
> Content-Disposition).
>  
> Hopefully this should work. But it's definitely less elegant than the 
> original (flawed) suggestion.
>  
> -Amit
>  


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