RE: [WEB SECURITY] Universal XSS with PDF files: highly dangerous
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@xxxxxxxxx]
Sent: Thursday, January 04, 2007 4:38 PM
To: bugtraq@xxxxxxxxxxxxxxxxx; 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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