BugTraq
Re: [WEB SECURITY] Universal XSS with PDF files: highly dangerous Jan 04 2007 09:38PM
Amit Klein (aksecurity gmail com) (2 replies)
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
>

[ reply ]
RE: [WEB SECURITY] Universal XSS with PDF files: highly dangerous Jan 09 2007 08:40PM
Tom Spector (t spector f5 com)
RE: [WEB SECURITY] Universal XSS with PDF files: highly dangerous Jan 04 2007 10:46PM
Guy Podjarny (gpodjarny watchfire com) (1 replies)
Re: [WEB SECURITY] Universal XSS with PDF files: highly dangerous Jan 04 2007 10:58PM
Amit Klein (aksecurity gmail com)


 

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