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