Cyrill Brunschwiler wrote:
>
> Compass worked out an advanced technical paper which explains the
> recently identified Adobe Acrobat Plug-in vulnerability. The document
> highlights the numerous useless remediation trials. Furthermore, you
> will experience why even the Open Web Application Security Project
> (OWASP) proposed solution seldom meets the requested security
> requirements.
>
> The full featured report is prepared for download at...
> http://www.csnc.ch/ (Anti-PDF-XSS Actions 9. Februar 2007)
>
Interesting paper. Let's see:
Regarding the "OWASP solution", it states:
"Unfortunately, the client IP address in entry server and proxy
environments is always the same and therefore an attack during the ten
seconds timeout is still possible.".
This is only correct for users behind NAT/proxy. And the attacker should
be behind the same proxy/NAT with the victims. I wouldn't say that this
"seldom meets the requested security requirements", especially when no
better solution is around (hold on, I'm getting to that).
Now comes the interesting part:
"Currently it looks like that the only safe way to protect from the
Universal PDF XSS attack, is to create a secure random token which is
bound to the client’s session ID and attached to every requested PDF
document link. [...] The Definitive Solution Explained"
I must disagree here. The proposed solution is flawed. Moreover, the
approach is flawed. Let's take a quick stab at the proposed solution.
Here is an attack against it:
1. Attacker (from its own machine, at his/her leisure) goes to
http://any.whe.re/file.pdf
2. Attacker is redirected to (say) http://any.whe.re/file.pdf?t=12345#X,
and receives a Set-Cookie: jsessionid=67890
3. Attacker sends a malicious link to the victim:
http://any.whe.re/file.pdf;jsessionid=67890?t=12345#a=javascript:alert(document.cookie)
4. The victim's browser requests
http://any.whe.re/file.pdf;jsessionid=67890?t=12345 from the server
(filter).
5. The filter considers this a legitimate request, because it ties the
"t" to a session (67890) in which this t was produced. The filter has no
idea that the session identifier was found in the URL and not in a
cookie, and the filter has no idea that the session was actually created
for a different client.
Note that the attack makes use of a J2EE application engine feature -
its willingness to accept URL session ID (even if the session ID was
originally provided via a cookie).
OK, that was a trivial attack, and circumventing it within the current
code is pretty simple - only allow session IDs inside cookies. But in
the meanwhile we saw an important observation: a secure solution must
take into account the ability of an attacker to communicate with the
server and pass to the victim session-specific data. In this respect, I
find the following statement from the paper questionable:
"In the sample filter code, session handling is based on Cookies but the
code could easily be rewritten to allow URL rewriting which would
support clients that reject Cookies as well."
In light of the above attack, it would make life much easier for an
attacker, and quite hard for the filter - as now the filter can't demand
that the session ID be presented to it via a cookie. How is the filter
going to protect itself from that attack?
Moving on to a pure cookie solution. To begin with, such solution was
already suggested in the past
(http://www.webappsec.org/lists/websecurity/archive/2007-01/msg00064.html),
and shown to be vulnerable to a similar attack
(http://www.webappsec.org/lists/websecurity/archive/2007-01/msg00065.html).
The attacker now needs to work harder. In step 1, he/she got a cookie
(jsessionid), but in step 2 the session ID cannot be used in the URL. It
has to be presented to the site inside a Cookie HTTP request header.
That's where some flash techniques kick in. Flash allows pretty liberal
manipulation of the HTTP request to an arbitrary site. So an attacker
can send the victim a Flash link (in the attacker's site), or a simple
link (to the attacker's site, showing HTML with Flash object). This
Flash will send out the request to the PDF-hosting server with the
Cookie header. Using trivial methods such as
http://www.securityfocus.com/archive/1/441014 doesn't seem to work with
the Cookie header, but more advanced techniques such as Rapid7's
http://www.rapid7.com/advisories/R7-0026.jsp and my own
http://www.securityfocus.com/archive/1/443391 enable crafting a complete
HTTP request. There's a bit of a complication here, since the forged
request is the second one - the browser has to be forced to send another
HTTP request so that the response will match it. But at the end of the
day, the result is that it's possible to forge requests with Cookie header.
Note: unlike anti-DNS pinning techniques, the Flash tricks don't provide
access to the response, yet do operate within the target's domain. So
the XSS vectors are relevant.
We need to understand the sad fact that browser requests can be
completely forged. Hence, solutions should be demonstrated to resist
attacks that forge complete requests.
Thanks,
-Amit
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Received on Feb 13 2007
Intro
