Wednesday, May 16, 2007

Determining sites trusted by NoScript

With the (relatively) new XSS filter added to NoScript it has become possible to determine whether a site is trusted or untrusted by seeing if NoScript decides to take action - of course this is not 100% accurate, since all these features can be turned off, but if they are turned off then you can execute some attacks anyway.

Open Redirect

The easiest method to determine whether a site is trusted is to use an open redirect, because you have control over where a site is sending a user, and as such can use the following two pieces of code to check whether is trusted:

<iframe src=""></iframe>

if (strpos($_SERVER['QUERY_STRING'], "=") !== FALSE) {
print "Untrusted!";
} else {
print "Trusted!<br />\n";
print "For ID: ".$_SERVER['QUERY_STRING'];


The server generated id is not even actually necessary since you can just store the info in a PHP session, but it is possible to 'attack' people who even have cookies disabled.

Non-Open Redirects

Now, a much more difficult task is attacking non-open redirects. Two things which limit this are:
  • We cannot use Javascript to determine where you've been.

  • We cannot actually pass a parameter to where we will be redirected.

The solution to the first problem lies here:

The solution to the second problem is a bit trickier, and relies on the 'usual' implementation of these systems, rather than a versatile technique.

Most systems which perform closed redirection have URLs which look like:

Where the id value is first run through intval(), then put directly into an SQL statement.

Now, we can exploit this fact by sending people to a URL like this:

Whereby if the is trusted then we will not be redirected anywhere because the URL will be filtered to look like this:

And since the id does not begin with a digit, the value of the variable, once put through intval() is 0, and either no redirect, or a completely different redirect will be done.

If the site is untrusted though, then the URL will be unfiltered, and when the id is run through intval, it will evaluate to 123, and so the user will be redirected to the usual place.

Controlled Off-Site Resources

If you can control any off-site resources such as images which a site embeds, then simply putting an ID in one parameter, and <script> in another, will cause the referer to be different on Trusted and Untrusted sites.

Generic Method

The methods above though all rely on a webapp having some kind of specific web feature, and while they're interesting (and that is the reason I included them), a generic method will prove far more useful.

And this generic method is extremely simple; just use the non-javascript history hack to find out what exact URLs a person has been to.

If you send a user to:

And the history hack says the user has been to

Then the site is obviously trusted.


These techniques can be used to either gather data about the user ala the Master Recon Tool (Mr. T) or the Black Dragon Project, or more importantly aid an attacker in bypassing NoScript's filters by finding which sites are trusted, and then either using those sites as a means of propagation (i.e. Emailing/PM-ing a user a link), or by sending a user to a persistent or DOM Based XSS on one of those sites.

Final notes

Since we do not have Javascript we need to be a bit tricky on how to actually use the info. The cleanest method is if you have a persistent or DOM based XSS in another site, then instead of simply sending data back to your server to be analysed you can have the CSS only History hack render an iframe which loads an iframe from your server, which redirects the user to the persistent or DOM based XSS in the trusted site.

The other method is having a iframe which refreshes every 5 seconds, which sends a request to the server, and the server will then try to aggregate all the data its collected, and act on it.

Why would you want to do this rather than sending the user to every single persistent and DOM based XSS condition you have?

Beats me; I just thought this was interesting.....

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