Make a web application penetration test is becoming tricky due modern browsers native anti-xss filtering facilities (they only work for reflected cross site scripting).

Firefox and Google Chrome leave attack pattern in the resulting HTML code but they don’t render it.

Microsoft Internet Explorer uses its internal XSS Filter library.

This one can disabled server side by the web application.

To be honest, Microsoft is one of the IT companies that provides a lot of documentation despite their anti-opensource crousades in the past. In this msdn blog post it is explained how IE will react when it detects a reflected cross site scripting. A test page is provided for people who wants to play with this feature.

Since this page is vulnerable you can either check how the browser you’re using right now it behaves when a XSS is exploited under your feet.

Post author, Eric Lawrence, says in his post:

Pages that have been secured against XSS via server-side logic may opt-out of this protection using a HTTP response header: X-XSS-Protection: 0

Wait. Is it possible to disable the filter server side just with an HTTP header in the response? Are you kidding at me?

Too much power without control

Using HTTP response to disable browser security feature is a non sense, for a number of reasons:

  • what if the server-side logic introduced to mitigate reflected cross site scripting is weak and it doesn’t block some attack payload?
  • what if new attack payloads have been discovered during the ages?
  • what if the server-side logic can be either disabled or it’s not used in some circumstances?
  • what if a rogue HTTP response header it has been injected by an attacker to disable your client security filter?

Let’s exploit it

Do you remember casper, the tiny transparent proxy I wrote some months ago? I first wrote this tool to quick check asynchronous calls a friend web site made to evaluate performances.

casper extends WEBrick::HTTPProxyServer class. Making a transparent proxy this way is trivial programming excercise.

It’s fun, but not that much. Let’s add to casper some offensive super power.

In a separated branch I implemented an injection code to disable XSS Filter protection for Microsoft Internet Explorer.

The real magic is in the lib/casper/disable_ie_xss_protection.rb source code. I opened the WEBrick::HTTPResponse class back again and I added a disable_ie_xss_protection method playing with HTTP response headers sent back to the browser.

``` ruby lib/casper/disable_ie_xss_protection.rb class WEBrick::HTTPResponse

def disable_ie_xss_protection self[“X-XSS-Protection”]= 0 end


Seven lines of code counting two empty lines I added for readability purposes.

In the Casper::Proxy class, I tell WEBrick to honor also response handlers in
the initialize method:

``` ruby lib/casper.rb
def initialize(config={})
  @req_count = 0
  @trace_domain = ""
  @trace_domain = config[:trace] if config[:trace] and ! config[:trace].empty?

  config[:Port] = 8080 if ! config[:Port]
  config[:AccessLog] = []
  config[:ProxyContentHandler] = do |req, res| 
    res.disable_ie_xss_protection if config[:disable_ie_xss_protection]
    log_requests(req, res) 


I added a command line flag for casper script in order the user to turn the xss filtering avoidance on: ``` ruby bin/casper

opts = [ ‘–help’, ‘-h’, GetoptLong::NO_ARGUMENT ], [ ‘–version’, ‘-v’, GetoptLong::NO_ARGUMENT ], [ ‘–trace’, ‘-T’, GetoptLong::REQUIRED_ARGUMENT], [ ‘–port’, ‘-p’, GetoptLong::REQUIRED_ARGUMENT ], [ ‘–disable-ie-xss’, ‘-x’, GetoptLong::NO_ARGUMENT ] )

opts.each do |opt, arg| case opt

when ‘–disable_ie_xss_protection’ options[:disable_ie_xss_protection] = true end end


Off by one

Bringing browser to the next level adding security features it is a great move from vendors. Adding the capability to turn off these security feature with a server side HTTP response it is a poor decision, and I suspect it’s more marketing driven than a real users request.

On the other side with a bunch of ruby lines of code I turned casper to be an attacking tool and this is the direction I’m moving so far.

Some more advanced injection patterns will be added in the future and I will integrate casper with cross, ciphersurfer, links and other gems.

This melting pot will be the dynamic saketesting engine and it will be used with static analysis engine I will release just for ruby and javascript languages right now to create an hibryd security scanner.

If you want to read more about Cross Site Scripting, I suggest you to go to this page about this kind of attack

Enjoy it!

Borderline image credits to Lynne Hand