How did I bypass everything in modsecurity evasion challenge?

First of all, at the moment this challenge is ongoing since last year (2013) and you may have already heard about it. Here is the link to this challenge: http://www.modsecurity.org/demo/demo-deny-noescape.html

I was very late to the party as other challengers could have defeated it easier in September 2013: http://blog.spiderlabs.com/2013/09/modsecurity-xss-evasion-challenge-results.html. These bypasses have been fixed now and it should not be possible to use those techniques to win the challenge.

As a result, the challenge is much harder than what it was in September and probably would be tougher in future. Fortunately, I managed to bypass all three different groups of the protections separately. Unfortunately, I could not find a single payload to bypass everything at the same time so I could not claim the prize just like other previous challengers! You can tell me first if you found a way to bypass them all though ;)

Here is what I did to bypass the XSS protections in this challenge for future reference:

XSS detection based on 5 different restricted blacklists! In my humble opinion, you may not be able to use these blacklists at the same time in a live web application because of the high number of false-positives. For example, it currently detects the following strings as attack vectors: “dude, I am online =))”, “<~form”, or “test this src for me”.

I thought I may be able to bypass the protection by using a flash file and an “object” tag, but “<object data=…” was blocked by different protections…

However, I used the following features to bypass these protections:

•  An “object” tag does not need to be closed for this scenario.
• A null character with an alphanumeric character after the “object” tag would make it undetectable by different protection methods in this challenge (e.g. “<object%00something”).
• “allowScriptAccess” and “data” attributes were allowed!

So the following is a bypass method for the XSS Defense #1:

And as the page writes the payload twice, I could also use the following to bypass the protections:

The defense #2 and #3 are about bypassing the white-list protections. You can read more about them in the modsecurity challenge page.

I was thinking to bypass DomPurify by using DOM clobbering attack (http://www.thespanner.co.uk/2013/05/16/dom-clobbering/) but it had some protections against it:

In this code, it checks “elm.attributes.item” to be a function but it does not verify the “elm.attributes” to have the correct type (it has been fixed now). As a result, I managed to bypass its “_sanitizeAttributes()” function by using a “form” tag with two “input” elements with the “name” attributes equal to “attributes”. If I was using just one “input” element, “elm.attributes.item” would not be a function, and therefore it was detectable; however, with more than one element, “elm.attributes.item” would be a function and “attributes.length” would be a numerical value so there would be no error in JavaScript, and this causes confusion for “currentNode.attributes[attr].name” in the following code to point to the “input” elements instead of the real form’s attributes which is what we need. Therefore, I could bypass the protection that DomPurify had in the “_sanitizeAttributes” function without causing any error by using DOM clobbering technique:

The bypass code is as follows:

It turned out that this also bypasses MentalJS sandbox by causing confusion as a result of DOM clobbering in the code below:

Apart from the previous bypass for MentalJS, I also found out that it is possible to bypass its sandbox by using “innerHTML” to build a script tag instead of using “createTextNode”:

I also found the following interesting tricks in MentalJS but they did not lead me to any new bypasses:

Keeping script “src” to an external js file (normally MentalJS removes them):

Keeping a non-whitelisted tag by clobbering the “removeChild” function (this will cause a JavaScript error but this is not a problem as it is in a try/catch statement):

If you are still interested, the following link is the conversation about bypassing MentalJS in slackers forum and it is really awesome:

http://sla.ckers.org/forum/read.php?2,29090,52131,page=13

MentalJS has been written by Gareth Heyes (@garethheyes‎) who was supporting me to bypass his tool to make it more secure.