Monday, April 07, 2014

Bypassing the XSS filter using function reassignment

The XSS filter introduced in IE8 is a really powerful defence against XSS. I tested the filter for a number of years and found various bypasses one of which I would like to share with you now. You can read more about the filter and its goal in the following blog post.


There have been numerous public bypasses of the filter however very few within the intended scope of the filter. The filter blocks reflected XSS in HTML context, script, style and event context. It does not support attacks that use multiple parameters or same origin requests. Once you are aware of the intended scope the difficulty of bypassing the filter is very high.

Function reassignment

This bypass was fixed in later versions of Internet Explorer but still works in compatibility mode. You can use the vector in a penetration test by forcing the target site into compatibility mode using an iframe with an EmulateIE7 meta element as shown below.

<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />

This loads IE in emulate mode and the entire JavaScript engine will revert to an older mode enabling the vector to function. We need to setup a page with the target input inside a function argument in order to demonstrate the bypass. As you can see below the parameter "x" appears inside a string which calls the function "x".

<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
function x() {

$x = isset($_GET['x']) ? $_GET['x'] : '';
x('<?php echo $x?&gtl');

In older versions of Internet Explorer it's possible to redefine a function within its calling arguments. This is very useful for bypassing the filter when your XSS hole executes within a function argument. To see how this works we pass a GET request to "x" within a payload that redefines the function "x" to alert and uses an argument before our break out string to pass to the function. The GET request looks like this:

The output of the page now looks like this:

<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
function x() {


"1" is inserted at the start of the argument then we break out of the string and redefine the function "x" to alert then finish up by completing the string. The alert function only accepts 1 argument so our other arguments are ignored and alert(1) executes successfully.


As mentioned previously this vector was patched in later versions of IE however it will still work where a target site is in compatibility mode or you can force it into the older mode using iframes. The newer JavaScript engines in IE will not allow you to redefine functions within arguments. To protect against this vector you can force your site into standards mode by specifying a doctype or using the X-UA-Compatible header or meta element in edge mode. Preventing your site from being framed is also a good idea using the X-Frames-Option header and of course fixing the actual XSS hole in the first place is preferred.


Saturday, April 05, 2014

How I Hacked Your Router

Some time ago a friend in infosec asked me to do a strange thing.  He asked me to hack him.  We will call him Bill, for the sake of anonymity.  Other names and places have been changed to protect the innocent.  Vendor names have been kept to incriminate the guilty.

Hacking a large corporation is easy(ish).  They have information assets that may span the globe, and despite investments in various protection technologies, it's just hard to keep track of all that stuff.  It requires Zen-like discipline to rigorously follow the cycle of scan-patch-repeat day after day, on all assets in an organization, without fail.

Hacking a person can be tough.  It's true that blackhats have the advantage in terms of the asymmetric nature of information security.  Sometimes it only takes one bug.  But the attack surface area of a single individual is quite small compared to a corporation.  In addition, most people trust large vendors with their information and the cloud vendors typically do a decent job of protecting people.

I started with basic recon.  I like to use Maltego, along with sites like,, pipl search, and other tools to enumerate online presence.  There's also the classics like Google+, Facebook and Linkedin.  It helps to have a fake profile on Facebook for this kind of work.  A good bait profile should be tuned to your target.  It will help when extracting additional information via social engineering.

In terms of online presence, password reset questions are good low hanging fruit.  I've seen webmail accounts asking for information that you can pull right out of the target's Facebook profile.  I'm sure most people don't even make the connection; they may have written their reset questions 5 years ago.  None of this stuff was going to work in this case though.  My target was an infosec nerd, and he was expecting me.

Time to take the fight to him.  First, I checked to see if he is hosting anything on his home Internet connection.  He may have been doing this and not even know it.  Many apps and devices use UPnP to punch holes in consumer-grade firewalls without much fanfare.  Sometimes all it takes is a NAS or media server to open up a backdoor.  To find his home IP address, I used a Skype resolver, such as  It worked brilliantly, so I scanned his IP address (and a few neighboring IPs) to see if I could find any services.  No dice though… I'm sure he figured I would do this.

Next up, 802.11.  Wireless networks are a great attack vector.  I have two Radeon 6990′s in an i7 rig that chews through WPA hashes.  I use a Markov predictive wordlist generator to feed guesses to oclHashcat.  It can achieve an 80% average crack rate over an 8 hour time frame.

So I set about to Bill's address with various Alfa wifi cards in tow.  While in this case I actually know Bill's address, I may have been able to get this information via recon or social engineering.  It's not exactly a secret.  After successfully capturing a WPA handshake, I ran the cracker for a week.  Still no dice.  This would probably work on most people, but Bill is an infosec guy.  His WPA key is probably >32 characters long.

At this point you're probably wondering why I didn't just spear-phish him with a Java 0-day and go have my victory beer.  The answer is simple — I know my target.  He has mastered the mantra of scan-patch-repeat.  Java isn't even installed.  And if I did have a browser 0-day in my back pocket, I would have used it to win the pwn2own last week.

After my visit to Bill's place, I did come away with one useful piece of information.  The wireless MAC address (BSSID) of his router: 06:A1:51:E3:15:E3.  Since I have the OUI (the first 3 bytes of the MAC), I know that it's a Netgear router.  I also know that Netgear routers have some issues, but Bill was running the latest firmware.  That doesn't mean that all the vulnerabilities were patched in the latest firmware though.  The only way to be sure was to buy a Netgear router and test it myself.

Determining the exact model is probably not possible (not remotely anyway).  Consumer devices may have a lot of variation between different models as the reference platforms come from SoC vendors such as Broadcom and Atheros.  I know that Bill is a bit frugal, so I went with the WNDR3400v3 — the entry level unit.

After reading about some of the vulnerabilities this device has had in the past, I created two Metasploit modules.  In the first module, I would use a CSRF bug to POST to the UPnP interface and punch a hole to access the telnet service of the router itself.  This issue likely exists in numerous other devices and is worth emphasizing:

If you can spoof UPnP requests via CSRF, you can turn the entire network inside-out.

That's an important point.  I was opening up a single port.  You can use Ajax requests from the victim's browser to configure NAT entries for every IP in a subnet, effectively disabling the firewall.  There are hard limits to the number of UPnP NAT entries of course, but most devices will allow enough entries to map a few key ports for a hundred hosts or so.

In order to trick Bill into connecting to my exploit, I sent him an email with an embedded link.  Cobalt Strike has a tool to copy an existing email (headers and all), which makes this basically turn-key.  All you need to do is modify the links.  So what email does everyone always click?  What would work even against an infosec guy?  Linkedin invites.

Now before I sent the email, I needed a follow up payload.  By default, the telnet port is enabled on Netgear routers, but the service is unresponsive.  You have to connect to the port and send a special unlock key.  Public exploits exist for this flaw, but I wrote another MSF modulebecause I love my Ruby (and Metasploit).

Bill clicked the link.  As soon as I saw the callback, I triggered the second module and logged into the router via telnet.  Once I obtained root access to the router, I immediately changed the DNS settings to point to a DNS server that I control.

Controlling DNS is a powerful thing; it effectively provides you with on-demand man-in-the-middle.  There are plenty of MITM attack vectors, but I like Evilgrade for stealth.  Evilgrade has been out for years, and still works great (some modifications necessary).  It took about a week before Bill decided to upgrade notepad++ to the new version.  When he did, he was fed a backdoored version that gave me a Meterpreter shell on his computer.  I immediately emailed him a few screen shots and a keystroke log, and he unplugged his computer a few minutes later.

For my efforts, I was rewarded with a six-pack of Ruby ale.  I do love my Ruby.