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Re: Cross site scripting explained



In-Reply-To: <195f0718f5f1.18f5f1195f07@xxxxxxxxxxxx>

http://www.idefense.com/idpapers/XSS.pdf
http://www.cgisecurity.net/articles/xss-faq.shtml

Those papers are by Endler and Zeno...they should get
you informed.  If you don?t feel like reading, I'll try
to sum up the basic concepts for you and everybody else.

In general there are two types of XSS attacks,
transient and permanent.

Lets say you have an E-commerce site named example.com.
Example.com uses their own type of session cookie to
maintain state when a customer makes transactions. An
example of a transient attack would be if I knew bob
currently has the example.com cookie on his system. If
I sent an IM to bob with a link that was specially
crafted w/ an XSS attack payload that sent bob?s
example.com cookie to a cookie collecting script at
bobs-evil-wife.com. So now bob?s wife can use his
cookie to session hijack his example.com account and do
what she pleases on bob?s account.

Ok now lets say I have a message board, I want users to
make colorful posts so I allow HTML to be put into
posts, but I unfortunately I allowed everything
including javascript. An Evil user comes along and
inserts script into a post that when loaded,
automatically posts "I am a luser" to every message
board on the site, or it could do anything else the
evil user wants to do on behalf of all the visitors
that loaded the site up and were members of the board.

Here are examples from this month of XSS attacks:
http://online.securityfocus.com/archive/1/303226/2002-12-06/2002-12-12/0
http://online.securityfocus.com/archive/1/303542/2002-12-13/2002-12-19/0
http://online.securityfocus.com/archive/1/303545/2002-12-13/2002-12-19/0


Sadly this type of hole is extremly easy to find in any
 non-trival website...I've found hundreds all over
major sites on the web. The developers just don't care
much about them though because the second part of the
attack, the user interaction, is difficult to
accomplish. There has been much debate regarding if
these types of vulns should be allowed on bugtraq. IMHO
the disclosure of these types of attacks should be
"moved" to webappsec list.

Cheers,

-Slow2Show-  <-- graduating Friday woo hoo!!
University of Florida 

>Can anyone explain to me or point me to a paper that
explains exactly 
>what cross site scripting is, and how it could be
useful/cause 
>problems for someone?  Thanks.
>
>Mike