From: RSnake (rsnakeshocking.com)
Date: Wed Nov 02 2005 - 11:05:43 CST

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Allowing src in images doesn't prevent all types XSS, as you can have it
point to a valid looking .jpg or .gif like http://somwhere.com/xss.jpg
with an HTaccess file like:

Redirect 302 /xss.jpg http://victimsite.com/admin.asp&deleteuser

Or you can set up an alias to have it point to a cgi script that has
more intelligence (if you're coming up with the idea to go and validate
if it is a real image file). If you haven't yet, please check out my
XSS page for lots of ways around common filters, like the one below:
http://ha.ckers.org/xss.html

On Wed, 2 Nov 2005, Jeff Robertson wrote:

> This is exactly the sort of thing I'm looking for. Anyone know of any
> libraries (preferably in Java) that already do this?
>
> Jeff Robertson
> Manager of Web Application Security
> Digital Insight
>
>
>> -----Original Message-----
>> From: Sverre H. Huseby [mailto:shhthathost.com]
>> Sent: Wednesday, November 02, 2005 10:52
>> To: Jeff Robertson
>> Cc: 'webappsecsecurityfocus.com'
>> Subject: Re: whitelisting HTML tags
>>
>>
>> [Jeff Robertson]
>>
>> | I need to tell my development to limit the HTML tags allowed in
>> | input to a subset that can't be used for XSS. Any guidelines for
>> | this?
>>
>> You need three levels of whitelisting:
>>
>> * For allowed _tags_
>>
>> * For allowed _attributes_ for the allowed tags (separate attribute
>> whitelist for each tag)
>>
>> To avoid e.g. onload, onclick and stuff
>>
>> If you allow an "img" tag, you could allow the "src" and "alt"
>> attributes, and discard the rest.
>>
>> * For allowed _attribute_values_ for the allowed attributes
>>
>> To avoid e.g. href="javascript:..."
>>
>> You would allow src="http:..." and src="ftp:", and discard the
>> rest.
>>
>>
>> Sverre.
>>
>> --
>> shhthathost.com My web security book: Innocent Code
>> http://shh.thathost.com/ http://innocentcode.thathost.com/
>>
>

-R

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