Re: Content monitorting in Application Security

[Jeremiah Grossman <jeremiah () whitehatsec com>]

Sounds like common web site functionality and the resulting security 
challenge.

Here are techniques that may help...

1) When receiving an uploaded file of any kind, use various parser 
libraries to sanity check the actual format of data. Ensuring the file 
being uploaded is what it claims to be. With the incoming file 
extension and content type header in agreement. jpeg's should be 
formatted like jpegs, mp3's like mp3's, html like html and so on.

2) If you plan on handling files beyond plain text, such as zips and 
exe's, you may consider using some type of A/V product as well. A nice 
security add-on that can be useful depending on the situation.

3) This following method is strictly about XSS and HTML/JavaScript 
content.

While its fairly easy to filter all HTML tags from a file to prevent 
XSS, its exponentially harder to separate HTML from executable 
client-side code (JavaScript). Especially when the HTML is freeform and 
most tags need to be supported on the web site. I've long said its a 
slippery slope to support use-submitted HTML, but sometimes it can't be 
helped.

There are a few things than can do help mitigate the risk of the 
uploaded files.

	a. Filter out potentially malicious HTML tags or only allows a strict 
set of safe HTML tags.
	b. Filter out potentially malicious tag attributes or only allows a 
strict set of safe tag attributes.

	* The either or is a  give and take of security vs. 
functionality/ease-of-use.

	Depending on the programming language you are using, there might be 
some libraries available that could help make this process easier. I 
haven't used them, but I noticed there are libraries available for Perl.

http://cpan.uwinnipeg.ca/dist/HTML-StripScripts
http://cpan.uwinnipeg.ca/dist/HTML-Scrubber-StripScripts

There might be some available if you use some other language.


best of luck!


jeremiah-






On Friday, January 7, 2005, at 04:55  AM, Alfred Hitchcock wrote:
> Hi All,
> I have a major doubt it would be of great help if anybody can provide 
> solution to this.
> I have a web page which allows to upload files such as jpeg and html 
> files.
> Is there any mechanisms which can detect malicious html files. E.g. if 
> a html page has got a malicious java script such as alert('xss') then 
> how can we check these things. One more point to be noted here is that 
> uploading of file can be done by any user.