Re: Preventing cross site scripting

["Tim Greer" <chatmaster () charter net>]

Thanks for clarifying and being patient with my inquires about your ideas
and what you mean. I better understand what you meant now. Yes, indeed,
there's no need (or point) to deal with anything *but* the tags--not just
the text. This is specifically what I had meant about using regex's to solve
and render based on the conditions of the checks, though there's other ways
to deal with it, like you said. Sounds fun, do update us when you have
something formed. I look forward to seeing other ideas for dealing with this
topic. :-)
--
Regards,
Tim Greer  chatmaster () charter net
Server administration, security, programming, consulting.


----- Original Message -----
From: "Laurian Gridinoc" <laur () grapefruitdesign com>
To: "Tim Greer" <chatmaster () charter net>
Cc: <webappsec () securityfocus com>
Sent: Friday, June 20, 2003 8:36 PM
Subject: Re: Preventing cross site scripting


> On Sat, 2003-06-21 at 00:49, Tim Greer wrote:
> > But you can't. You have to look at it as text and determine what
characters
> > will be dangerous. HTML is only a markup language, there's no dictionary
> > type matches. You would have a very large index as well if you attempted
to
> > determine what was valid. That is okay, and is reasonable if done
> > properly... not the problem. The problem is XSS and how someone can
insert
> > characters or values into otherwise valid HTML tags to cause the
problem.
> You look at it as text just until you separate the markup from the rest,
> then you treat markup and latter the remaining text content. There is no
> point in processing the attributes or the text content of an object tag
> when I want to drop it from start.
>
> Treating the HTML as text (treating it all in the same step - tags and
> values and content) is what was Yahoo making last year - and the ended
> up in replacing `evil' stuff not only in the tags but also in the text
> content.
> [The word "medieval" (since it contains the javascript command "eval" is
> converted in Yahoo mail to "medireview".]
> http://www.ntk.net/2002/07/12/
> http://www.ntk.net/2002/07/12/yahoo.txt
> This is also a nice example of how wrong a blacklist filter may be.
>
> > The only way to determine if it's valid and safe, barring a lot of
static
> > assumptions and basically having a huge whitelist,
>
> The huge whitelist is starting with the HTML DTD which defines what and
> where is allowed, and the first filtering occurs when Tidy parses the
> html document according to the standard which is a whitelist check after
> all.
>
> >  would be to simply strip
> > out or refuse to render any HTML tag that has any character in it that
could
> > pose the potential to insert something to create an XSS attack.
>
> Tidy won't strip by this, it will just proper escape what isn't allow to
> remain there in that format resolving almost all of the XSS attacks
> which are based on breaking the syntax.
>
> > Only so many HTML tags would allow for someone to do this in reality.
The
> > one's that do, since any tag element and value can be in any combination
in
> > a tag and be valid, so it requires some very specific checks and some
just
> > simply denying it, since it would be too open for faults.
>
> not any combination, the DTD restricts it by specifying what is allowed.
>
> > Anyway, like you
> > said, people sent emails in HTML (I personally would either not render
any
> > email with HTML or only safe tags and screw the people that want to send
> > HTML-ized email),
>
> most are not aware of what is the mail client doing :) and anyway richer
> text formats may enhance communication.
>
> > so it can get rather involved, unless you simply remove
> > those 4 or 6 vital characters from within a specific tag that could
cause
> > the problem.
> > And, why would someone need the characters in a tag anyway? You can
check
> > this all, allowing the special characters only in what must be valid
places.
> this is what the parser is doing.
>
> > Even a string with multiple single or double quotes. It's just as
effective
> > and much simpler this way. Text is what creates the markup language,
after
> > all, and thus you can't treat it as a language only and be safe. You are
> > going to have to do a lot more work and have to modify it for each newly
> > implemented tag in D?HTML, as well as for anything that could be an
*XML,
> > PHP, etc. type of tag.
>
> these are particular cases, you may support (allow) additional tags by
> defining them in the DTD; PHP on the other hand uses processing
> instructions (<?php ... ?>) rather than it's own namespace for tags
> which I consider it bad as concept.
>
> > > > Nonetheless, if you develop anything along the lines you
> > > > speak, please let me know, I'd like to check it out and what you're
> > doing.
> > > When I'll have another examples I'll post'em to the list.
>
> An working example will say more than any code listing, I'll be happy to
> assemble one from already running stuff.
>
> Cheers,
>
> --
> Laurian Gridinoc
> Chief Developer
> GRAPEFRUIT DESIGN
>
> tel/fax: +40.232.233068
> tel/fax: +1.646.349.2916
> mobile: +40.745.304379
> e-mail: laur () gd ro
> www.grapefruitdesign.com
> www.gd.ro