Re: CROSS SITE-SCRIPTING Protection with PHP

[Dan Kaminsky <dan () doxpara com>]

> >  * Automatically providing tamper control (eg. message digests) to
> >    data that are not supposed to be tampered with.
> >    
>
> And you verify that the digest isn't changed *how*?  (Hint - how do you
> keep your attacker from handing you a piece of data along with a digest that
> matches?
>  
For remotely computed data / hashes, you can't -- thus the folly of 
trusting MD5 hashes on critical files downloaded off of untrusted 
servers.  If somebody can modify the tarball, they can probably modify 
the hash too.

But for cookie-style fields that are reflected back to stateful systems, 
you can certainly provide tamper control using the HMAC (Hashed Message 
Authentication) construction.  Essentially, the HMAC adds a symmetric 
secret to the resulting hash -- mere possession of data becomes 
insufficient to compute a new hash; one must possess the secret as well. 
So people can either pass back the valid data, or nothing at all.  Elegant.

Encrypting your state and embedding it into a cookie(or URL, hideously 
enough) is actually a great way to trade computation for bandwidth; at 
the cost of a bit of bandwidth, your applications can be completely 
stateless.  As Caezar pointed out a few days ago to me, a brilliant 
variant of this for systems with *huge* amounts of state is simply to 
pass to the clients a small HMAC'd handle to some state cached on a 
central state-cache, which then upon receipt of a validated handle sends 
to the stateless backend web request plus the relatively large amount of 
data the server needed to process the request.  The load balanced server 
remains stateless, the central state cache does nothing but hash, 
validate, store, and forward (all trivial ops), and the wire doesn't get 
flooded with garbage we didn't want to keep around.

As for a global site scripting solution...hmmm, I can imagine one tag 
I'd like in web browsers:  "After this point, no scripting allowed, DOM 
is dead, go away."  Think about it -- it's just priv dropping.

--Dan
www.doxpara.com