OSEC

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Subject: Re: recent 'cross site scripting' CERT advisory
From: Ari Gordon-Schlosberg (regsNEBCORP.COM)
Date: Mon Feb 07 2000 - 17:55:00 CST

[Bill Thompson <billDIAL.PIPEX.COM>]
> One form of protection from a truly *cross-site* attack that I didn't
> see mentioned in the CERT advisory is the trusty "HTTP_REFERER"
> check. But then, with so many sites using affiliate programs to get
> their search boxes and book-buying links distributed across the Web,
> there may be few major e-commerce sites that block requests based on
> the referral source.

HTTP_REFERER is trivial to spoof, and it's likely that anyone perpetrating
a sophisticated attack would laugh at having to spoof the Referer: header.
It's a form of trusting the client, which is a big, huge, no-no. It's okay
if you're trying to protect from someone seeing a page that should
register for (like downloading a white paper), because it's not worth an
attackers trouble to circumvent something like. But Referer: should never
be used as a security measure. Hell, anyone with telnet can spoof a Refer:
URL. 

--
Ari							there is no spoon
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