full-disclosure - Pros and cons of 'Access-Control-Allow-Origin' header?

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Message-ID: <CAFZ5_sC-cO7mNn7VYv2Hucbibr5yPvNfpQfNXWRAYb2OCxMfrw@mail.gmail.com>
Date: Thu, 23 Feb 2012 00:07:08 +0530
From: David Blanc <davidblanc1975@...il.com>
To: full-disclosure@...ts.grok.org.uk
Subject: Pros and cons of 'Access-Control-Allow-Origin'
	header?

Does 'Access-Control-Allow-Origin' header provide any benefits in
defending against cross site scripting attacks?

Doesn't 'Access-Control-Allow-Origin' header make any XSS flaw
trivially exploitable? For example, if an attacker finds an XSS flaw
in a web application, he can now inject a JavaScript with
XMLHttpRequest that sends a request to attacker's web server which
serves resources with the HTTP header "Access-Control-Allow-Origin:
*". The browser would see this header and fetch the resource from the
attacker's web server.

Isn't the web a safer place without this header?

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