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Re: Microsoft IE cookies readable via about: URLS
From: "Nick FitzGerald" <nick () virus-l demon co uk>
Date: Sat, 10 Nov 2001 02:51:21 +1200

Jouko Pynnonen <jouko () solutions fi> wrote:

Microsoft Internet Explorer has a vulnerability which allows a malicious
website to access any cookie in the browser's memory or those stored on
disk. Cookies are used by web sites for storing preferences, statistics
and tracking users, but also for storing more sensitive information such
as session keys and even usernames and passwords. Cookies are used by
many (probably most) online banks, webmail systems, and other sites
requiring user authentication.

Access to cookies may allow an attacker to retrieve passwords or other
sensitive information, or hijack authenticated web sessions.

What makes this possible are certain features of "about:" URL handling of
IE. For some reason, an URL starting with "about:" can contain html code
that will be interpreted by the browser. For instance entering the URL
"about:<h1>hello</h1>" brings up a page with the heading "hello". The URL
may contain JavaScript as well. Going to the following location with IE
causes an alert box to be displayed:

about:<script language=JavaScript>alert('ALERT');</script>

This was hinted at in Andrew Clover's message of 19 October that 
pointed out the about: protocol is, by default, in the Internet 
security zone *and* about: URLs can have cookies.  This is a neat, 
though unsurprising, extension of that.  Andrew's post is in the 
archives at:


Finally, the about URL may have a hostname placed after the colon, and IE
uses that hostname when determining the cookies to use:

about://www.anydomain.fi/<script language=JavaScript>alert(document.cookie);</script>

The above URL would result in IE displaying cookies of www.anydomain.fi
in the alert box, assuming that the site has been visited and it has set
a cookie which hasn't expired.

A malicious website can have a piece of JavaScript redirecting the
browser to an about: URL similar to the one above, and do anything with
the cookie information of any selected domain. Instead of showing an
alert box, the JavaScript code might just pass the cookie contents to a
script or a CGI program which could quietly store the information to a
file and then redirect the browser elsewhere or show some seemingly
harmless web content.

A web page for testing the vulnerability can be found at


You can type in an address of a website that uses cookies, (without
"http://";) and it will tell you if your browser is vulnerable to the
problem. For a relatively harmless test case try typing the address
www.google.com in the box (assuming you've visited Google before).

At least IE versions 6 and 5.50 appear to be vulnerable, but it looks
like some older versions like 5.00 isn't, at least in the way described
above. It interprets the html and JavaScript, but doesn't have any cookie
data in document.cookie.

A vulnerability with the same impact came public in May 2000, see

Microsoft was contacted November 1st. Their response was quick and they
are producing a patch to be released soon (if not already released).

That's interesting, given they seemed to think there was no problem 
(despite the flaw being obvious to the rest of the world) back when 
Andrew mentioned it...

Until then, you can protect yourself from the vulnerability by disabling
cookies (at Tools -> Internet options -> Security -> Customize) or by
switching to another browser such as Opera or Netscape, which don't
appear to have the same about: URL features.

A better workaround (assuming that you feel cookies are "relatively 
useful" and would rather not turn them off) is to put about: URLs 
into the Restricted Sites zone, as detailed in Andrew Clover's 
followup to his own post:


In short, create a DWORD value named "about" under:

   HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap\ProtocolDefaults 

and set it to 4.

I just tested this against your test page and with the above value
set, the test tells me "No cookies found for site...". 
Interestingly, this registry change seems to have almost immediate
effect -- i.e. it did not require a restart or logout/login or even 
an IE exit/restart (I did this on Win2K) but occasionally, when 
running the test page over and over alternating back and forward 
between having the above value set and not present (the default), the 
page would work as if the registry value had not yet been changed.

Nick FitzGerald
Computer Virus Consulting Ltd.
Ph/FAX: +64 3 3529854

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