Re: 'cross site scripting' CERT advisory and MS

From: David LeBlanc (dleblancat_private)
Date: Wed Feb 16 2000 - 09:39:56 PST

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    I wanted to reply to this, and make a clarification -
    
    At 08:57 PM 2/14/00 -0500, Rishi Lee Khan wrote:
    >There is an easy way to open a web page using and email client using HTML
    >parsing ... simply put in the <head> tag <meta http-equiv="REFRESH"
    >content="0;URL=http://www.yourpagehere.com">
    
    Tried it, and it doesn't seem to work.  Created an HTML mail with this
    embedded, opened it in Outlook, and no refresh.  Did a Save As to dump it
    out to file, opened it with IE, got the refresh.  I'm not saying it can't
    be made to work, but I can't do it, and it seems like a decent test, since
    I am getting it to refresh in IE.
    
    >Marc Slemko wrote:
    >> So while disabling all the "features" that you can when reading HTML mail
    >> is definitely recommended and protects you against a lot of attacks, it is
    >> not a complete solution.  I seriously doubt that all the ways of
    >> exploiting this issue without using scripting languages have been
    >> discovered.
    
    Now for the clarification:
    
    I am NOT trying to solve the general problem of all the bad things that
    either can happen, or are theoretically possible once you plug in the
    network cable.  I am trying to solve the specific problem of cross-site
    scripting attacks being delivered by e-mail.
    
    What I recommend specifically for using Outlook (probably also applies to
    other mail readers using IE as a HTML viewer) is:
    1) Set it to run in the Restricted Sites zone
    2) Edit the Restricted Sites zone into what I call maximum paranoia mode -
    turn EVERYTHING off.  IIRC, cookies are off to begin with, but this gets
    them turned off for sure.
    
    Am I now saying that if you do this, you're safe?  Absolutely not.  You're
    never safe.  A meteorite could come through the roof, or you could get hit
    with an evil bug that isn't publicly known yet.  Anything can happen.  No
    one expects the Spanish Inquisition!  I _am_ saying that there are a whole
    bunch of things that I _know_ can get you that now won't get you.
    
    Am I saying that HTML mail is a great idea, and that applying these
    settings makes it all safe and cozy?  To quote Marc, "NO, NO, NO!!!"  IMHO,
    it isn't a great idea, but lots of people use it, and I can't turn it off
    in the mail reader I use at work, so I think these settings make it a much
    more reasonable risk.
    
    Speaking of which, there are still 3 things that I know of to worry about:
    1) Embedded URLs in HTML mail - these will invoke the browser IF you click
    on them, and the effect will depend on a lot of other issues.  You're also
    now most likely running in the Internet zone, so different settings apply.
    Personally, I take a look at them before clicking on them, or just type
    them in.
    
    2) HTML attachments - these aren't governed by the mail reader, but by the
    browser.  Make the browser settings you think are appropriate.
    
    3) Things I don't know about.  No telling what sort of nastiness is lurking
    out there.  Definately worry about this one.  I don't think security
    problems on the Internet are a passing phase - we're all in for a wild ride.
    
    
    David LeBlanc
    dleblancat_private
    



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