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Re: questions about MIME and XSS in email messages
From: Javier Fernandez-Sanguino <jfernandez () germinus com>
Date: Thu, 04 May 2006 13:18:00 +0200

offset dijo:
I have a couple questions about the limitations of XSS regarding
email messages/MIME types.

As I don't see any answer to this question I'll jump in.

1)  Is it an RFC standard that if no MIME type is defined in the
header that the mail client default to plaintext?  Or is this
implementation specific by the authors of the email clients (ie. not
ruled by RFC rules)

AFAIK, RFC1521 covers how MIME headers should be interpreted by clients. From section 7.2 (page 27):

"   A body part is NOT to be interpreted as actually being an RFC 822
   message.  To begin with, NO header fields are actually required in
   body parts.  A body part that starts with a blank line, therefore, is
   allowed and is a body part for which all default values are to be
   assumed.  In such a case, the absence of a Content-Type header field
   implies that the corresponding body is plain US-ASCII text.  The only
   header fields that have defined meaning for body parts are those the
   names of which begin with "Content-".  All other header fields are
   generally to be ignored in body parts. "

2)  Are there are any scenarios where its possible to trick an email
client into processing text/html when the header doesnt indicate a
MIME type and appears to be plaintext even though the XSS code is
shown (due to lack of input/output validation by the client).

You can trick users to use a link that includes XSS code by using some http link that does not give away the trick. It will be seen as plain text, but you can trick users to "follow" it. With a little bit of social engineering, you could make naive users take the link from the text and copy & paste it to a browser, thus triggering the XSS attack.

Links that might not give away the attack include, for example, a tinyurl.com link (notice that it might be a violation of its terms of service) or a server you control that forces a 302 redirect in the WWW browser.

HTH

Javier

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