questions about MIME and XSS in email messages

From: offset (offset@svcroot.net)
Date: Thu Apr 13 2006 - 18:13:01 EDT

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

In my experimentation, if I can control the email header by inserting the following (or the application does this for me), I can get the email client to process the HTML.
- - - - - - -
...
<other email header information left out>
...
Content-Transfer-Encoding: quoted-printable
Content-Type: text/html

<body here with xss html>
- - - - - - -

Any other email messages without putting in a MIME type in the email header is rendered by the email client as text/plain (which essentially disabled any XSS processing).

My questions to the list:

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)

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).

Thanks in advance,

-off

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