James Galvin notes that MIME/PEM specifies that the MIME headers in
the signed portion of a message are included in the digital signature
and can't be tampered with. Since the discussion in this thread is
all very relevant, let me revise my example to show that the problem
is still there:
Content-Type: multipart/signed; protocol="pem"; hashalg="md5";
boundary="Signed Message"
Note: please send refunded monies to Jeff Thompson
--Signed Message
Content-Type: text/plain
We have returned item #4425 for full refund.
--Signed Message
Content-Type: application/signature
<pemsig>
--Signed Message--