[ietf-dkim] Features that could be reconsidered as part of the bis process

[Murray S. Kucherawy]

On Wed, 20 May 2009, Steve Atkins wrote:
> Another use case is to use l= to sign a text part of an email, but not 
> to sign an attachment. In that case I can obviously replace the 
> attachment with my own content, but depending on the details of the 
> email structure I may well be able to replace the text section as 
> rendered to the user as well.

Indeed, Outlook will opt to render an HTML part over a text part whenever 
given the choice.  Thus, if you sign only the text/plain portion of a 
message and an attacker appends a text/html part, the unsigned HTML 
version will be rendered even if completely different from the text/plain 
part, and DKIM would give that a thumbs-up.