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[ietf-dkim] New Issue: Include new "known message replay" threat?

2006-03-09 08:37:49

I think I mentioned this before but maybe didn't explain well. (And
I didn't see it when combing the current issues, so I feel free to
raise it again, otherwise I couldn't:-)

Chosen message replay as currently defined requires the attacker to
get a specific message signed, and that's fine. The idea here is
slightly different in that we lower the barrier for the attacker and
only require him to be able to make some guesses about the message
that'll get signed.

An example, example.com sign everything; attacker knows some (innocent)
users in that domain; attacker wants to get a message containing "update
your account", with a hypertext link, so he sends a mail to a bunch of
these users with that HTML fragment hoping that one will reply, in such
a way that the resulting mail is usable for his nefarious purposes.

Different people will respond differently, but all reply messages will
be signed, and some may be usable as if the attacker had chosen the text
himself.  I'm sure better examples can be derived (and I offer to buy a
beer for the best in Dallas), the point is that the attacker might be
able to achieve the same effect as a chosen message replay but with
just a little more effort and without any internal collusion, zombies
or whatever.  That would presumably mean that this attack would be
mounted against enterprise domains more often than chosen message
replay.

Some kind of egress content filtering is probably the best counter
against this, and I don't see any changes for DKIM base resulting.
But if there's agreement that this is a different threat, then I think
it'd merit inclusion.

OTOH, this could be included just as a variant of chosen message
replay, which'd be easier on the editor since it'd only add a
paragraph to that section.

Stephen.


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