(Catching up on mail)
On September 27, 2005 at 14:53, Jim Fenton wrote:
ftp://ftpeng.cisco.com/fenton/draft-fenton-dkim-threats-00.html
My initial comments, sorry if some may be dups:
* Introduction implies a different goal for DKIM than what the
draft spec states. Here, it only mentions DKIM being used to
associate domain responsibility for a message vs "the sender of
the message was authorized to use a given email address."
* Bad actors in claimed originator's unit may be technically
outside of the scope of DKIM, but it can affect its adoption.
I.e. If domains are unable, or unwilling, to control bad
actors in their unit, then DKIM will be useless overhead.
An obvious example is free email services like Yahoo, Hotmail, etc.
Using a replay attack as described in section 6.3, DKIM signatures
of such domains can be useless, hurting the effectiveness of all
DKIM signatures.
* 4.3 implies the benefit of having MUA-based DKIM verification.
* The document talks about "origin addresses", implying that DKIM
signatures are mainly applicable for such usages. I.e. DKIM
signatures are for use by originating domains and not necessarily
any domain that wants to "claim responsibility" for a message.
This goes back to previous threads about DKIM scope and who should,
and should not, sign and when signing should occur.
I think there needs to be clear text somewhere on the scope of DKIM.
This will help determine the value DKIM offers and the security
threats to it.
* 5.2.1 does not state explicitly how DKIM is effective in dealing
with attacks mentioned in second paragraph. As noted in past
discussions, mainly related to SSP, DKIM, as currently defined,
has holes allowing forgery to go undetected.
* 5.2.3 should mention the "window" of such, and similiar, attacks.
Is simple revocation (either via key or Doug's opaque ID method)
sufficient to minimize the damage and deter attackers?
* Attacks on canonicalization methods is not mentioned. I.e. Bad
actors may exploit weakness in specific canonicalization methods to
allow messages to pass signature validation but contain different
content from what was originally signed.
* 6.3 should mention the use of complementary technologies, or
possible extensions to DKIM. To provide protection against replay
as it is happening, envelope-based technologies will need to be
employed. I'm not sure that systems that rely on reacting to the
attack after it has happened will be effective enough in deterring
attackers.
--ewh
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