Earl Hood wrote:
(Catching up on mail)
Welcome back!
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."
Good point; this was written somewhat after the base and SSP drafts and
whatever description we use, it should be consistent.
* 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.
Sure; the point here was that there are other mechanisms that complement
DKIM within the claimed originator's and recipients units. DKIM isn't
generally useful in these places, and other things need to be deployed
to make this effective.
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.
I think of this as a separate issue from the previous paragraph, but
yes, replay is a significant concern.
* 4.3 implies the benefit of having MUA-based DKIM verification.
That's only one way to deal with it. Equally good is to have MTA-based
DKIM verification and control the spoofing of Authentication-Results
headers.
* 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 can see where you get that impression, but the emphasis on origin
addresses is just because such a claim has more easily described value
than a claim of responsibility from some other domain.
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.
I'm guessing this should go in the introduction?
* 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.
I'll add some more detail. The effectiveness comes from the fact that,
if addresses from an address book were being used as From addresses on
messages, the attacker would not normally be able to sign on behalf of
these addresses.
* 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?
I expect the replay to happen effectively instantaneously, rendering the
revocation techniques that have been discussed ineffective.
* 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.
The list of attacks in section 6 is not intended to be exhaustive. But
canonicalization attacks should perhaps be in the Security
Considerations of the base draft.
* 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.
By "envelope-based" I presume you mean SPF, Sender ID Framework, and/or
CSV. We should not create a normative dependency on any of these in the
DKIM specs, but I think they're worth mentioning in the threat analysis.
-Jim
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