Earl Hood wrote:
Welcome back!(Catching up on mail) Good point; this was written somewhat after the base and SSP drafts and whatever description we use, it should be consistent.On September 27, 2005 at 14:53, Jim Fenton wrote:ftp://ftpeng.cisco.com/fenton/draft-fenton-dkim-threats-00.htmlMy 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." 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.* 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. I think of this as a separate issue from the previous paragraph, but yes, replay is a significant concern.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. 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.* 4.3 implies the benefit of having MUA-based DKIM verification. 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.* 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'm guessing this should go in the introduction?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'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.* 5.2.1 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 expect the replay to happen effectively instantaneously, rendering the revocation techniques that have been discussed ineffective.* 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? 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.* 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. 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.* 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. -Jim |
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