> (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." I believe this to simply be a case of the draft specs being a bit behind the times. > * 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. I think this is a point worth mentioning. > * 4.3 implies the benefit of having MUA-based DKIM verification. I think the implication relies on a premise that is unlikely to be true in practice, specifically, that there won't be some mechanism in place to detect and deal with bad actors within an administrative domain. For example, I've seen a marked increase in the number of setups that require authenticated connections from all clients to send mail. But regardless, I think this is all outside the scope of the current effort. I suppose we could note that other mechanisms may come into play within an organization to thwart attacks within an organization, but that's about as far as we need to go. > * 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. I have no problem with this, although I am somewhat at a loss as to what form such test would take. > * 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. It may be difficult to nail this down now given the immaturity of the relevant specifications. > * 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? Seems reasonable to me. > * 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. This most certainly needs to be present. > * 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. I really don't think we should be discussing additional technologies here. Ned _______________________________________________ ietf-dkim mailing list http://dkim.org
