Hi Doug,
Thanks for making the effort - I do appreciate it & will
post a substantive response later on today (gotta go
inflict myself on some unfortunate students in the
meantime:-)
Cheers,
S.
Douglas Otis wrote:
On Jan 9, 2006, at 12:02 PM, Stephen Farrell wrote:
I'd love to see you write that text up that could be used in the
threats draft. I've yet to see it in a usable form.
Terminology:
The terms "open-ended" and "closed" authorization are defined as:
A basic function of email authorization referenced by way of an
identity is to influence the acceptance or rejection of a message.
The term "closed" indicates acceptance is based upon the identity
being found within a defined set of identifiers. When acceptance
does not require that the identity be contained within a defined
set, this is described as open-ended authorization. This
definition is not altered by the rating of messages once they are
accepted.
SSP 'o=' Qualifiers:
"~" Signs some (open)
"-" All signed & allow other signatures. (open)
"!" All signed. (closed)
"." Never sends mail. (closed)
"^" Check user specific policy (deferred)
3. SSP Related Threats
3.1 Risks associated with the misuse of "open-ended" authorizations
Administrators often block abusive messages using lists of sources
with a history of sending abusive messages. Within email, the client
IP address or verified host-name could be used to fairly identify
sources. Assuming a mechanism will deal with abusive replays, even
the DKIM signature could be fairly used.
Alas, an administrator may also consider acceptance granted by an
email-address authorization as verification of this as a source
identifier. This strategy has the effect of holding the email-
address domain owner culpable for authorizations that permit
acceptance of abusive messages. When the authorization is open-
ended, the email-address domain owner is therefore exposed to unfair
accruals of abuse based upon authorization.
3.2 Disruption caused by "closed" authorizations
When closed authorizations are used, mediators or users obtaining
access from other providers will likely be outside the set of
identifiers contained within the authorization. Closed
authorizations will therefore disrupt common practices such as
posting to list servers, use of e-invites, and other similar
services.
3.3 Accommodating "closed" policies at the mediator
When the mediator is a list server, one technique to ensure delivery
may be to modify the header being checked to reference a different
authorization record. One form of this technique may introduce
multiple From email-addresses where the first address conforms to the
identity of the list-server. A similar technique could be used to
overcome closed authorizations imposed by providers where the user
may also utilize two From addresses. This could be needed when the
second address is recognizable to the recipient, but otherwise
prohibited by closed authorization.
3.4 Increased overhead checking multiple From addresses
The From header within a message may contain any number of addresses.
Some consider use of multiple addresses a valid means to overcome
limitations of an authorization mechanism. Alternatively, some wish
to check authorizations for every From address to preclude this
strategy being used to overcome the limitations imposed by
authorizations. Multiple From addresses could be confusing for the
recipient and poorly handled by the email applications. Precluding
acceptance of any From address that would be in conflict with the
specific email-address authorization further increases the overhead
associated with searching for authorizations.
3.5 Coercive ratings when not publishing an authorization record
Email-address authorization provides advantages for large domains.
Large domains are much less sensitive to abuse histories as they are
often excluded from block-lists due to their size. However, smaller
domains are much more prone to being negatively impacted by unfair
accruals.
Down-rating domains without email-address authorization by larger
domains is a technique used to coerce other domains into publishing
authorizations. Open-ended authorizations are needed to permit
current practices expected by customers, but then these
authorizations may fall prey to bad actors who will utilize these
authorizations for their abuse. When these smaller domains become
placed within block-lists, there will be an exodus over to the larger
domains. Coercing the use of the email-address authorization also
mitigates the overhead associated with searching for these records.
3.6 Exploitation of "open-ended" authorization being unfairly
attributed to the mail-address domain owner
When messages obtain improved ratings which depend upon the email-
address having been authorized, then open-ended authorization records
will allow bad actor to use these authorization records to improve
upon their message acceptance ratings. To ensure messages are
accepted after passing through other mediators, an open-ended
authorization is required of the email-address domain owner.
Unfortunately, the email-address domain owner is unable to control
whether their authorization is seen as a "weak" form of
authentication and subsequently used to accrue abuse from all
permitted sources. As a result of message ratings based upon
authorization, open-ended authorizations, and the assumption of
authorization being a "weak" identifier, the email-address domain
owner may find their domain subsequently block-listed.
3.7 Overhead of email-address authorization retrivial
The overhead related to a defensive strategy should not increase the
burden of the recipient as opposed to that of the sender.
Unfortunately, walking up label trees searching for email-address
authorization records imposes a relatively high overhead. This
overhead is kept high as few lookups return an authorization record
and therefore the lack of a record will be retained only briefly
within the DNS cache.
3.8 Label depth found in abusive email versus legitimate email
Bad actors take advantage of an evolving structure of top, second,
third, and forth level domains. Often bad actors create a series of
random labels above some domain to make it difficult to filter, as
the significant level where the direct registration is made becomes
difficult to determine algorithmically. This practice tends to
increase the number of labels found in abusive messages.
3.9 Dictionary attacks of local-part authorizations
Defensive programs currently defend against dictionary attacks being
attempted at the SMTP server. DNS however is not normally designed
to identify such searches, and with the lower latency of DNS, these
searches can be more productive at determining valid email-addresses
when user specific authorizations are being published.
-Doug
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