pre-workgroup J. Fenton
Internet-Draft Cisco Systems, Inc.
Expires: April 24, 2006 October 21, 2005
Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)
draft-fenton-dkim-threats-01
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Copyright (C) The Internet Society (2005).
Abstract
This document provides an analysis of some threats against Internet
mail that are intended to be addressed by signature-based mail
authentication, in particular DomainKeys Identified Mail. It
discusses the nature and location of the bad actors, what their
capabilities are, and what they intend to accomplish via their
attacks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Bad Actors . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Capabilities of the Bad Actors . . . . . . . . . . . . . . . . 4
3.1. General capabilities . . . . . . . . . . . . . . . . . . . 4
3.2. Advanced capabilities . . . . . . . . . . . . . . . . . . 4
4. Location of the Bad Actors . . . . . . . . . . . . . . . . . . 5
4.1. Externally-located Bad Actors . . . . . . . . . . . . . . 5
4.2. Within Claimed Originator's Administrative Unit . . . . . 6
4.3. Within Recipient's Administrative Unit . . . . . . . . . . 6
5. Representative Bad Acts . . . . . . . . . . . . . . . . . . . 7
5.1. Use of Arbitrary Identities . . . . . . . . . . . . . . . 7
5.2. Use of Specific Identities . . . . . . . . . . . . . . . . 7
5.2.1. Exploitation of Social Relationships . . . . . . . . . 8
5.2.2. Identity-Related Fraud . . . . . . . . . . . . . . . . 8
5.2.3. Reputation Attacks . . . . . . . . . . . . . . . . . . 8
6. Attacks on Message Signing . . . . . . . . . . . . . . . . . . 9
6.1. Unsigned Messages . . . . . . . . . . . . . . . . . . . . 9
6.2. Use of Throw-Away Addresses . . . . . . . . . . . . . . . 9
6.3. Message Replay . . . . . . . . . . . . . . . . . . . . . . 10
6.4. Control of Key Management . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. Informative References . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Glossary . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Appendix C. Edit History . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
DomainKeys Identified Mail (DKIM) [I-D.allman-dkim-base] defines a
simple, low cost, and effective mechanism by which email messages can
be cryptographically signed, permitting a signing domain to claim
responsibility for the use of a given email address. Message
recipients can verify the signature by querying the signer's domain
directly to retrieve the appropriate public key, and thereby confirm
that the message was attested to by a party in possession of the
private key for the signing domain.
Once the attesting party or parties have been established, the
recipient may evaluate the message in the context of additional
information such as locally-maintained whitelists, shared reputation
services, and/or third-party accreditation. The description of these
mechanisms is outside the scope of this effort. By applying a
signature, a good player will be able to associate a positive
reputation with the message, in hopes that it will receive
preferential treatment by the recipient.
This effort is not intended to address threats associated with
message confidentiality nor does it intend to provide a long-term
archival signature.
2. The Bad Actors
The problem space being addressed by DKIM is characterized by a wide
range of attackers in terms of motivation, sophistication, and
capabilities.
At the low end of the spectrum are bad actors who may simply send
email, perhaps using one of many commercially available tools, which
the recipient does not want to receive. These tools may or may not
falsify the origin address of messages, and may, in the future, be
capable of generating message signatures as well.
At the next tier are what would be considered "professional" senders
of unwanted email. These attackers would deploy specific
infrastructure, including Mail Transfer Agents (MTAs), registered
domains and possibly networks of compromised computers ("zombies") to
send messages, and in some cases to harvest addresses to which to
send. These senders often operate as commercial enterprises and send
messages on behalf of third parties.
The most sophisticated and financially-motivated senders of messages
are those who stand to receive substantial financial benefit, such as
from an email-based fraud scheme. These attackers can be expected to
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employ all of the above mechanisms and in addition attacks on
Internet infrastructure itself, such as DNS cache-poisoning attacks
and IP routing attacks via compromised network routing elements.
3. Capabilities of the Bad Actors
3.1. General capabilities
In general, the bad actors described above should be expected to have
access to the following:
1. An extensive corpus of messages from domains they might wish to
impersonate
2. Knowledge of the business aims and model for domains they might
wish to impersonate
3. Access to public key and associated authorization records
published by the domain
and the ability to do at least some of the following:
1. Submit messages to MTAs at multiple locations in the Internet
2. Construct arbitrary message headers, including those claiming to
be mailing lists, resenders, and other mail agents
3. Sign messages on behalf of potentially-untraceable domains under
their control
4. Generate substantial numbers of either unsigned or apparently-
signed messages which might be used to attempt a denial of
service attack
5. Resend messages which may have been previously signed by the
domain
6. Transmit messages using any envelope information desired
3.2. Advanced capabilities
As noted above, certain classes of bad actors may have substantial
financial motivation for their activities, and therefore should be
expected to have more capabilities at their disposal. These include:
1. Manipulation of IP routing. This could be used to submit
messages from specific IP addresses or difficult-to-trace
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addresses, or to cause diversion of messages to a specific
domain.
2. Limited influence over portions of DNS using mechanisms such as
cache poisoning. This might be used to influence message
routing, or to cause falsification of DNS-based key or policy
advertisements.
3. Access to significant computing resources, perhaps through the
conscription of worm-infected "zombie" computers. This could
allow the bad actor to perform various types of brute-force
attacks.
4. Ability to "wiretap" some existing traffic, perhaps from a
wireless network.
Either of the first two of these mechanisms could be used to allow
the bad actor to function as a man-in-the-middle between sender and
recipient, if that attack is useful.
4. Location of the Bad Actors
In the following discussion, the term "administrative unit", taken
from [I-D.crocker-email-arch], is used to refer to a portion of the
email path that is under common administration. The originator and
recipient typically develop trust relationships with the
administrative units that send and receive their email, respectively,
to perform the signing and verification of their messages.
Bad actors or their proxies can be located anywhere in the Internet.
Bad actors within the administrative unit of the claimed originator
and/or recipient domain have capabilities beyond those elsewhere, as
described in the below sections. Bad actors can also collude by
acting in multiple locations simultaneously (a "distributed bad
actor").
4.1. Externally-located Bad Actors
DKIM focuses primarily on bad actors located outside of the
administrative units of the claimed originator and the recipient.
These administrative units frequently correspond to the protected
portions of the network adjacent to the originator and recipient. It
is in this area that the trust relationships required for
authenticated message submission do not exist and do not scale
adequately to be practical. Conversely, within these administrative
units, there are other mechanisms such as authenticated message
submission that are wasier to deploy and more likely to be used than
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DKIM.
External bad actors are usually attempting to exploit the "any to
any" nature of email which motivates most recipient MTAs to accept
messages from anywhere for delivery to their local domain. They may
generate messages without signatures, with incorrect signatures, or
with correct signatures from domains with little traceability. They
may also pose as mailing lists, greeting cards, or other agents which
legitimately send or re-send messages on behalf of others.
4.2. Within Claimed Originator's Administrative Unit
Bad actors in the form of rogue or unauthorized users or malware-
infected computers can exist within the administrative unit
corresponding to a message's origin address. Since the submission of
messages in this area generally occurs prior to the application of a
message signature, DKIM is not directly effective against these bad
actors. Defense against these bad actors is dependent upon other
means, such as proper use of firewalls, and mail submission agents
that are configured to authenticate the sender.
In the special case where the administrative unit is non-contiguous
(e.g., a company that communicates between branches over the external
Internet), DKIM signatures can be used to distinguish between
legitimate externally-originated messages and attempts to spoof
addresses in the local domain.
4.3. Within Recipient's Administrative Unit
Bad actors may also exist within the administrative unit of the
message recipient. These bad actors may attempt to exploit the trust
relationships which exist within the unit. Since messages will
typically only have undergone DKIM verification at the administrative
unit boundary, DKIM is not effective against messages submitted in
this area.
For example, the bad actor may attempt to apply a header such as
Authentication-Results [I-D.kucherawy-sender-auth-header] which would
normally be added (and spoofing of which would be detected) at the
boundary of the administrative unit. This could be used to falsely
indicate that the message was authenticated successfully.
As in the originator case, these bad actors are best dealt with by
controlling the submission of messages within the administrative
unit. Depending on the characteristics of the administrative unit,
cryptographic methods may or may not be needed to accomplish this.
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5. Representative Bad Acts
One of the most fundamental bad acts being attempted is the delivery
of messages which are not authorized by the alleged originating
domain. As described above, these messages might merely be unwanted
by the recipient, or might be part of a confidence scheme or a
delivery vector for malware.
5.1. Use of Arbitrary Identities
This class of bad acts includes the sending of messages which aim to
obscure the identity of the actual sender. In some cases the actual
sender might be the bad actor, or in other cases might be a third-
party under the control of the bad actor (e.g., a compromised
computer).
DKIM is effective in mitigating against the use of addresses not
controlled by bad actors, but is not effective against the use of
addresses they control. In other words, the presence of a valid DKIM
signature does not guarantee that the signer is not a bad actor. It
also does not guarantee the accountability of the signer, since that
is limited by the extent to which domain registration requires
accountability for its registrants. However, accreditation and
reputation systems can be used to enhance the accountability of DKIM-
verified addresses and/or the likelihood that signed messages are
desirable.
5.2. Use of Specific Identities
A second major class of bad acts involves the assertion of specific
identities in email.
Note that some bad acts involving specific identities can sometimes
be accomplished, although perhaps less effectively, with similar
looking identities that mislead some recipients. For example, if the
bad actor is able to control the domain "examp1e.com" (note the "one"
between the p and e), they might be able to convince some recipients
that a message from admin@examp1e.com is really admin@example.com.
Similar types of attacks using internationalized domain names have
been hypothesized where it could be very difficult to see character
differences in popular typefaces. Similarly, if example2.com was
controlled by a bad actor, the bad actor could sign messages from
bigbank.example2.com which might also mislead some recipients. To
the extent that these domains are controlled by bad actors, DKIM is
not effective against these attacks, although it could support the
ability of reputation and/or accreditation systems to aid the user in
identifying them.
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5.2.1. Exploitation of Social Relationships
One reason for asserting a specific origin address is to encourage a
recipient to read and act on particular email messages by appearing
to be an acquaintance or previous correspondent that the recipient
might trust. This tactic has been used by email-propagated malware
which mail themselves to addresses in the infected host's address
book. In this case, however, the sender's address may not be
falsified, so DKIM would not be effective in defending against this
act.
It is also possible for address books to be harvested and used by an
attacker to send messages from elsewhere. DKIM would be effective in
mitigating these acts by limiting the scope of origin addresses for
which a valid signature can be obtained when sending the messages
from other locations.
5.2.2. Identity-Related Fraud
Bad acts related to email-based fraud often, but not always, involve
the transmission of messages using specific origin addresses of other
entities as part of the fraud scheme. The use of a specific address
of origin sometimes contributes to the success of the fraud by
convincing the recipient that the message was actually sent by the
alleged sender.
To the extent that the success of the fraud depends on or is enhanced
by the use of a specific origin address, the bad actor may have
significant financial motivation and resources to circumvent any
measures taken to protect specific addresses from unauthorized use.
5.2.3. Reputation Attacks
Another motivation for using a specific origin address in a message
is to harm the reputation of another, commonly referred to as a "joe-
job". For example, a commercial entity might wish to harm the
reputation of a competitor, perhaps by sending unsolicited bulk email
on behalf of that competitor. It is for this reason that reputation
systems must be based on an identity that is, in practice, fairly
reliable.
Reputation attacks of this sort are sometimes based on the
retransmission (often referred to as a "replay") of a legitimately
sent message. DKIM provides little protection against such acts,
although the key used to sign the original instance of the message
can be revoked, which limits the time window available for such
attacks. Other reputation attacks, involving the fabrication and
transmission of a fictitious message, are addressed by DKIM since the
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bad actor would not, without inside assistance, be able to obtain a
valid signature for the fabricated message.
6. Attacks on Message Signing
Bad actors can be expected to exploit all of the limitations of
message authentication systems. They are also likely to be motivated
to degrade the usefulness of message authentication systems in order
to hinder their deployment. Some representatives of these categories
of bad acts are described below. Additional postulated attacks are
described in the Security Considerations section of [I-D.allman-dkim-
base].
6.1. Unsigned Messages
Messages without signatures may be sent in an effort to exploit the
incremental deployment of message signatures. In many cases, a
recipient may not be able to make a determination about unsigned
messages from a domain, and therefore will need to accept the message
(although perhaps at a lower delivery priority). This situation is
mitigated by the use of the DKIM Sender Signing Policy (SSP)
[I-D.allman-dkim-ssp] that indicates whether or not a given domain
signs all of its messages. Nevertheless, the possibility of
signature breakage due to legitimate modification of the message may
limit the ability of SSP to dictate harsh treatment of messages
without valid signatures.
Messages with invalid signatures may also be introduced by bad
actors. The intent may be to make the message appear as though it
was legitimately sent, but "broken" in transit, i.e. that the message
was modified, rendering the signature invalid. At least until the
causes of signature breakage are well understood, messages with
invalid signatures need to be evaluated as if the invalid signature
isn't present at all.
6.2. Use of Throw-Away Addresses
Bad actors may also introduce messages with valid signatures on
behalf of domains they control, perhaps "throw-away" domains
registered under false pretenses which are difficult to trace. In
other words, the existence of a message signature does not imply that
the message is "good". The use of such domains will undoubtedly give
rise to domain-based accreditation and reputation systems. Until
these are available, local reputation, mostly in the form of
whitelists, can be maintained by domains to improve the
deliverability of email from domains with which they have business or
other relationships.
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Accreditation and reputation, or even local whitelists, require a
reliable identity on which to base their assertion, and in the case
of reputation on which to base any feedback reports. Message signing
provides an identity which is intended to be sufficiently reliable
for this purpose, and it (or some other reliable mechanism) is
necessary for accreditation and reputation systems to operate.
Modification of messages by mailing lists and other legitimate agents
requires that a mechanism be created for signing of messages by other
than the originating domain. This provides a bad actor with an
additional avenue through which it might attempt to circumvent
message authentication. A bad actor might attempt to pose as a
mailing list which modifies a message and adds its own signature
taking responsibility for the message. If this signature is from an
untraceable domain, little assertion of the legitimacy of the message
is provided by this signature. For this reason, accreditation,
reputation, and local reputation in the form of white lists is at
least as important for these signatures from third parties as they
are for origination address signatures.
6.3. Message Replay
Message replay is a term used to describe the retransmission of
already-signed messages. Here the bad actor obtains an account with
a domain such as a consumer ISP, and sends an undesirable message to
an external address controlled by the bad actor or an accomplice.
That message, having now obtained a signature, is forwarded to other
recipients without the authorization of the signing domain. It is
closely related to one of the reputation attacks described above.
This bad act is basically indistinguishable from a number of
acceptable acts, such as the transparent forwarding of messages by a
recipient to multiple addresses. For this reason, DKIM is not
particularly effective at detecting and eliminating this bad act.
Prompt key revocation may mitigate this problem; however, since
verification typically occurs as messages are received by recipient
domains, time is of the essence.
Other means to mitigate this bad act include the use of content
filtering on messages being signed, and business models which enforce
more accountability for subscribers whose messages are to be signed
by DKIM.
6.4. Control of Key Management
In cases where the Bad Actor is in control of the DNS zone for a
domain's keyspace subdomain (_domainkey), or is able to influence the
records in that zone, message signing may present a new opportunity
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to interfere with the receipt of legitimate messages or to sign
messages not illegitimately. This could occur if the DNS provider
for the domain is not reliable or if the security measures used by
the DNS provider are breached by the bad actor. DKIM is fully
dependent on the key information which it is provided by DNS, so
independent means such as audits of the key records would be required
to mitigate this threat.
7. IANA Considerations
This document defines no items requiring IANA assignment.
8. Security Considerations
This document describes the security threat environment in which
DomainKeys Identified Mail (DKIM) is expected to provide some
benefit.
9. Informative References
[I-D.allman-dkim-base]
Allman, E., "DomainKeys Identified Mail (DKIM)",
draft-allman-dkim-base-00 (work in progress), July 2005.
[I-D.allman-dkim-ssp]
Allman, E., "DKIM Sender Signing Policy",
draft-allman-dkim-ssp-00 (work in progress), July 2005.
[I-D.crocker-email-arch]
Crocker, D., "Internet Mail Architecture",
draft-crocker-email-arch-04 (work in progress),
March 2005.
[]
Kucherawy, M., "Message Header for Indicating Sender
Authentication Status",
draft-kucherawy-sender-auth-header-02 (work in progress),
May 2005.
Appendix A. Glossary
Origin address - The address on an email message, typically the RFC
2822 From: address, which is associated with the alleged author of
the message and is displayed by the recipient's MUA as the source of
the message.
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Appendix B. Acknowledgements
The author wishes to thank Phillip Hallam-Baker, Eliot Lear, Tony
Finch, Dave Crocker, Barry Leiba, Arvel Hathcock, Eric Allman, and
Jon Callas for valuable suggestions and constructive criticism of
earlier versions of this draft.
Appendix C. Edit History
Changes since -00 draft:
o Changed beginning of introduction to make it consistent with -base
draft.
o Clarified reasons for focus on externally-located bad actors.
o Elaborated on reasons for effectiveness of address book attacks.
o Described attack time windows with respect to replay attacks.
o Added discussion of attacks using look-alike domains.
o Added section on key management attacks.
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Author's Address
Jim Fenton
Cisco Systems, Inc.
MS SJ-24/2
170 W. Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5914
Email: fenton@cisco.com
URI:
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