pre-workgroup                                                  J. Fenton
Internet-Draft                                       Cisco Systems, Inc.
Expires:  March 30, 2006                              September 26, 2005

    Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   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

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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  . . . . . . . . . . . . . . . . . . .  6
     5.1.  Use of Arbitrary Identities  . . . . . . . . . . . . . . .  6
     5.2.  Use of Specific Identities . . . . . . . . . . . . . . . .  7
       5.2.1.  Exploitation of Social Relationships . . . . . . . . .  7
       5.2.2.  Identity-Related Fraud . . . . . . . . . . . . . . . .  7
       5.2.3.  Reputation Attacks . . . . . . . . . . . . . . . . . .  8
   6.  Attacks on Message Signing . . . . . . . . . . . . . . . . . .  8
     6.1.  Unsigned Messages  . . . . . . . . . . . . . . . . . . . .  8
     6.2.  Use of throw-away addresses  . . . . . . . . . . . . . . .  9
     6.3.  Message replay . . . . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Glossary  . . . . . . . . . . . . . . . . . . . . . . 11
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

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1.  Introduction

   Message signing, as exemplified by DomainKeys Identified Mail (DKIM)
   [I-D.allman-dkim-base], is a mechanism to allow an email sender or
   intermediary to assert responsibility for an email message in transit
   by means of a digital signature.

   Once the responsible 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

   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 "zombie" networks 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
   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.

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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

   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

   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
       addresses, or to cause diversion of messages to a specific

   2.  Limited influence over portions of DNS using mechanisms such as
       cache poisoning.  This might be used to influence message

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       routing, or to cause falsification of DNS-based key or policy

   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

   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.

4.1.  Externally-located Bad Actors

   DKIM focuses primarily on bad actors located external to the
   administrative units of the claimed originator and the 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.

   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.

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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 with the administrative unit of the message
   recipient.  These bad actors may attempt to exploit the trust
   relationships which happen within the unit.  Since messages will
   typically have undergone DKIM verification at the administrative unit
   boundary, DKIM is not effective against messages submitted in this

   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
   using a mechanism like SMTP AUTH.

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

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   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

   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

5.2.  Use of Specific Identities

   A second major class of bad acts involves the assertion of specific
   identities in email.

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 worms
   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

   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.

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

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   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

   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,
   except that the key used to sign the original instance of the message
   can be revoked.  Other reputation attacks, involving the fabrication
   and transmission of a fictitious message, are addressed by DKIM since
   the 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-

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

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   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.

   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.

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   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.

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

9.  Informative References

              Allman, E., "DomainKeys Identified Mail (DKIM)",
              draft-allman-dkim-base-00 (work in progress), July 2005.

              Allman, E., "DKIM Sender Signing Policy",
              draft-allman-dkim-ssp-00 (work in progress), July 2005.

              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.

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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.

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.

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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

   Phone:  +1 408 526 5914

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