Network Working Group J. Fenton

Network Working Group J. Fenton
Request for Comments: 4686 Cisco Systems, Inc.
Category: Informational September 2006

Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

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
1.1. Terminology and Model ......................................3
1.2. Document Structure .........................................5
2. The Bad Actors ..................................................6
2.1. Characteristics ............................................6
2.2. Capabilities ...............................................6
2.3. Location ...................................................8
2.3.1. Externally-Located Bad Actors .......................8
2.3.2. Within Claimed Originator's Administrative Unit .....8
2.3.3. Within Recipient's Administrative Unit ..............9
3. Representative Bad Acts .........................................9
3.1. Use of Arbitrary Identities ................................9
3.2. Use of Specific Identities ................................10
3.2.1. Exploitation of Social Relationships ...............10
3.2.2. Identity-Related Fraud .............................11
3.2.3. Reputation Attacks .................................11
3.2.4. Reflection Attacks .................................11
4. Attacks on Message Signing .....................................12
4.1. Attacks against Message Signatures ........................12
4.1.1. Theft of Private Key for Domain ....................13
4.1.2. Theft of Delegated Private Key .....................13
4.1.3. Private Key Recovery via Side Channel Attack .......14
4.1.4. Chosen Message Replay ..............................14
4.1.5. Signed Message Replay ..............................16
4.1.6. Denial-of-Service Attack against Verifier ..........16
4.1.7. Denial-of-Service Attack against Key Service .......17
4.1.8. Canonicalization Abuse .............................17
4.1.9. Body Length Limit Abuse ............................17
4.1.10. Use of Revoked Key ................................18
4.1.11. Compromise of Key Server ..........................18
4.1.12. Falsification of Key Service Replies ..............19
4.1.13. Publication of Malformed Key Records
and/or Signatures .................................19
4.1.14. Cryptographic Weaknesses in Signature Generation ..20
4.1.15. Display Name Abuse ................................21
4.1.16. Compromised System within Originator's Network ....21
4.1.17. Verification Probe Attack .........................21
4.1.18. Key Publication by Higher-Level Domain ............22
4.2. Attacks against Message Signing Practices .................23
4.2.1. Look-Alike Domain Names ............................23
4.2.2. Internationalized Domain Name Abuse ................23
4.2.3. Denial-of-Service Attack against Signing
Practices ..........................................24
4.2.4. Use of Multiple From Addresses .....................24
4.2.5. Abuse of Third-Party Signatures ....................24
4.2.6. Falsification of Sender Signing Practices Replies ..25

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4.3. Other Attacks .............................................25
4.3.1. Packet Amplification Attacks via DNS ...............25
5. Derived Requirements ...........................................26
6. Security Considerations ........................................26
7. Informative References .........................................27
Appendix A. Acknowledgements ......................................28

1. Introduction

The DomainKeys Identified Mail (DKIM) protocol is being specified by
the IETF DKIM Working Group. The DKIM protocol defines a 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. This document
addresses threats relative to two works in progress by the DKIM
Working Group, the DKIM signature specification [DKIM-BASE] and DKIM
Sender Signing Practices [DKIM-SSP].

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 the IETF DKIM Working Group
effort. By applying a signature, a good player enables a verifier 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.

1.1. Terminology and Model

An administrative unit (AU) is the portion of the path of an email
message 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.

The origin address is 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 Mail User Agent
(MUA) as the source of the message.

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The following diagram illustrates a typical usage flowchart for DKIM:

+---------------------------------+
| SIGNATURE CREATION |
| (Originating or Relaying AU) |
| |
| Sign (Message, Domain, Key) |
| |
+---------------------------------+
| - Message (Domain, Key)
|
[Internet]
|
V
+---------------------------------+
+-----------+ | SIGNATURE VERIFICATION |
| | | (Relaying or Delivering AU) |
| KEY | | |
| QUERY +--->| Verify (Message, Domain, Key) |
| | | |
+-----------+ +----------------+----------------+
| - Verified Domain
+-----------+ V - [Report]
| SENDER | +----------------+----------------+
| SIGNING | | |
| PRACTICES +--->| SIGNER EVALUATION |
| QUERY | | |
| | +---------------------------------+
+-----------+

DKIM operates entirely on the content (body and selected header
fields) of the message, as defined in RFC 2822 [RFC2822]. The
transmission of messages via SMTP, defined in RFC 2821 [RFC2821], and
such elements as the envelope-from and envelope-to addresses and the
HELO domain are not relevant to DKIM verification. This is an
intentional decision made to allow verification of messages via
protocols other than SMTP, such as POP [RFC1939] and IMAP [RFC3501]
which an MUA acting as a verifier might use.

The Sender Signing Practices Query referred to in the diagram above
is a means by which the verifier can query the alleged author's
domain to determine their practices for signing messages, which in
turn may influence their evaluation of the message. If, for example,
a message arrives without any valid signatures, and the alleged
author's domain advertises that they sign all messages, the verifier
might handle that message differently than if a signature was not
necessarily to be expected.

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1.2. Document Structure

The remainder of this document describes the problems that DKIM might
be expected to address, and the extent to which it may be successful
in so doing. These are described in terms of the potential bad
actors, their capabilities and location in the network, and the bad
acts that they might wish to commit.

This is followed by a description of postulated attacks on DKIM
message signing and on the use of Sender Signing Practices to assist
in the treatment of unsigned messages. A list of derived
requirements is also presented, which is intended to guide the DKIM
design and review process.

The sections dealing with attacks on DKIM each begin with a table
summarizing the postulated attacks in each category along with their
expected impact and likelihood. The following definitions were used
as rough criteria for scoring the attacks:

Impact:

High: Affects the verification of messages from an entire domain
or multiple domains

Medium: Affects the verification of messages from specific users,
Mail Transfer Agents (MTAs), and/or bounded time periods

Low: Affects the verification of isolated individual messages
only

Likelihood:

High: All email users should expect this attack on a frequent
basis

Medium: Email users should expect this attack occasionally;
frequently for a few users

Low: Attack is expected to be rare and/or very infrequent

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2. The Bad Actors

2.1. Characteristics

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, that
the recipient does not want to receive. These tools typically allow
one to 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 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
employ all of the above mechanisms and additionally may attack the
Internet infrastructure itself, including DNS cache-poisoning attacks
and IP routing attacks.

2.2. 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 keys and associated authorization records
associated with the domain

and the ability to do at least some of the following:

1. Submit messages to MTAs and Message Submission Agents (MSAs) at
multiple locations in the Internet

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2. Construct arbitrary message header fields, including those
claiming to be mailing lists, resenders, and other mail agents

3. Sign messages on behalf of domains under their control

4. Generate substantial numbers of either unsigned or apparently-
signed messages that might be used to attempt a denial-of-service
attack

5. Resend messages that may have been previously signed by the
domain

6. Transmit messages using any envelope information desired

7. Act as an authorized submitter for messages from a compromised
computer

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
domain.

2. Limited influence over portions of DNS using mechanisms such as
cache poisoning. This might be used to influence message routing
or to falsify advertisements of DNS-based keys or signing
practices.

3. Access to significant computing resources, for example, 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 eavesdrop on 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 author and
recipient, if that attack is useful.

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2.3. Location

Bad actors or their proxies can be located anywhere in the Internet.
Certain attacks are possible primarily 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 from multiple locations (a
"distributed bad actor").

It should also be noted that with the use of "zombies" and other
proxies, externally-located bad actors may gain some of the
capabilities of being located within the claimed originator's or
recipient's administrative unit. This emphasizes the importance of
appropriate security measures, such as authenticated submission of
messages, even within administrative units.

2.3.1. 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 easier to deploy and more likely to be used than
DKIM.

External bad actors are usually attempting to exploit the "any to
any" nature of email that 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 that
legitimately send or resend messages on behalf of others.

2.3.2. 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 Message Submission Agents
that are configured to authenticate the author.

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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.

2.3.3. 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 that 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 spoof a header field
indicating the results of verification. This header field would
normally be added by the verifier, which would also detect spoofed
header fields on messages it was attempting to verify. This could be
used to falsely indicate that the message was authenticated
successfully.

As in the originator case, these bad actors can be dealt with by
controlling the submission of messages within the administrative
unit. Since DKIM permits verification to occur anywhere within the
recipient's administrative unit, these threats can also be minimized
by moving verification closer to the recipient, such as at the Mail
Delivery Agent (MDA), or on the recipient's MUA itself.

3. Representative Bad Acts

One of the most fundamental bad acts being attempted is the delivery
of messages that are not intended to have been sent 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.

3.1. Use of Arbitrary Identities

This class of bad acts includes the sending of messages that aim to
obscure the identity of the actual author. 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).

Particularly when coupled with sender signing practices that indicate
the domain owner signs all messages, DKIM can be effective in
mitigating against the abuse of addresses not controlled by bad

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actors. DKIM is not effective against the use of addresses
controlled by bad actors. 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
DKIM does not attempt to identify the signer individually, but rather
identifies the domain that they control. Accreditation and
reputation systems and locally-maintained whitelists and blacklists
can be used to enhance the accountability of DKIM-verified addresses
and/or the likelihood that signed messages are desirable.

3.2. 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 [email protected] is really from
[email protected]. 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.

DKIM is effective against the use of specific identities only when
there is an expectation that such messages will, in fact, be signed.
The primary means for establishing this is the use of Sender Signing
Practices (SSP), which will be specified by the IETF DKIM Working
Group.

3.2.1. 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
that mail themselves to addresses in the infected host's address
book. In this case, however, the author's address may not be
falsified, so DKIM would not be effective in defending against this
act.

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It is also possible for address books to be harvested and used by an
attacker to post messages from elsewhere. DKIM could 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.

3.2.2. 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
helping convince the recipient that the message was actually sent by
the alleged author.

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.

When signatures are verified by or for the recipient, DKIM is
effective in defending against the fraudulent use of origin addresses
on signed messages. When the published sender signing practices of
the origin address indicate that all messages from that address
should be signed, DKIM further mitigates against the attempted
fraudulent use of the origin address on unsigned messages.

3.2.3. 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.

3.2.4. Reflection Attacks

A commonly-used tactic by some bad actors is the indirect
transmission of messages by intentionally mis-addressing the message
and causing it to be "bounced", or sent to the return address (RFC
2821 envelope-from address) on the message. In this case, the
specific identity asserted in the email is that of the actual target
of the message, to whom the message is "returned".

DKIM does not, in general, attempt to validate the RFC2821.mailfrom
return address on messages, either directly (noting that the mailfrom

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address is an element of the SMTP protocol, and not the message
content on which DKIM operates), or via the optional Return-Path
header field. Furthermore, as is noted in Section 4.4 of RFC 2821
[RFC2821], it is common and useful practice for a message's return
path not to correspond to the origin address. For these reasons,
DKIM is not effective against reflection attacks.

4. 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. Both the signature mechanism itself and
declarations made regarding use of message signatures (referred to
here as Sender Signing Practices or SSP) can be expected to be the
target of attacks.

4.1. Attacks against Message Signatures

The following is a summary of postulated attacks against DKIM
signatures:

+---------------------------------------------+--------+------------+
| Attack Name | Impact | Likelihood |
+---------------------------------------------+--------+------------+
| Theft of private key for domain | High | Low |
| Theft of delegated private key | Medium | Medium |
| Private key recovery via side channel attack| High | Low |
| Chosen message replay | Low | M/H |
| Signed message replay | Low | High |
| Denial-of-service attack against verifier | High | Medium |
| Denial-of-service attack against key service| High | Medium |
| Canonicalization abuse | Low | Medium |
| Body length limit abuse | Medium | Medium |
| Use of revoked key | Medium | Low |
| Compromise of key server | High | Low |
| Falsification of key service replies | Medium | Medium |
| Publication of malformed key records and/or | High | Low |
| signatures | | |
| Cryptographic weaknesses in signature | High | Low |
| generation | | |
| Display name abuse | Medium | High |
| Compromised system within originator's | High | Medium |
| network | | |
| Verification probe attack | Medium | Medium |
| Key publication by higher-level domain | High | Low |
+---------------------------------------------+--------+------------+

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4.1.1. Theft of Private Key for Domain

Message signing technologies such as DKIM are vulnerable to theft of
the private keys used to sign messages. This includes "out-of-band"
means for this theft, such as burglary, bribery, extortion, and the
like, as well as electronic means for such theft, such as a
compromise of network and host security around the place where a
private key is stored.

Keys that are valid for all addresses in a domain typically reside in
MTAs that should be located in well-protected sites, such as data
centers. Various means should be employed for minimizing access to
private keys, such as non-existence of commands for displaying their
value, although ultimately memory dumps and the like will probably
contain the keys. Due to the unattended nature of MTAs, some
countermeasures, such as the use of a pass phrase to "unlock" a key,
are not practical to use. Other mechanisms, such as the use of
dedicated hardware devices that contain the private key and perform
the cryptographic signature operation, would be very effective in
denying export of the private key to those without physical access to
the device. Such devices would almost certainly make the theft of
the key visible, so that appropriate action (revocation of the
corresponding public key) can be taken should that happen.

4.1.2. Theft of Delegated Private Key

There are several circumstances where a domain owner will want to
delegate the ability to sign messages for the domain to an individual
user or a third party associated with an outsourced activity such as
a corporate benefits administrator or a marketing campaign. Since
these keys may exist on less well-protected devices than the domain's
own MTAs, they will in many cases be more susceptible to compromise.

In order to mitigate this exposure, keys used to sign such messages
can be restricted by the domain owner to be valid for signing
messages only on behalf of specific addresses in the domain. This
maintains protection for the majority of addresses in the domain.

A related threat is the exploitation of weaknesses in the delegation
process itself. This threat can be mitigated through the use of
customary precautions against the theft of private keys and the
falsification of public keys in transit. For example, the exposure
to theft can be minimized if the delegate generates the keypair to be
used, and sends the public key to the domain owner. The exposure to
falsification (substitution of a different public key) can be reduced
if this transmission is signed by the delegate and verified by the
domain owner.

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4.1.3. Private Key Recovery via Side Channel Attack

All popular digital signature algorithms are subject to a variety of
side channel attacks. The most well-known of these are timing
channels [Kocher96], power analysis [Kocher99], and cache timing
analysis [Bernstein04]. Most of these attacks require either
physical access to the machine or the ability to run processes
directly on the target machine. Defending against these attacks is
out of scope for DKIM.

However, remote timing analysis (at least on local area networks) is
known to be feasible [Boneh03], particularly in server-type platforms
where the attacker can inject traffic that will immediately be
subject to the cryptographic operation in question. With enough
samples, these techniques can be used to extract private keys even in
the face of modest amounts of noise in the timing measurements.

The three commonly proposed countermeasures against timing analysis
are:

1. Make the operation run in constant time. This turns out in
practice to be rather difficult.

2. Make the time independent of the input data. This can be
difficult, but see [Boneh03] for more details.

3. Use blinding. This is generally considered the best current
practice countermeasure, and while not proved generally secure is
a countermeasure against known timing attacks. It adds about
2-10% to the cost of the operation and is implemented in many
common cryptographic libraries. Unfortunately, Digital Signature
Algorithm (DSA) and Elliptic Curve DSA (ECDSA) do not have
standard methods though some defenses may exist.

Note that adding random delays to the operation is only a partial
countermeasure. Because the noise is generally uniformly
distributed, a large enough number of samples can be used to average
it out and extract an accurate timing signal.

4.1.4. Chosen Message Replay

Chosen message replay refers to the scenario where the attacker
creates a message and obtains a signature for it by sending it
through an MTA authorized by the originating domain to
himself/herself or an accomplice. They then "replay" the signed
message by sending it, using different envelope addresses, to a
(typically large) number of other recipients.

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Due to the requirement to get an attacker-generated message signed,
chosen message replay would most commonly be experienced by consumer
ISPs or others offering email accounts to clients, particularly where
there is little or no accountability to the account holder (the
attacker in this case). One approach to solving this problem is for
the domain to only sign email for clients that have passed a vetting
process to provide traceability to the message originator in the
event of abuse. At present, the low cost of email accounts (zero)
does not make it practical for any vetting to occur. It remains to
be seen whether this will be the model with signed mail as well, or
whether a higher level of trust will be required to obtain an email
signature.

A variation on this attack involves the attacker sending a message
with the intent of obtaining a signed reply containing their original
message. The reply might come from an innocent user or might be an
automatic response such as a "user unknown" bounce message. In some
cases, this signed reply message might accomplish the attacker's
objectives if replayed. This variation on chosen message replay can
be mitigated by limiting the extent to which the original content is
quoted in automatic replies, and by the use of complementary
mechanisms such as egress content filtering.

Revocation of the signature or the associated key is a potential
countermeasure. However, the rapid pace at which the message might
be replayed (especially with an army of "zombie" computers), compared
with the time required to detect the attack and implement the
revocation, is likely to be problematic. A related problem is the
likelihood that domains will use a small number of signing keys for a
large number of customers, which is beneficial from a caching
standpoint but is likely to result in a great deal of collateral
damage (in the form of signature verification failures) should a key
be revoked suddenly.

Signature revocation addresses the collateral damage problem at the
expense of significant scaling requirements. At the extreme,
verifiers could be required to check for revocation of each signature
verified, which would result in very significant transaction rates.
An alternative, "revocation identifiers", has been proposed, which
would permit revocation on an intermediate level of granularity,
perhaps on a per-account basis. Messages containing these
identifiers would result in a query to a revocation database, which
might be represented in DNS.

Further study is needed to determine if the benefits from revocation
(given the potential speed of a replay attack) outweigh the
transactional cost of querying a revocation database.

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4.1.5. Signed Message Replay

Signed message replay refers to the retransmission of already-signed
messages to additional recipients beyond those intended by the author
or the original poster of the message. The attacker arranges to
receive a message from the victim, and then retransmits it intact but
with different envelope addresses. This might be done, for example,
to make it look like a legitimate sender of messages is sending a
large amount of spam. When reputation services are deployed, this
could damage the author's reputation or that of the author's domain.

A larger number of domains are potential victims of signed message
replay than chosen message replay because the former does not require
the ability for the attacker to send messages from the victim domain.
However, the capabilities of the attacker are lower. Unless coupled
with another attack such as body length limit abuse, it isn't
possible for the attacker to use this, for example, for advertising.

Many mailing lists, especially those that do not modify the content
of the message and signed header fields and hence do not invalidate
the signature, engage in a form of signed message replay. The use of
body length limits and other mechanisms to enhance the survivability
of messages effectively enhances the ability to do so. The only
things that distinguish this case from undesirable forms of signed
message replay is the intent of the replayer, which cannot be
determined by the network.

4.1.6. Denial-of-Service Attack against Verifier

While it takes some computing resources to sign and verify a
signature, it takes negligible computing resources to generate an
invalid signature. An attacker could therefore construct a "make
work" attack against a verifier, by sending a large number of
incorrectly-signed messages to a given verifier, perhaps with
multiple signatures each. The motivation might be to make it too
expensive to verify messages.

While this attack is feasible, it can be greatly mitigated by the
manner in which the verifier operates. For example, it might decide
to accept only a certain number of signatures per message, limit the
maximum key size it will accept (to prevent outrageously large
signatures from causing unneeded work), and verify signatures in a
particular order. The verifier could also maintain state
representing the current signature verification failure rate and
adopt a defensive posture when attacks may be under way.

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4.1.7. Denial-of-Service Attack against Key Service

An attacker might also attempt to degrade the availability of an
originator's key service, in order to cause that originator's
messages to be unverifiable. One way to do this might be to quickly
send a large number of messages with signatures that reference a
particular key, thereby creating a heavy load on the key server.
Other types of DoS attacks on the key server or the network
infrastructure serving it are also possible.

The best defense against this attack is to provide redundant key
servers, preferably on geographically-separate parts of the Internet.
Caching also helps a great deal, by decreasing the load on
authoritative key servers when there are many simultaneous key
requests. The use of a key service protocol that minimizes the
transactional cost of key lookups is also beneficial. It is noted
that the Domain Name System has all these characteristics.

4.1.8. Canonicalization Abuse

Canonicalization algorithms represent a tradeoff between the survival
of the validity of a message signature and the desire not to allow
the message to be altered inappropriately. In the past,
canonicalization algorithms have been proposed that would have
permitted attackers, in some cases, to alter the meaning of a
message.

Message signatures that support multiple canonicalization algorithms
give the signer the ability to decide the relative importance of
signature survivability and immutability of the signed content. If
an unexpected vulnerability appears in a canonicalization algorithm
in general use, new algorithms can be deployed, although it will be a
slow process because the signer can never be sure which algorithm(s)
the verifier supports. For this reason, canonicalization algorithms,
like cryptographic algorithms, should undergo a wide and careful
review process.

4.1.9. Body Length Limit Abuse

A body length limit is an optional indication from the signer of how
much content has been signed. The verifier can either ignore the
limit, verify the specified portion of the message, or truncate the
message to the specified portion and verify it. The motivation for
this feature is the behavior of many mailing lists that add a
trailer, perhaps identifying the list, at the end of messages.

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When body length limits are used, there is the potential for an
attacker to add content to the message. It has been shown that this
content, although at the end, can cover desirable content, especially
in the case of HTML messages.

If the body length isn't specified, or if the verifier decides to
ignore the limit, body length limits are moot. If the verifier or
recipient truncates the message at the signed content, there is no
opportunity for the attacker to add anything.

If the verifier observes body length limits when present, there is
the potential that an attacker can make undesired content visible to
the recipient. The size of the appended content makes little
difference, because it can simply be a URL reference pointing to the
actual content. Receiving MUAs can mitigate this threat by, at a
minimum, identifying the unsigned content in the message.

4.1.10. Use of Revoked Key

The benefits obtained by caching of key records opens the possibility
that keys that have been revoked may be used for some period of time
after their revocation. The best examples of this occur when a
holder of a key delegated by the domain administrator must be
unexpectedly deauthorized from sending mail on behalf of one or more
addresses in the domain.

The caching of key records is normally short-lived, on the order of
hours to days. In many cases, this threat can be mitigated simply by
setting a short time-to-live (TTL) for keys not under the domain
administrator's direct control (assuming, of course, that control of
the TTL value may be specified for each record, as it can with DNS).
In some cases, such as the recovery following a stolen private key
belonging to one of the domain's MTAs, the possibility of theft and
the effort required to revoke the key authorization must be
considered when choosing a TTL. The chosen TTL must be long enough
to mitigate denial-of-service attacks and provide reasonable
transaction efficiency, and no longer.

4.1.11. Compromise of Key Server

Rather than by attempting to obtain a private key, an attacker might
instead focus efforts on the server used to publish public keys for a
domain. As in the key theft case, the motive might be to allow the
attacker to sign messages on behalf of the domain. This attack
provides the attacker with the additional capability to remove
legitimate keys from publication, thereby denying the domain the
ability for the signatures on its mail to verify correctly.

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In order to limit the ability to sign a message to entities
authorized by the owner of a signing domain, a relationship must be
established between the signing address and the location from which a
public key is obtained to verify the message. DKIM does this by
publishing either the public key or a reference to it within the DNS
hierarchy of the signing domain. The verifier derives the location
from which to retrieve the public key from the signing address or
domain. The security of the verification process is therefore
dependent on the security of the DNS hierarchy for the signing
domain.

An attacker might successfully compromise the host that is the
primary key server for the signing domain, such as the domain's DNS
master server. Another approach might be to compromise a higher-
level DNS server and change the delegation of name servers for the
signing domain to others under the control of the attacker.

This attack can be mitigated somewhat by independent monitoring to
audit the key service. Such auditing of the key service should occur
by means of zone transfers rather than queries to the zone's primary
server, so that the addition of records to the zone can be detected.

4.1.12. Falsification of Key Service Replies

Replies from the key service may also be spoofed by a suitably
positioned attacker. For DNS, one such way to do this is "cache
poisoning", in which the attacker provides unnecessary (and
incorrect) additional information in DNS replies, which is cached.

DNSSEC [RFC4033] is the preferred means of mitigating this threat,
but the current uptake rate for DNSSEC is slow enough that one would
not like to create a dependency on its deployment. In the case of a
cache poisoning attack, the vulnerabilities created by this attack
are both localized and of limited duration, although records with
relatively long TTL may persist beyond the attack itself.

4.1.13. Publication of Malformed Key Records and/or Signatures

In this attack, the attacker publishes suitably crafted key records
or sends mail with intentionally malformed signatures, in an attempt
to confuse the verifier and perhaps disable verification altogether.
This attack is really a characteristic of an implementation
vulnerability, a buffer overflow or lack of bounds checking, for
example, rather than a vulnerability of the signature mechanism
itself. This threat is best mitigated by careful implementation and
creation of test suites that challenge the verification process.

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4.1.14. Cryptographic Weaknesses in Signature Generation

The cryptographic algorithms used to generate mail signatures,
specifically the hash algorithm and digital signature generation and
verification operations, may over time be subject to mathematical
techniques that degrade their security. At this writing, the SHA-1
hash algorithm is the subject of extensive mathematical analysis that
has considerably lowered the time required to create two messages
with the same hash value. This trend can be expected to continue.

One consequence of a weakness in the hash algorithm is a hash
collision attack. Hash collision attacks in message signing systems
involve the same person creating two different messages that have the
same hash value, where only one of the two messages would normally be
signed. The attack is based on the second message inheriting the
signature of the first. For DKIM, this means that a sender might
create a "good" message and a "bad" message, where some filter at the
signing party's site would sign the good message but not the bad
message. The attacker gets the good message signed, and then
incorporates that signature in the bad message. This scenario is not
common, but could happen, for example, at a site that does content
analysis on messages before signing them.

Current known attacks against SHA-1 make this attack extremely
difficult to mount, but as attacks improve and computing power
becomes more readily available, such an attack could become
achievable.

The message signature system must be designed to support multiple
signature and hash algorithms, and the signing domain must be able to
specify which algorithms it uses to sign messages. The choice of
algorithms must be published in key records, and not only in the
signature itself, to ensure that an attacker is not able to create
signatures using algorithms weaker than the domain wishes to permit.

Because the signer and verifier of email do not, in general,
communicate directly, negotiation of the algorithms used for signing
cannot occur. In other words, a signer has no way of knowing which
algorithm(s) a verifier supports or (due to mail forwarding) where
the verifier is. For this reason, it is expected that once message
signing is widely deployed, algorithm change will occur slowly, and
legacy algorithms will need to be supported for a considerable
period. Algorithms used for message signatures therefore need to be
secure against expected cryptographic developments several years into
the future.

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4.1.15. Display Name Abuse

Message signatures only relate to the address-specification portion
of an email address, while some MUAs only display (or some recipients
only pay attention to) the display name portion of the address. This
inconsistency leads to an attack where the attacker uses a From
header field such as:

From: "Dudley DoRight" <[email protected]>

In this example, the attacker, [email protected], can sign the
message and still convince some recipients that the message is from
Dudley DoRight, who is presumably a trusted individual. Coupled with
the use of a throw-away domain or email address, it may be difficult
to hold the attacker accountable for using another's display name.

This is an attack that must be dealt with in the recipient's MUA.
One approach is to require that the signer's address specification
(and not just the display name) be visible to the recipient.

4.1.16. Compromised System within Originator's Network

In many cases, MTAs may be configured to accept and sign messages
that originate within the topological boundaries of the originator's
network (i.e., within a firewall). The increasing use of compromised
systems to send email presents a problem for such policies, because
the attacker, using a compromised system as a proxy, can generate
signed mail at will.

Several approaches exist for mitigating this attack. The use of
authenticated submission, even within the network boundaries, can be
used to limit the addresses for which the attacker may obtain a
signature. It may also help locate the compromised system that is
the source of the messages more quickly. Content analysis of
outbound mail to identify undesirable and malicious content, as well
as monitoring of the volume of messages being sent by users, may also
prevent arbitrary messages from being signed and sent.

4.1.17. Verification Probe Attack

As noted above, bad actors (attackers) can sign messages on behalf of
domains they control. Since they may also control the key service
(e.g., the authoritative DNS name servers for the _domainkey
subdomain), it is possible for them to observe public key lookups,
and their source, when messages are verified.

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One such attack, which we will refer to as a "verification probe", is
to send a message with a DKIM signature to each of many addresses in
a mailing list. The messages need not contain valid signatures, and
each instance of the message would typically use a different
selector. The attacker could then monitor key service requests and
determine which selectors had been accessed, and correspondingly
which addressees used DKIM verification. This could be used to
target future mailings at recipients who do not use DKIM
verification, on the premise that these addressees are more likely to
act on the message contents.

4.1.18. Key Publication by Higher-Level Domain

In order to support the ability of a domain to sign for subdomains
under its administrative control, DKIM permits the domain of a
signature (d= tag) to be any higher-level domain than the signature's
address (i= or equivalent). However, since there is no mechanism for
determining common administrative control of a subdomain, it is
possible for a parent to publish keys that are valid for any domain
below them in the DNS hierarchy. In other words, mail from the
domain example.anytown.ny.us could be signed using keys published by
anytown.ny.us, ny.us, or us, in addition to the domain itself.

Operation of a domain always requires a trust relationship with
higher-level domains. Higher-level domains already have ultimate
power over their subdomains: they could change the name server
delegation for the domain or disenfranchise it entirely. So it is
unlikely that a higher-level domain would intentionally compromise a
subdomain in this manner. However, if higher-level domains send mail
on their own behalf, they may wish to publish keys at their own
level. Higher-level domains must employ special care in the
delegation of keys they publish to ensure that any of their
subdomains are not compromised by misuse of such keys.

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4.2. Attacks against Message Signing Practices

The following is a summary of postulated attacks against signing
practices:

+---------------------------------------------+--------+------------+
| Attack Name | Impact | Likelihood |
+---------------------------------------------+--------+------------+
| Look-alike domain names | High | High |
| Internationalized domain name abuse | High | High |
| Denial-of-service attack against signing | Medium | Medium |
| practices | | |
| Use of multiple From addresses | Low | Medium |
| Abuse of third-party signatures | Medium | High |
| Falsification of Sender Signing Practices | Medium | Medium |
| replies | | |
+---------------------------------------------+--------+------------+

4.2.1. Look-Alike Domain Names

Attackers may attempt to circumvent signing practices of a domain by
using a domain name that is close to, but not the same as, the domain
with signing practices. For instance, "example.com" might be
replaced by "examp1e.com". If the message is not to be signed, DKIM
does not require that the domain used actually exist (although other
mechanisms may make this a requirement). Services exist to monitor
domain registrations to identify potential domain name abuse, but
naturally do not identify the use of unregistered domain names.

A related attack is possible when the MUA does not render the domain
name in an easily recognizable format. If, for example, a Chinese
domain name is rendered in "punycode" as xn--cjsp26b3obxw7f.com, the
unfamiliarity of that representation may enable other domains to more
easily be mis-recognized as the expected domain.

Users that are unfamiliar with internet naming conventions may also
mis-recognize certain names. For example, users may confuse
online.example.com with online-example.com, the latter of which may
have been registered by an attacker.

4.2.2. Internationalized Domain Name Abuse

Internationalized domain names present a special case of the look-
alike domain name attack described above. Due to similarities in the
appearance of many Unicode characters, domains (particularly those
drawing characters from different groups) may be created that are
visually indistinguishable from other, possibly high-value domains.
This is discussed in detail in Unicode Technical Report 36 [UTR36].

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Surveillance of domain registration records may point out some of
these, but there are many such similarities. As in the look-alike
domain attack above, this technique may also be used to circumvent
sender signing practices of other domains.

4.2.3. Denial-of-Service Attack against Signing Practices

Just as the publication of public keys by a domain can be impacted by
an attacker, so can the publication of Sender Signing Practices (SSP)
by a domain. In the case of SSP, the transmission of large amounts
of unsigned mail purporting to come from the domain can result in a
heavy transaction load requesting the SSP record. More general DoS
attacks against the servers providing the SSP records are possible as
well. This is of particular concern since the default signing
practices are "we don't sign everything", which means that SSP
failures result in the verifier's failure to heed more stringent
signing practices.

As with defense against DoS attacks for key servers, the best defense
against this attack is to provide redundant servers, preferably on
geographically-separate parts of the Internet. Caching again helps a
great deal, and signing practices should rarely change, so TTL values
can be relatively large.

4.2.4. Use of Multiple From Addresses

Although this usage is never seen by most recipients, RFC 2822
[RFC2822] permits the From address to contain multiple address
specifications. The lookup of Sender Signing Practices is based on
the From address, so if addresses from multiple domains are in the
From address, the question arises which signing practices to use. A
rule (say, "use the first address") could be specified, but then an
attacker could put a throwaway address prior to that of a high-value
domain. It is also possible for SSP to look at all addresses, and
choose the most restrictive rule. This is an area in need of further
study.

4.2.5. Abuse of Third-Party Signatures

In a number of situations, including mailing lists, event
invitations, and "send this article to a friend" services, the DKIM
signature on a message may not come from the originating address
domain. For this reason, "third-party" signatures, those attached by
the mailing list, invitation service, or news service, frequently
need to be regarded as having some validity. Since this effectively
makes it possible for any domain to sign any message, a sending

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domain may publish sender signing practices stating that it does not
use such services, and accordingly that verifiers should view such
signatures with suspicion.

However, the restrictions placed on a domain by publishing "no
third-party" signing practices effectively disallows many existing
uses of email. For the majority of domains that are unable to adopt
these practices, an attacker may with some degree of success sign
messages purporting to come from the domain. For this reason,
accreditation and reputation services, as well as locally-maintained
whitelists and blacklists, will need to play a significant role in
evaluating messages that have been signed by third parties.

4.2.6. Falsification of Sender Signing Practices Replies

In an analogous manner to the falsification of key service replies
described in Section 4.1.12, replies to sender signing practices
queries can also be falsified. One such attack would be to weaken
the signing practices to make unsigned messages allegedly from a
given domain appear less suspicious. Another attack on a victim
domain that is not signing messages could attempt to make the
domain's messages look more suspicious, in order to interfere with
the victim's ability to send mail.

As with the falsification of key service replies, DNSSEC is the
preferred means of mitigating this attack. Even in the absence of
DNSSEC, vulnerabilities due to cache poisoning are localized.

4.3. Other Attacks

This section describes attacks against other Internet infrastructure
that are enabled by deployment of DKIM. A summary of these
postulated attacks is as follows:

+--------------------------------------+--------+------------+
| Attack Name | Impact | Likelihood |
+--------------------------------------+--------+------------+
| Packet amplification attacks via DNS | N/A | Medium |
+--------------------------------------+--------+------------+

4.3.1. Packet Amplification Attacks via DNS

Recently, there has been an increase in denial-of-service attacks
involving the transmission of spoofed UDP DNS requests to openly-
accessible domain name servers [US-CERT-DNS]. To the extent that the
response from the name server is larger than the request, the name
server functions as an amplifier for such an attack.

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DKIM contributes indirectly to this attack by requiring the
publication of fairly large DNS records for distributing public keys.
The names of these records are also well known, since the record
names can be determined by examining properly-signed messages. This
attack does not have an impact on DKIM itself. DKIM, however, is not
the only application that uses large DNS records, and a DNS-based
solution to this problem will likely be required.

5. Derived Requirements

This section lists requirements for DKIM not explicitly stated in the
above discussion. These requirements include:

The store for key and SSP records must be capable of utilizing
multiple geographically-dispersed servers.

Key and SSP records must be cacheable, either by the verifier
requesting them or by other infrastructure.

The cache time-to-live for key records must be specifiable on a
per-record basis.

The signature algorithm identifier in the message must be one of
the ones listed in a key record for the identified domain.

The algorithm(s) used for message signatures need to be secure
against expected cryptographic developments several years in the
future.

6. Security Considerations

This document describes the security threat environment in which
DomainKeys Identified Mail (DKIM) is expected to provide some
benefit, and it presents a number of attacks relevant to its
deployment.

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7. Informative References

[Bernstein04] Bernstein, D., "Cache Timing Attacks on AES",
April 2004.

[Boneh03] Boneh, D. and D. Brumley, "Remote Timing Attacks are
Practical", Proc. 12th USENIX Security Symposium,
2003.

[DKIM-BASE] Allman, E., "DomainKeys Identified Mail (DKIM)
Signatures", Work in Progress, August 2006.

[DKIM-SSP] Allman, E., "DKIM Sender Signing Practices", Work in
Progress, August 2006.

[Kocher96] Kocher, P., "Timing Attacks on Implementations of
Diffie-Hellman, RSA, and other Cryptosystems",
Advances in Cryptology, pages 104-113, 1996.

[Kocher99] Kocher, P., Joffe, J., and B. Yun, "Differential Power
Analysis: Leaking Secrets", Crypto '99, pages 388-397,
1999.

[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version
3", STD 53, RFC 1939, May 1996.

[RFC2821] Klensin, J., "Simple Mail Transfer Protocol",
RFC 2821, April 2001.

[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and
S. Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.

[US-CERT-DNS] US-CERT, "The Continuing Denial of Service Threat
Posed by DNS Recursion".

[UTR36] Davis, M. and M. Suignard, "Unicode Technical Report
#36: Unicode Security Considerations", UTR 36,
July 2005.

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Appendix A. Acknowledgements

The author wishes to thank Phillip Hallam-Baker, Eliot Lear, Tony
Finch, Dave Crocker, Barry Leiba, Arvel Hathcock, Eric Allman, Jon
Callas, Stephen Farrell, Doug Otis, Frank Ellermann, Eric Rescorla,
Paul Hoffman, Hector Santos, and numerous others on the ietf-dkim
mailing list for valuable suggestions and constructive criticism of
earlier versions of this document.

Author's Address

Jim Fenton
Cisco Systems, Inc.
MS SJ-9/2
170 W. Tasman Drive
San Jose, CA 95134-1706
USA

Phone: +1 408 526 5914
EMail: [email protected]

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Full Copyright Statement

Copyright (C) The Internet Society (2006).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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