pre-workgroup J. Fenton
Internet-Draft Cisco Systems, Inc.
Expires: June 22, 2006 December 19, 2005
Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)
draft-fenton-dkim-threats-02
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Copyright Notice
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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology and Model . . . . . . . . . . . . . . . . . . 4
2. The Bad Actors . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Characteristics . . . . . . . . . . . . . . . . . . . . . 5
2.2. Capabilities . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Location . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.1. Externally-located Bad Actors . . . . . . . . . . . . 7
2.3.2. Within Claimed Originator's Administrative Unit . . . 8
2.3.3. Within Recipient's Administrative Unit . . . . . . . . 8
3. Representative Bad Acts . . . . . . . . . . . . . . . . . . . 9
3.1. Use of Arbitrary Identities . . . . . . . . . . . . . . . 9
3.2. Use of Specific Identities . . . . . . . . . . . . . . . . 9
3.2.1. Exploitation of Social Relationships . . . . . . . . . 10
3.2.2. Identity-Related Fraud . . . . . . . . . . . . . . . . 10
3.2.3. Reputation Attacks . . . . . . . . . . . . . . . . . . 10
4. Attacks on Message Signing . . . . . . . . . . . . . . . . . . 11
4.1. Attacks Against Message Signatures . . . . . . . . . . . . 12
4.1.1. Theft of Private Key for Domain . . . . . . . . . . . 12
4.1.2. Theft of Delegated Private Key . . . . . . . . . . . . 13
4.1.3. Private Key Recovery via Timing Attack . . . . . . . . 13
4.1.4. Chosen Message Replay . . . . . . . . . . . . . . . . 13
4.1.5. Signed Message Replay . . . . . . . . . . . . . . . . 14
4.1.6. Denial-of-Service Attack Against Verifier . . . . . . 15
4.1.7. Denial-of-Service Attack Against Key Service . . . . . 15
4.1.8. Canonicalization Abuse . . . . . . . . . . . . . . . . 15
4.1.9. Body Length Limit Abuse . . . . . . . . . . . . . . . 16
4.1.10. Use of Revoked Key . . . . . . . . . . . . . . . . . . 16
4.1.11. Compromise of Key Server . . . . . . . . . . . . . . . 17
4.1.12. Falsification of Key Service Replies . . . . . . . . . 17
4.1.13. Publication of Malformed Key Records and/or
Signatures . . . . . . . . . . . . . . . . . . . . . . 17
4.1.14. Cryptographic Weaknesses in Signature Generation . . . 18
4.1.15. Display Name Abuse . . . . . . . . . . . . . . . . . . 18
4.1.16. Compromised System Within Originator's Network . . . . 19
4.2. Attacks Against Message Signing Policy . . . . . . . . . . 19
4.2.1. Look-Alike Domain Names . . . . . . . . . . . . . . . 19
4.2.2. Internationalized Domain Name Abuse . . . . . . . . . 19
4.2.3. Denial-of-Service Attack Against Signing Policy . . . 20
4.2.4. Use of Multiple From Addresses . . . . . . . . . . . . 20
5. Derived Requirements . . . . . . . . . . . . . . . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. Informative References . . . . . . . . . . . . . . . . . . . . 21
Appendix A. Glossary . . . . . . . . . . . . . . . . . . . . . . 22
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 22
Appendix C. Edit History . . . . . . . . . . . . . . . . . . . . 22
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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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.
1.1. Terminology and Model
The following diagram illustrates a typical usage flowchart for DKIM:
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+---------------------------------+
| SIGNATURE CREATION |
| (Originating or Relaying ADMD) |
| |
| Sign (Message, Domain, Key) |
| |
+---------------------------------+
| - Message (Domain, Key)
|
[Internet]
|
V
+---------------------------------+
+-----------+ | SIGNATURE VERIFICATION |
| | | (Relaying or Delivering ADMD) |
| KEY | | |
| QUERY +...>| Verify (Message, Domain, Key) |
| | | |
+-----------+ +----------------+----------------+
| - Verified Domain
+-----------+ V - [Report]
| | +----------------+----------------+
| SIGNER | | |
| PRACTICES +...>| SIGNER EVALUATION |
| QUERY | | |
| | +---------------------------------+
+-----------+
Definitions of some terms used in this document may be found in
Appendix A.
Placeholder for some discussion of 2821 vs. 2822 solutions, etc.
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, 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.
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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
employ all of the above mechanisms and additionally may attack the
Internet infrastructure itself, e.g., DNS cache-poisoning attacks; IP
routing attacks via compromised network routing elements.
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
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
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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 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.
2.3. Location
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.
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 in multiple locations
simultaneously (a "distributed bad actor").
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
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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 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.
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 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.
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 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
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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.
3. 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.
3.1. 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.
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 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
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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.
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
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.
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 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.
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.
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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 (often referred
to as Sender Signing Policy, Sender Signing Practices or SSP) can be
expected to be the target of attacks.
The sections below begin with a table summarizing the postulated
attacks in each category along with their expected impact and
likelihood. The following criteria were used in scoring the attacks
against these criteria:
Impact:
High: Affects the verification of messages by an entire domain or
multiple domains
Medium: Affects the verification of messages by specific users, MTAs,
and/or bounded time periods
Low: Affects the verification of isolated individual messages only
Likelihood:
High: All users of DKIM should expect this attack on a frequent basis
Medium: Users of DKIM 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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4.1. Attacks Against Message Signatures
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 timing 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 | High | Medium |
| service | | |
| 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 | Medium | Medium |
| network | | |
+---------------------------------------------+--------+------------+
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, including 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 which are valid for all addresses in a domain typically reside
in MTAs which 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.
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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.
4.1.3. Private Key Recovery via Timing Attack
Timing attacks are a technique whereby the private key is recovered
by observing the time required to sign a series of messages. It
requires both the ability to submit messages for signing as well as
the ability to accurately measure the time required to compute the
signature.
In most cases, an MTA has are enough variables (system load, clock
resolution, queuing delays, etc.) to prevent the signing time from
being measured accurately enough to be useful for a timing attack.
Furthermore, while some domains, e.g., consumer ISPs, would allow an
attacker to submit messages for signature, with many other domains
this is difficult. Other mechanisms, such as mailing lists hosted by
the domain, might be paths by which an attacker might submit messages
for signature, and should also be considered as possible vectors for
timing attacks.
4.1.4. Chosen Message Replay
Chosen Message Replay (CMR) 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 him/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.
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 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
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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.
Revocation of the signature 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 presents a problem
revoking some signatures and not others. To this end, "revocation
identifiers" have been proposed which would permit more fine-grained
revocation, 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 the
revocation database.
4.1.5. Signed Message Replay
Signed Message Replay (SMR) refers to the retransmission of already-
signed messages to additional recipients beyond those intended by the
sender. 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 originator's
reputation.
A larger number of domains are potential victims of SMR than of CMR,
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 which do not modify the content
of the message and signed headers and hence do not invalidate the
signature, engage in a form of SMR. The only things that distinguish
this case from undesirable forms of SMR is the intent of the
replayer, which cannot be determined by the network.
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4.1.6. Denial-of-Service Attack Against Verifier
While it takes some compute resources to sign and verify a signature,
it takes negligible compute 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.
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 which 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 which 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 which would have
permitted attackers, in some cases, to alter the meaning of a
message.
Message signatures which 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
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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 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 which add a
trailer, perhaps identifying the list, at the end of messages.
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. Recipients need to use means to, at a minimum,
identify 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 which 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 for keys not under the domain
administrator's direct control (assuming, of course, that control of
the time-to-live 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
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theft and the time 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.
The host which is the primary key server, such as a DNS master server
for the domain, might be compromised. Another approach might be to
change the delegation of key servers at the next higher domain level.
This attack can be mitigated somewhat by independent monitoring to
audit the key service. However, it may be difficult to detect the
publication of additional keys by such means until the selector(s)
added by the attackers are known.
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. Fortunately, the
vulnerabilities created by this attack are both localized and of
limited duration, although records with relatively long TTL may be
created with cache poisoning.
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 the public-key encryption/
decryption 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
which has considerably lowered the time required to create two
messages with the same hash value. This trend can be expected to
continue.
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, rather than in the
signature itself, to ensure that an attacker is not able to create
signatures using algorithms weaker than the domain wishes to permit.
Due to the fact that 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, nor (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.
4.1.15. Display Name Abuse
Message signatures only relate to the address-specification portion
of an email address, which 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 an From
header field such as:
From: "Dudley DoRight" <whiplash@example.org>
In this example, the attacker, whiplash@example.org, 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 bring the attacker to account for the use of another's display
name.
This is an attack which must be dealt with in the recipient's MUA.
One approach is to require that the signer's address specification
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(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
which 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.
4.2. Attacks Against Message Signing Policy
Summary of postulated attacks against signing policy:
+---------------------------------------------+--------+------------+
| Attack Name | Impact | Likelihood |
+---------------------------------------------+--------+------------+
| Look-alike domain names | High | High |
| Internationalized domain name abuse | High | Medium |
| Denial-of-service attack against signing | Medium | Medium |
| policy | | |
| Use of multiple From addresses | Low | Medium |
+---------------------------------------------+--------+------------+
4.2.1. Look-Alike Domain Names
Attackers may attempt to circumvent signing policy of a domain by
using a domain name which is close to, but not the same as the domain
with a signing policy. 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.
4.2.2. Internationalized Domain Name Abuse
Internationalized domain names present a special case of the look-
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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 which are
visually indistinguishable from other, possibly high-value domains.
This is discussed in detail in Unicode TR 36 [UTR36]. 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
policy of other domains.
4.2.3. Denial-of-Service Attack Against Signing Policy
Just as the publication of public keys by a domain can be impacted by
an attacker, so can the publication of Sender Signing Policy (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 policy
is "we don't sign everything", which means that SSP, in effect, fails
open.
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 policy should rarely change, so TTL values
can be relatively large.
4.2.4. Use of Multiple From Addresses
Although this usage is rare, RFC 2822 [RFC2822] permits the From
address to contain multiple address specifications. The lookup of
Sender Signing Policy is based on the From address, so if addresses
from multiple domains are in the From address, the question arises
which signing policy 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.
5. Derived Requirements
This section, as yet incomplete, is an attempt to capture a set of
requirements for DKIM from the above discussion. These requirements
include:
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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 algorithm(s) used by the signing domain associated with a
given key must be specified independently of the signature itself.
6. IANA Considerations
This document defines no items requiring IANA assignment.
7. Security Considerations
This document describes the security threat environment in which
DomainKeys Identified Mail (DKIM) is expected to provide some
benefit, and presents a number of attacks relevant to its deployment.
8. Informative References
[I-D.allman-dkim-base]
Allman, E., "DomainKeys Identified Mail (DKIM)",
draft-allman-dkim-base-01 (work in progress),
October 2005.
[I-D.allman-dkim-ssp]
Allman, E., "DKIM Sender Signing Policy",
draft-allman-dkim-ssp-01 (work in progress), October 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.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
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[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[UTR36] Davis, M. and M. Suignard, "Unicode Security
Considerations", UTR 36, July 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.
More definitions to be added.
Appendix B. Acknowledgements
The author wishes to thank Phillip Hallam-Baker, Eliot Lear, Tony
Finch, Dave Crocker, Barry Leiba, Arvel Hathcock, Eric Allman, Jon
Callas, and Stephen Farrell 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.
Changes since -01 draft:
o Reorganized description of bad actors.
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o Greatly expanded description of attacks against DKIM and SSP.
o Added "derived requirements" section.
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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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