Individual D. Otis
Internet-Draft D. Rand
Intended status: Informational Trend Micro
Expires: December 6, 2013 June 04, 2013
DKIM is Harmful as Specified
draft-otis-dkim-harmful-02
Abstract
Currently, email lacks conventions ensuring SMTP clients can be
identified by an authenticated domain. Unfortunately many hope to
use DKIM as an alternative, but it is independent of intended
recipients and domains accountable for sending the message. This
means DKIM is poorly suited for establishing abuse assessments for
unsolicited messaging of commercial email otherwise known as SPAM,
nor was this initially DKIM's intent. DKIM lacks message context
essential to ensure fair assessment and to ensure this assessment is
not poisoned (Who initiated the transaction and to whom).
DKIM was instead intended to establish increased levels of trust
based upon valid DKIM signatures controlling acceptance and what a
user sees within the FROM header field. But DKIM failed to guard
against pre-pended header fields where any acceptance based on valid
DKIM signatures is sure to exclude header field spoofing, especially
that of the FROM. This weakness allows malefactors to exploit DKIM
signature acceptance established by high-volume DKIM domains to spoof
ANY other domain, even when prohibited within the Signer's network.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 6, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Exploiting Trust . . . . . . . . . . . . . . . . . . . . . . . 5
3. Maintaining Trust . . . . . . . . . . . . . . . . . . . . . . 6
4. Responding to Defects and Exploitation . . . . . . . . . . . . 6
5. Conflating DKIM Fragments with Email Messages . . . . . . . . 7
6. SMTP Can't . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. DKIM Vulnerability . . . . . . . . . . . . . . . . . . . . . . 9
8. Barriers to an Authenticated Domain . . . . . . . . . . . . . 10
9. Domains as a Basis for Managing Traffic . . . . . . . . . . . 10
10. XMPP Shows the Way Forward . . . . . . . . . . . . . . . . . . 11
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
12. Security Considerations . . . . . . . . . . . . . . . . . . . 11
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
14. References - Informative . . . . . . . . . . . . . . . . . . . 12
Appendix A. DKIM Examples . . . . . . . . . . . . . . . . . . . . 14
Appendix B. Stats . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
Currently, IPv4 address reputation provides the primary basis for
defending open SMTP services (acceptance without prior arrangement).
Use of IP addresses in this role completely breaks down when dealing
with IPv6 [RFC2460] due to data requirements and an inability to
defend detection of subscription violations. There are currently
18,209,237,415,985,168 /64 equivalent IPv6 prefixes routed.
[v6-BGP-Rpts]. In comparison, for IPv4 there are 2,614,654,096 IP
addresses routed. While IPv4 is reaching its maximum, IPv6 has about
0.1% of the available /64 prefix routed and this continues to grow
rapidly. Unlike IPv4, there is no practical means to scan reverse
DNS namespace within IPv6 since each /64 prefix may contain any
number of PTR records ranging up to 184,000,000,000,000,000,000.
A technique commonly employed to automate IPv4 address categorization
of suitable hosts is to check whether reverse PTR records appear to
represent valid hostnames. Those that represent 4 decimal numbers
are often considered unacceptable, for example. Our processing of
reverse DNS namespace in cooperation with network providers now
excludes about 38%, or about 1,000,000,000 IPv4 addresses. Comparing
IPv6 /64 prefixes with the remainder of routable IPv4 addresses shows
there are 11.3 million times more IPv6 /64 prefixes needing
categorization. In addition, there is no practical means to
facilitate this effort.
IP address reputation requires logging associated connections to
permit review. Whether describing reputations as only positive or
only negative, errant exclusion or inclusion of either poses similar
risk. Tracking currently routed IPv6 /64 prefixes using a single bit
requires 6 million billion bytes or 5,650 Terra-bytes just to track
simple use. Even feedback by IPv6 address prefix will expose
mailboxes that detect subscription policy violations.
Some also suggest there will not be a significant increase in the
number of servers running over IPv6 and since their overall number
should be comparable, email should still be dealing with a similar
number of IP addresses. Unlike IPv4, IPv6 does not constrain the
number of IP addresses assigned to a network interface. This feature
allows each connection from a server to originate from a different IP
address, perhaps one for each user. The potential increase allowed
by IPv6 may prove explosive, even those only from good actors.
Organizations such as M3AAWG hope to use DKIM as a required
acceptance requirement to better ensure a domain identity offers a
basis for acceptance to replace that of the IP address used by SMTP
clients. Due to the understandable IPv6 reputation services'
inability to scale, domain based alternatives are being sought. Some
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at least understand DKIM is unable to support negative reputation
schemes. Reliance on a mechanism unable to sustain close scrutiny of
negative assertions makes sustained differentiation of positive and
negative views less tenable.
At this April's Rocky Mountain IPv6 Task Force summit in Denver, the
second day government track sessions raised concerns about there
being no means available to defend SMTP over IPv6. There are
proposals within the IETF aimed at establishing DKIM as a basis for
reputation schemes in the Repute WG (i.e. section 3.2 of
[I-D.ietf-repute-email-identifiers] which introduces DKIM domains
being used along with SMTP client IP addresses and rfc5321.helo also
identifying the SMTP client. Identifying the SMTP client encompasses
both "Who Initiated" and "To Whom" message elements to support fair
negative assertions. However, DKIM does not encompass this essential
information. In addition, DKIM's inability to detect invalid
prefixed header fields also means any positive DKIM reputation
assertion can prove highly harmful by increasing trust in possible
deceptions.
2. Exploiting Trust
Trust established by a signing domain is being exploited to mislead
recipients about who authored a message. DKIM's trust related
function may be generalized as better ensuring delivery to in-boxes
as opposed to junk folders or silent discard. It is also apparent
receivers expect signature validation ensures invalid header fields
have not been pre-fixed. While it is possible for signing domains to
support this expectation by including non-existent header fields in a
list of those added to the signature's hash, few implement this
feature.
Perhaps signers consider such double listing wasteful of storage
resources or they assume the validation process makes these checks
without the non-intuitive double listing of header fields not
permitted to repeat anyway. When a domain is very large, errant
filtering is likely to entail costly customer support which affords
them greater latitude and who are also likely sensitive to wasting
their storage resources.
Regardless of possible underlying motivations, it is clear checks for
valid message structure remains a general expectation of DKIM's
validation process that simply does not occur. Not every domain is
seeking to establish the same level of trust, where those not
checking for pre-fixed header fields having greater latitude place
all other domains at risk. Checking message structure is explicitly
not to be handled by the transport. Modification to SMTP
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implementations such as Sendmail, Exim, or Postfix and the like are
neither appropriate, nor likely beneficial within a relevant time
frame.
Larger domains often obtain their size by offering relatively easy
access. These domains afford malefactors a simple method to have
their deceptive messages reach their victim's in-box. DKIM's
validation process not explicitly ensuring against invalid repeated
header fields together with of limited hash inclusion permitting
abuse of other domains makes DKIM harmful. DKIM validation MUST be
modified to ensure against invalid repeated header fields so that
trust established by a signing domain is not exploited to mislead
recipients about who authored a message.
3. Maintaining Trust
Not every subsystem or protocol layer should be expected to repeat
previous security checks to establish proper layering, however
critical checks important for enforcing new relationships within a
message should not be assumed, especially those involving a trivial
effort. With high levels of abuse resulting from email's open
nature, delegating checks in a structured manner better conserves
essential resources. However, email's highly distributed store and
forward protocol could not function if rigid message structures were
enforced by the transport. Such enforcement does not scale, and may
impede necessary change. New authentication or presentation
requirements may involve small structural adjustments. For example,
internationalization introduced a format negotiation not assured to
survive beyond the next hop.
4. Responding to Defects and Exploitation
As with aviation, the success of email has risen to great heights.
As within the world of aviation, faults threatening security, that
when discovered, demand our attention and diligence to effect repair.
Email has become an integral component in general commerce and the
maintenance of security such as reporting system failures, break-in
attempts, and facilitating account access recovery.
Reporting or predicting failure should not be viewed as exhibiting a
lack of respect for achieved accomplishments. Noting and repairing
faults only signify the importance of email's prominent role. As
with most security related protocols, responding to noted defects is
fairly common. Not responding to discovered defects in a security
related protocol would be shocking. Simply publishing this draft
appears to have already increase the level of multiple FROM header
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field abuse seen where it is now at 21% of signed DKIM messages.
5. Conflating DKIM Fragments with Email Messages
DKIM signs only fragments of an email message, so it is more proper
to refer to "DKIM Signed Fragments", and not "DKIM Signed Messages".
Normal DKIM signature validation offers a simple PASS/FAIL
associating it with a specific domain. When a recipient receives a
PASS status, only the last FROM header field message fragment is
ensured to have been included in the DKIM signature process. Other
message fragments, including the message body, are optional and may
not have been included. The FROM header field is normally visible
UNLESS there are multiple FROM header fields. In which case, the
signed FROM header field fragment is likely invisible, as is the DKIM
signature fragments that hide which other message fragments had been
encompassed by the DKIM signature process.
DKIM's trust related role is to better ensure message delivery to a
user's in-box. Unless DKIM ensures this trust is not used to
perpetrate deception, no positive assertions regarding a DKIM domain
is safe. As a result, DKIM can not be used with either positive or
negative reputation assertions in its current form.
The FROM header field is the Author identifier in section 11.1 of
[I-D.kucherawy-dmarc-base]. The DMARC specification offers normative
language that a message SHOULD be rejected when multiple FROM header
fields are detected. This requirement would not be necessary or
impose protocol layer violations if DKIM did not offer valid
signature results when repeated header fields violate [RFC5322].
[RFC5322] declaring a message structure invalid does not preclude the
occurrence of invalid messages, and [RFC5321] clearly states it will
not enforce [RFC5322] message structure due to practical constraints.
Instead of relying on optional policies such as DMARC that make
partial guesses that ignore DATE or SUBJECT spoofing for example,
critical violations of the message structure that pertain to
enhancing trust can be enforced by DKIM simply defining the
associated signatures invalid. Unlike DMARC, proper signature
definition does not cross protocol layers, especially since no other
layer enforces [RFC5322].
Since multiple DKIM signatures can occur, simple annotation of which
fragments and domains associate with a valid signature is precluded.
The ONLY message fragment ensured by a DKIM signature is the FROM
header field. Just as DMARC concluded only the FROM header field is
closely observed by recipients, DKIM initially reached this
conclusion as well. While no absolute assurance of header field
validity is asserted, the domain together with it's reputation
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permits recipients to increase their trust in what is observed in the
FROM header field. This trust further increases when the DKIM domain
is authoritative for the FROM header field domain.
When acceptance is predicated on the DKIM signature, as occurs with
DMARC, preserving trust associated with the FROM header field in
conjunction with the DKIM domain is destroyed whenever multiple FROM
header fields are permitted by not invalidating these DKIM
signatures. DMARC over reaches when rejecting email based upon
message format as with [RFC6854] group syntax in the FROM header
field. DMARC should not be required to second guess whether a DKIM
signature is valid.
6. SMTP Can't
SMTP [RFC5321] recommends against rejecting messages based upon
perceived defects in the message structure. This liberal acceptance
permits evolutionary change in message specifications starting at
[RFC0822] that was based on [RFC0733] replaced by [RFC2822] and again
by [RFC5322], [RFC6152], [RFC6532], and [RFC6854]; the second to last
paragraph in section 3 of [RFC5321] provides a definitive statement
messages should not be rejected due to perceived defects in the
[RFC0822] message structure. The initial reference to [RFC0822] in
this paragraph offers two foot notes with the second referencing the
latest version of [RFC0822] which is [RFC5322] which itself has
recently been updated. The impact of initially removing text
specifically indicating which header fields are not to repeat is
unknown. This information was implied within the then-new ABNF
notation. Clarifying text for this requirement did not return until
the [RFC0822] revision 19 years later which also indicates this
specification's success at providing a foundation that allowed email
to flourish.
There are many SMTP servers that have been in operation for decades
with years passing between security patches. Such an accomplishment
is most remarkable considering the volume of traffic being handled,
often from highly malicious sources. This amazing stability and
scalability with high levels of security would not have been possible
if SMTP had been expected to validate message formats.
Expecting SMTP to validate message formats to protect against
vulnerabilities pertaining to protocols such as DKIM does not scale.
The general use of DKIM permits signature checks subsequent to
acceptance where only the status of signatures determines internal
placement. As such, it becomes critical to ensure a DKIM signature
is never declared valid having malformed header field stacks. To
accomplish this, the DKIM specification must change.
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7. DKIM Vulnerability
DKIM permits a vulnerability by not checking the message header field
stack for invalid repeats when signing or verifying a signature. The
DKIM signature process must walk both down and then up the header
field stack while selecting the header fields to be included in the
hash process of the signature. The DKIM process will even ignore
prefixed FROM header fields which is the only header field always
included.
The WG concluded that "listing non-existent header fields as signed"
hack added in non-normative language together with opinions that
checking for invalid repeated header fields is not to be considered
DKIM's problem. See section 8.15 of [RFC6376] where this issue was
expressed as not an attack against the trust DKIM intends to convey,
and thus not a concern for DKIM. Nevertheless, improperly formed
messages may display only the first of multiple header fields that,
as a result of erroneous assumptions of there being no invalid
repeated header fields, the prefixed header fields are likely to be
displayed in lieu of those signed while not impacting DKIM's
signature validity.
DKIM incorrectly assumed the header field stack's starting condition,
which DKIM itself is best able to determine, and is an option in the
OpenDKIM implementation. This is likely to astonish most recipients
that DKIM failed to make a robust effort to maintain the trust it is
attempting to convey. Three members of the WG authored proposed
changes aimed specifically at addressing this issue [DKIM-MH-Attack].
At the time, some expressed concerns about whether this might set
back DKIM's standardization process. As such, DKIM Signers may sign
malformed messages (e.g., violate [RFC5322]) and be in compliance
with DKIM specifications. In addition, receivers may verify these
messages as having valid signatures despite multiple instances of a
header field only permitted to occur once and also be in compliance
with DKIM specifications. See addendum for examples of the possible
abuse permitted.
Use of DKIM on such messages exposes a vulnerability in the
evaluation process. Rather than ensuring essential checks are made
prior to producing a result, a wasteful hack was later suggested
where extra non-existent header fields could be included in the list
of signed header fields. Any pre-pended header field added after
signing would thereby change resulting hashes and invalidate the
signature. Not all domains are attempting to achieve the same level
of trust and may be more sensitive to incurring incremental storage
requirements. Some domains may even inadvertently sign invalid
repeated header fields because this check had not been required in
the DKIM process. These same DKIM domains are also likely to
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establish themselves as being Too Big To Block. These TBTB domains
can then be used to spoof other domains that may have otherwise
established a high level of trust by implementing the hack where, due
to this defect in DKIM, can still do nothing in their defense from
the perspective of now deceived recipients.
This vulnerability in DKIM represents an exploit allowing serious
attacks caused by erroneous assumptions made in DKIM's signature
process. There is also a header field, which because of its label,
may potentially mislead recipients into believing it contains valid
"Authentication-Results" [RFC5451]. Common phrases such as
"Authentication-Results", "pass", and "fail", rather than use of
result codes belies introductory claims this header is not intended
for direct human consumption.
8. Barriers to an Authenticated Domain
Some advocate use of DKIM as a means to obtain domain references
based on the increased prevalence of this protocol. DKIM is
independent of the domain actually sending the message and the
recipient by design. Unfortunately, DKIM also does not attempt to
protect against likely abuses that are also beyond the control of the
signing domain in which DKIM signature validity conveys no assurance
pre-fixed header fields have not changed what recipients see. As
such, DKIM signing domains can not be held accountable for incidents
of abuse appearing to violate subscription policies or that spoof
other domains.
Because of DKIM's vulnerability to header field spoofing, it would
not be safe to express positive reputations either. Any such
assurance could be exploited by malefactors to deceive those trusting
DKIM results. In short, a DKIM signed domain as currently defined,
can not be safely used in any context, other than the most rigid
exclusion of any unsigned content which is well beyond any existing
implementation. DKIM can not be safely used for email reputation as
currently defined.
9. Domains as a Basis for Managing Traffic
A manageable basis for assessments can leverage a smaller number of
related domains, compared to IPv6 or even IPv4 addresses. Although
technically the domain name space can be larger than the massively
large IPv6 address space, in practice it is not. One hundred
thousand domains control 90% of Internet traffic out of approximately
100 million domains active each month. The top 150 domains control
50% of the traffic, and the top 2,500 domains control 75%. This
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level of domain consolidation permits effective fast-path white-
listing. Improvements achieved using domains to consolidate the
threat landscape can easily justify added cryptographic
authentication burdens. Even APL resource records [RFC3123] can
authenticate EHLO using a single DNS transaction, but this would not
allow IPv6 email to be more easily managed when facing extensive use
of transitional technologies such as ISATAP, Teredo, 6to4, NAT64, and
DNS64, which cryptographic technology can offer.
10. XMPP Shows the Way Forward
In addition to SMTP [RFC5321] using StartTLS [RFC3207] XMPP [RFC6122]
uses StartTLS [RFC6120] over a different port with many of the
features used by web servers such as [RFC2560] as one means to
increase scalability. It seems plausible that by defining SMTP
access over a different port is where a new authentication and
international requirements can be resolved together. Of course, port
25 can be used where it might require StartTLS in the case of IPv6
connections.
Many administrators overlook a serious problem made much worse by
chatty protocols that impose processing delays. Examining server
logs will not reveal any problem either, because the limited resource
being consumed is the number of outstanding connections TCP is able
to support. Reaching this limit will prevent new connections from
being instantiated but this is not logged as an event. Over time
administrators may hear complaints email is not being delivered or
just see an ever growing percentage of spam.
11. IANA Considerations
This document requires no IANA consideration.
12. Security Considerations
This draft intends to describe serious security concerns raised with
use of DKIM that is exacerbated with IPv6 email. The contained
recommendations are expected to reduce these security concerns. To
better ensure security, the DKIM specification must change.
Recommendations [DKIM-MH-Attack] rejected by the DKIM WG were aimed
at repairing this defect by simply requiring the definition for a
valid DKIM signature to ensure no invalid repeated header fields are
present. It is also clear that the non-normative language describing
the non-intuitive approach of listing non-existent header fields has
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not been widely embraced, especially by the domains sensitive to
storage requirements. The overall storage requirement was one of
weighing factors in selecting between IIM and DKIM. IIM's inclusion
of the public key within the message was considered an unnecessary
waste of storage. It seems many also consider the prophylactic
listing of non-existent header fields an unnecessary waste as well.
Based upon the current data, the present DKIM specification did not
result in something that can retain trust, and that ultimately leads
to even greater protocol layer violations.
13. Acknowledgements
The authors wish to acknowledge valuable contributions from the
following: Dave Crocker.
14. References - Informative
[DKIM-MH-Attack]
http://trac.tools.ietf.org/wg/dkim/trac/ticket/24,
"Multiple-header-attack alternative proposal", April 2011.
[I-D.ietf-repute-email-identifiers]
Borenstein, N. and M. Kucherawy, "A Reputation Response
Set for Email Identifiers",
draft-ietf-repute-email-identifiers-07 (work in progress),
May 2013.
[I-D.kucherawy-dmarc-base]
Kucherawy, M., "Domain-based Message Authentication,
Reporting and Conformance (DMARC)",
draft-kucherawy-dmarc-base-00 (work in progress),
March 2013.
[RFC0733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
"Standard for the format of ARPA network text messages",
RFC 733, November 1977.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3123] Koch, P., "A DNS RR Type for Lists of Address Prefixes
(APL RR)", RFC 3123, June 2001.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6122] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Address Format", RFC 6122, March 2011.
[RFC6152] Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
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Service Extension for 8-bit MIME Transport", STD 71,
RFC 6152, March 2011.
[RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", RFC 6376,
September 2011.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012.
[RFC6854] Leiba, B., "Update to Internet Message Format to Allow
Group Syntax in the "From:" and "Sender:" Header Fields",
RFC 6854, March 2013.
[v6-BGP-Rpts]
http://bgp.potaroo.net/v6/as6447/, "BGP Routing Table
Analysis Reports/IPv6/AS6447 views", May 2013.
Appendix A. DKIM Examples
From Random User Tue Mar 12 12:07:37 2013
X-Apparently-To: just4spamdlr@yahoo.com via 72.30.237.8; Tue, 12 Mar 2013 12:08:37 -0700
Return-Path: <Fake.user@gmail.com>
Received-SPF: neutral (192.83.249.65 is neither permitted nor denied by domain of gmail.com)
A3RleHQvcGxhaW4DAzACA3RleHQvaHRtbAMDMQ--
X-YMailISG: Po8J_9cWLDuz5QIo_tChc7OagZYPBIscsK7APx8FMj835hEX
clyJxoQr6Ojy40ccEugqmkym_ayJu65fKm.KJY73k6aprxb9s7Bj6P32lpml
6yGzxWFYdNXCwcxHtFGdhKe3v7Tjh8x051jkxjIqfuS0vo8J5rZOr.Z__6vD
4wiGFDUwFHNUWAwuz_pwp7pZ5HCivuuuyszYVvH0eIFsrQ9crR.rrk_3EQU2
Xkv_fInlGDFR8fafFPMOgQ7QOrHhy0zQUbptDEFGdh1QVOyLwIpjwEC7264k
4MqxUH7zz_M5JOQzj6dJslH0.iz5y9Sgp6y6kTUHAVP2f_t1hMeRvf3F7WJ6
1yY2rZJALIME1CtiNKQJoDctzgGFRnh_5mo415MvUcEIH7qqS5RFgWtXEQpd
JIpyYlECDXVUcuASoLmzbuGSiCEVLq7f4EiBTAsaMwXJ07OgXBR.QYDw3VfA
Z0AcfnFrUVHNLZtLaFukQKzdk9c6SpHFHSuCAsvLPuZeRy4Ij5ndXd7viyCS
IkAHsnhG_u3.nZr3zUDFOrqw8sEKphobj6ZJ8KEXtuhr_tx.94abE1JRJYi5
fukj2h8y9s.K10ZxoTClaw41_DD8fxESbyfyTRPytiEXUdK1WEjgS3rAZ0TA
WPJPDr063xLYk20UY0V.N5J15lBCtqZcde_9pdXwxVySyXo1KEQOaH3TNRBZ
AKMFuCC7NF56aklkiUgk2EWm8iYoHsFez5_HtOz1zmc1dv4mNFOPTaNrXF2X
qjFiwfdUipupIlAEc6pIdv0_le.xvz1jnaewEOyxo4dKd2XLVvybLfsLY16U
FzLS9MJJ1wC0Cmf3G2SbOmT4ZiAvPjyv8QnHzbSDDDy3hqg8F0uEE03sJ5dm
on5FxOHZZ1wCH7DL1QAXpZYxYWKV.h3q69dKQMl6HbnmfT_WZQY4X8uKXqkZ
o34v.YmvJxHSRCSmhFpug1EstpJ4gHVitl_eJzT_n6xYQwhNAuMZ9uRjN2xE
1Lf7NpgzRf9bFvOpJAlyLoK5Xvxbx711cMgEUfGIha_JtL1P7hyfncRszHDv
txgUYzcsVvRyAyVvwDAM.TEBsFhAtqqwOibqo2l5xCBj2yXRbKJ0EOC1JDMs
HA--
X-Originating-IP: [192.83.249.65]
Authentication-Results: mta1225.mail.bf1.yahoo.com from=gmail.com; domainkeys=neutral (no sig);
Otis & Rand Expires December 6, 2013 [Page 14]
Internet-Draft DKIM-HARMFUL June 2013
from=gmail.com; dkim=pass (ok)
Received: from 127.0.0.1 (EHLO rdaver.bungi.com) (192.83.249.65)
by mta1225.mail.bf1.yahoo.com with SMTP; Tue, 12 Mar 2013 12:08:36 -0700
Received: by rdaver.bungi.com
via smail with stdio
id <m1UFUYr-00KeXPC@rdaver.bungi.com>
for Just4spamdlr@yahoo.com; Tue, 12 Mar 2013 12:08:33 -0700 (PDT)
(Smail-3.2.0.94 1997-Apr-22 #591 built 2011-Feb-5)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=gmail.com; s=20120113;
h=mime-version:x-received:date:message-id:subject:from:to
:content-type;
bh=PS9xMxYwwTGwWXbCd8bjBBm2rwb79wVOSDLhmp+k4b4=;
b=qnYVUccLSAi2DGJdUgDDIP9A3uPk3PaxgqhYLBn6xU382MsCi/ICFgKAoFPuwM7BvL
AuSuqL6P54cIJ3Pn36h2xmXy+ucNr5r5OqIY63rtvj6Apjr4uW1PzG47J7BGEiP9iwDZ
PLTzl9ZLpZXvZZpTCJOXUQP2HF8q6aivCblYZIQcCdVRCftG+A4z0+dEyTHbxoAMx9U3
GFISRRHcZ7k7GAyYmLrSr3fUTjvpa1YWoNK+IcSALC2tKVSW5FP1IQAT07f1e8+bOgHh
JleaQIw8b1Vjlzhs4hFKLdedmjQqjDJXVP/K3J+t/ggfYn4H547fu6Pb5syKZIiuPf1e
yJqA==
MIME-Version: 1.0
X-Received: by 10.220.221.143 with SMTP id ic15mr6773333vcb.32.1363115257152;
Tue, 12 Mar 2013 12:07:37 -0700 (PDT)
Received: by 10.52.70.169 with HTTP; Tue, 12 Mar 2013 12:07:37 -0700 (PDT)
Date: Tue, 12 Mar 2013 09:07:37 -1000
Message-ID: <CA+VnpPKv0s-p2nKkAkNHS4V2SxZehw_6S9QF5p1p2ji+FMof=Q@mail.gmail.com>
Subject: An example signed message
From: Random User <random.j.user.994@gmail.com>
To: just4spamdlr@yahoo.com
Content-Type: multipart/alternative; boundary=14dae9cdc33bb0ff5204d7bf00ff
Content-Length: 280
reporting valid signature
From Fake User Tue Mar 12 12:07:37 2013
X-Apparently-To: just4spamdlr@yahoo.com via 72.30.237.8; Tue, 12 Mar 2013 12:09:01 -0700
Return-Path: <Fake.user@gmail.com>
Received-SPF: neutral (192.83.249.65 is neither permitted nor denied by domain of gmail.com)
A3RleHQvcGxhaW4DAzACA3RleHQvaHRtbAMDMQ--
X-YMailISG: gFqc.ysWLDtqkdjDpSCH39uGWhgFfnsGdWobzNb5os6sP0We
_L38eAdX.VKZWQ2F75gFwoipcPyj4g0uKMm_vSayLjrnps9lBxMGLvtTE8kT
XYxIw6vZb4aFZ_jEcpoRntvJDkZQl4XSGWGakfmJ5G2blTWZ_i1BVkBvj0Sv
jEymvhoIXZTb_l8C0Jh69ot3MgrNBvjhrBmhCK3sziUtDPpKQPJb_lxCnYKN
O0SiArQ_TUXrCRFRNsyEiJxzVfSgJWIdsCV5BN3cp..NZ17X8fguB.YxNQjt
qjVcGMd4IjQioY.a4f1luQxuiCN1yWvYqiLpP6eOCQhMrHt9XOdk32HAXNuJ
GBraVtjrySTl9Db7PpRC46wlMs3iIUHl3z0d4o6293sMA5qFmnbczGoLRGFs
RUVlBJuRoJCSYZh5LOwbj0RPQNX2Nmw.LHwF7SY3XcZWFUjvUQQ2sdx63m_J
Otis & Rand Expires December 6, 2013 [Page 15]
Internet-Draft DKIM-HARMFUL June 2013
Mgy7JHAwBTVH6ytULsbXvu38a5GIYHccfNnDKVjtsrIg9qBDpVASHrRkncL0
MFLy5FHLb_XBW1TPztCFtlRViKr_HFxMob6aZIte6T57AMqlV2YAHwVNObwx
WE8ZWTkKNWbXqJYytd3vyuyAHfuseBFP_Jfmj0zVtg52EXpIlDiTANEOTamP
zeu23QbeRWJd_Gpz9bbGw_OorPdcV.WJOQ29DHpiYAQRgWjJNLjkd8dI.vuM
vs1Fr7LOiE3wRpSU5AW_hrR4anvGrnwSPOQaFmpNE0pl8n.Vomrp.5NU8cgU
QYI1UCSPoE_HK5Som2HMPYZFQv0pJSu1NeitXlRM3DHkIMvW4aVYqrHSNVjl
gGCFFx77c25QW.XAGtySBYWcTzcUlHP4fMa7Wli4u06C4N3pDPiQoXKOC10U
koXUMKFYmedaZYvEeQRPO3_8xHwKyZ.QInDsnQRwPFWYKvcWCJu4c5zxDMG4
h1AsyT3CM80nZXk8.ZGhzfTgo810Xjn_OJVgUfkG1z3..ReN990deaWJY8F5
_j6lRWLZZRzCMwOGpJ6I.jgaN5mNk38Kj6.NYLFCpMTEIt28jIRHD85cfpa3
iOL3drg1TIKQWrEhS9u3H29niQ_hjHbk7ys6uSJvowilRwO8eB2s.Wz0
X-Originating-IP: [192.83.249.65]
Authentication-Results: mta1266.mail.bf1.yahoo.com
from=gmail.com; domainkeys=neutral (no sig);
from=gmail.com; dkim=pass (ok)
Received: from 127.0.0.1 (EHLO rdaver.bungi.com) (192.83.249.65)
by mta1266.mail.bf1.yahoo.com with SMTP; Tue, 12 Mar 2013 12:09:00 -0700
Received: by rdaver.bungi.com
via smail with stdio
id <m1UFUZI-00KeXRC@rdaver.bungi.com>
for Just4spamdlr@yahoo.com; Tue, 12 Mar 2013 12:09:00 -0700 (PDT)
(Smail-3.2.0.94 1997-Apr-22 #591 built 2011-Feb-5)
From: Fake User <fake.user@gmail.com>
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=gmail.com; s=20120113;
h=mime-version:x-received:date:message-id:subject:from:to
:content-type;
bh=PS9xMxYwwTGwWXbCd8bjBBm2rwb79wVOSDLhmp+k4b4=;
b=qnYVUccLSAi2DGJdUgDDIP9A3uPk3PaxgqhYLBn6xU382MsCi/ICFgKAoFPuwM7BvL
AuSuqL6P54cIJ3Pn36h2xmXy+ucNr5r5OqIY63rtvj6Apjr4uW1PzG47J7BGEiP9iwDZ
PLTzl9ZLpZXvZZpTCJOXUQP2HF8q6aivCblYZIQcCdVRCftG+A4z0+dEyTHbxoAMx9U3
GFISRRHcZ7k7GAyYmLrSr3fUTjvpa1YWoNK+IcSALC2tKVSW5FP1IQAT07f1e8+bOgHh
JleaQIw8b1Vjlzhs4hFKLdedmjQqjDJXVP/K3J+t/ggfYn4H547fu6Pb5syKZIiuPf1e
yJqA==
MIME-Version: 1.0
X-Received: by 10.220.221.143 with SMTP id ic15mr6773333vcb.32.1363115257152;
Tue, 12 Mar 2013 12:07:37 -0700 (PDT)
Received: by 10.52.70.169 with HTTP; Tue, 12 Mar 2013 12:07:37 -0700 (PDT)
Date: Tue, 12 Mar 2013 09:07:37 -1000
Message-ID: <CA+VnpPKv0s-p2nKkAkNHS4V2SxZehw_6S9QF5p1p2ji+FMof=Q@mail.gmail.com>
Subject: An example signed message
From: Random User <random.j.user.994@gmail.com>
To: just4spamdlr@yahoo.com
Content-Type: multipart/alternative; boundary=14dae9cdc33bb0ff5204d7bf00ff
Content-Length: 280
spoofed DKIM with valid signature
Otis & Rand Expires December 6, 2013 [Page 16]
Internet-Draft DKIM-HARMFUL June 2013
Appendix B. Stats
DKIM total: 5063
DKIM pass: 4354
DKIM fail: 709
DKIM pass w/multiple from: 916 (about 21% on average)
An increase appears concurrent with the publication of this draft.
Looking at roughly a few hours of spam
Authors' Addresses
Douglas Otis
Trend Micro
10101 N. De Anza Blvd
Cupertino, CA 95014
USA
Phone: +1.408.257-1500
Email: doug_otis@trendmicro.com
Dave Rand
Trend Micro
10101 N. De Anza Blvd
Cupertino, CA 95014
USA
Phone: +1.408.257-1500
Email: dave_rand@trendmicro.com
Otis & Rand Expires December 6, 2013 [Page 17]