Advice for Safe Handling of Malformed Messages
draft-ietf-appsawg-malformed-mail-02
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (appsawg WG) | |
|---|---|---|---|
| Author | Murray Kucherawy | ||
| Last updated | 2012-06-27 (Latest revision 2012-05-19) | ||
| Replaces | draft-kucherawy-mta-malformed | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Reviews |
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| Stream | WG state | WG Document | |
| Document shepherd | Salvatore Loreto | ||
| IESG | IESG state | AD is watching | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Barry Leiba | ||
| Send notices to | appsawg-chairs@tools.ietf.org, draft-ietf-appsawg-malformed-mail@tools.ietf.org |
draft-ietf-appsawg-malformed-mail-02
APPSAWG M. Kucherawy
Internet-Draft Cloudmark, Inc.
Intended status: Informational May 19, 2012
Expires: November 20, 2012
Advice for Safe Handling of Malformed Messages
draft-ietf-appsawg-malformed-mail-02
Abstract
The email ecosystem has long had a very permissive set of common
processing rules in place, despite increasingly rigid standards
governing its components, ostensibly to improve the user experience.
The handling of these come at some cost, and various components are
faced with decisions about whether or not to permit non-conforming
messages to continue toward their destinations unaltered, adjust them
to conform (possibly at the cost of losing some of the original
message), or outright rejecting them.
This document includes a collection of the best advice available
regarding a variety of common malformed mail situations, to be used
as implementation guidance. It must be emphasized, however, that the
intent of this document is not to standardize malformations or
otherwise encourage their proliferation. The messages are manifestly
malformed, and the code and culture that generates them needs to be
fixed. Nevertheless, many malformed messages from otherwise
legitimate senders are in circulation and will be for some time, and,
unfortunately, commercial reality shows that we cannot simply reject
or discard them. Accordingly, this document presents recommendations
for dealing with them in ways that seem to do the least additional
harm until the infrastructure is tightened up to match the standards.
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
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on November 20, 2012.
Copyright Notice
Copyright (c) 2012 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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
1.1. The Purpose Of This Work . . . . . . . . . . . . . . . . . 4
1.2. Not The Purpose Of This Work . . . . . . . . . . . . . . . 4
2. Document Conventions . . . . . . . . . . . . . . . . . . . . . 4
2.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Internal Representations . . . . . . . . . . . . . . . . . . . 5
5. Invariate Content . . . . . . . . . . . . . . . . . . . . . . 5
6. Mail Submission Agents . . . . . . . . . . . . . . . . . . . . 6
7. Line Terminaton . . . . . . . . . . . . . . . . . . . . . . . 6
8. Header Anomalies . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Converting Obsolete and Invalid Syntaxes . . . . . . . . . 7
8.1.1. Host-Address Syntax . . . . . . . . . . . . . . . . . 7
8.1.2. Excessive Angle Brackets . . . . . . . . . . . . . . . 7
8.1.3. Unbalanced Angle Brackets . . . . . . . . . . . . . . 8
8.1.4. Unbalanced Parentheses . . . . . . . . . . . . . . . . 8
8.1.5. Unbalanced Quotes . . . . . . . . . . . . . . . . . . 8
8.2. Non-Header Lines . . . . . . . . . . . . . . . . . . . . . 8
8.3. Unusual Spacing . . . . . . . . . . . . . . . . . . . . . 10
8.4. Header Malformations . . . . . . . . . . . . . . . . . . . 10
8.5. Header Field Counts . . . . . . . . . . . . . . . . . . . 11
8.6. Missing Header Fields . . . . . . . . . . . . . . . . . . 12
8.7. Eight-Bit Data . . . . . . . . . . . . . . . . . . . . . . 12
9. MIME Anomalies . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Header Field Names . . . . . . . . . . . . . . . . . . . . 13
9.2. Missing MIME-Version Field . . . . . . . . . . . . . . . . 14
10. Body Anomalies . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Oversized Lines . . . . . . . . . . . . . . . . . . . . . 14
11. Security Considerations . . . . . . . . . . . . . . . . . . . 15
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
13.1. Normative References . . . . . . . . . . . . . . . . . . . 15
13.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 16
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1. Introduction
1.1. The Purpose Of This Work
The history of email standards, going back to [RFC822] and beyond,
contains a fairly rigid evolution of specifications. But
implementations within that culture have also long had an
undercurrent known formally as the robustness principle, but also
known informally as Postel's Law: "Be conservative in what you do, be
liberal in what you accept from others."
In general, this served the email ecosystem well by allowing a few
errors in implementations without obstructing participation in the
game. The proverbial bar was set low. However, as we have evolved
into the current era, some of these lenient stances have begun to
expose opportunities that can be exploited by malefactors. Various
email-based applications rely on strong application of these
standards for simple security checks, while the very basic building
blocks of that infrastructure, intending to be robust, fail utterly
to assert those standards.
This document presents some areas in which the more lenient stances
can provide vectors for attack, and then presents the collected
wisdom of numerous applications in and around the email ecosystem for
dealing with them to mitigate their impact.
1.2. Not The Purpose Of This Work
It is important to understand that this work is not an effort to
endorse or standardize certain common malformations. The code and
culture that introduces such messages into the mail stream needs to
be repaired, as the security penalty now being paid for this lax
processing arguably outweighs the reduction in support costs to end
users who are not expected to understand the standards. However, the
reality is that this will not be fixed quickly.
Given this, it is beneficial to provide implementers with guidance
about the safest or most effective way to handle malformed messages
when they arrive, taking into consideration the tradeoffs of the
choices available especially with respect to how various actors in
the email ecosystem respond to such messages in terms of handling,
parsing, or rendering to end users.
2. Document Conventions
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2.1. Key Words
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 [KEYWORDS]. However,
they only have that meaning in this document when they are presented
entirely in uppercase.
2.2. Examples
Examples of message content include a number within braces at the end
of each line. These are line numbers for use in subsequent
discussion, and are not actually part of the message content
presented in the example.
Blank lines are not numbered in the examples.
3. Background
The reader would benefit from reading [EMAIL-ARCH] for some general
background about the overall email architecture. Of particular
interest is the Internet Message Format, detailed in [MAIL].
Throughout this document, the use of the term "messsage" should be
assumed to mean a block of text conforming to the Internet Message
Format.
4. Internal Representations
Any agent handling a message could have one or two (or more) distinct
representations of a message it is handling. One is an internal
representation, such as a block of storage used for the header and a
block for the body. These may be sorted, encoded, decoded, etc., as
per the needs of that particular module. The other is the
representation that is output to the next agent in the handling
chain. This might be identical to the version that is input to the
module, or it might have some changes such as added or reordered
header fields, body modifications to remove malicious content, etc.
In some cases, advice is provided only for internal representations.
However, there is often occasion to mandate changes to the output as
well.
5. Invariate Content
Experience has shown that it is beneficial to ensure that, from the
first analysis agent at ingress into the destination Administrative
Management Domain (ADMD; see [EMAIL-ARCH]) to the agent that actually
affects delivery to the end user, the message each agent sees is
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identical. Absent this, it can be impossible for different agents in
the chain to make assertions about the content that correlate.
For example, suppose a handling agent records that a message had some
specific set of properties at ingress to the ADMD, then permitted it
to continue inbound. Some other agent alters the content for some
reason. The user, on viewing the delivered content, reports the
message as abusive. If the report is based on the set of properties
recorded at ingress, then the compliant effectively references a
message different from what the user saw, which could render the
complaint inactionable. Similarly, a message with properties that a
filtering agent might use to reject an abusive message could be
allowed to reach the user if an intermediate agent altered the
message in a manner that alters one of those properties, thwarting
detection of the abuse.
Therefore, agents comprising an inbound message processing
environment SHOULD ensure that each agent sees the same content, and
the message reaches the end user unmodified. An exception to this is
content that is identitfied as certainly harmful, such as some kind
of malicious executable software included in the message.
6. Mail Submission Agents
Within the email context, the single most influential component that
can reduce the presence of malformed items in the email system is the
Mail Submission Agent (MSA). This is the component that is
essentially the interface between end users that create content and
the mail stream.
The lax processing described earlier in the document creates a high
support and security cost overall. Thus, MSAs MUST evolve to become
more strict about enforcement of all relevant email standards,
especially [MAIL] and the [MIME] family of documents.
Relay Mail Transport Agents (MTAs) SHOULD also be more strict;
although preventing the dissemination of malformed messages is
desirable, the rejection of such mail already in transit also has a
support cost, namely the creation of a [DSN] that many end users
might not understand.
7. Line Terminaton
The only valid line separation sequence in messaging is ASCII 0x0D
("carriage return", or CR) followed by ASCII 0x0A ("line feed", or
LF), commonly referred to as CRLF. Common UNIX user tools, however,
typically only use LF for line termination. This means the protocol
has to convert LF to CRLF before transporting a message.
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Naive implementations can cause messages to be transmitted with a mix
of line terminations, such as LF everywhere except CRLF only at the
end of the message. According to [SMTP], this means the entire
message actually exists on a single line.
A "naked" CR or LF in a message has no reasonable justification, and
furthermore [MIME] presents mechanisms for encoding content that
actually does need to contain such an unusual character sequence.
Thus, handling agents MUST treat naked CRs and LFs as CRLFs when
interpreting the message.
8. Header Anomalies
This section covers common syntactical and semantic anomalies found
in headers of messages, and presents preferred mitigations.
8.1. Converting Obsolete and Invalid Syntaxes
There are numerous cases of obsolete header syntaxes that can be
applied to confound agents with variable processing. This section
presents some examples of these. Messages including them SHOULD be
rejected; where this is not possible, RECOMMENDED internal
interpretations are provided.
8.1.1. Host-Address Syntax
The following obsolete syntax:
To: <@example.net:fran@example.com>
should be interpreted as:
To: <fran@example.com>
8.1.2. Excessive Angle Brackets
The following over-use of angle brackets:
To: <<<user2@example.org>>>
should be interpreted as:
To: "<<" <user2@example.org>
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8.1.3. Unbalanced Angle Brackets
The following use of unbalanced angle brackets:
To: <another@example.net
To: second@example.org>
should be interpreted as:
To: <another@example.net>
To: second@example.org
8.1.4. Unbalanced Parentheses
The following use of unbalanced parentheses:
To: (Testing <fran@example.com>
To: Testing) <sam@example.com>
should be interpreted as:
To: (Testing) <fran@example.com>
To: "Testing)" <sam@example.com>
8.1.5. Unbalanced Quotes
The following use of unbalanced quotation marks:
To: "Joe <joe@example.com>
should be interpreted as:
To: "Joe <joe@example.com>"@example.net
where "example.net" is the domain name or host name of the handling
agent making the interpretation.
8.2. Non-Header Lines
It has been observed that some messages contain a line of text in the
header that is not a valid message header field of any kind. For
example:
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From: user@example.com {1}
To: userpal@example.net {2}
Subject: This is your reminder {3}
about the football game tonight {4}
Date: Wed, 20 Oct 2010 20:53:35 -0400 {5}
Don't forget to meet us for the tailgate party! {7}
The cause of this is typically a bug in a message generator of some
kind. Line {4} was intended to be a continuation of line {3}; it
should have been indented by whitespace as set out in Section 2.2.3
of [MAIL].
This anomaly has varying impacts on processing software, depending on
the implementation:
1. some agents choose to separate the header of the message from the
body only at the first empty line (i.e. a CRLF immediately
followed by another CRLF);
2. some agents assume this anomaly should be interpreted to mean the
body starts at line {4}, as the end of the header is assumed by
encountering something that is not a valid header field or folded
portion thereof;
3. some agents assume this should be interpreted as an intended
header folding as described above and thus simply append a single
space character (ASCII 0x20) and the content of line {4} to that
of line {3};
4. some agents reject this outright as line {4} is neither a valid
header field nor a folded continuation of a header field prior to
an empty line.
This can be exploited if it is known that one message handling agent
will take one action while the next agent in the handling chain will
take another. Consider, for example, a message filter that searches
message headers for properties indicative of abusive of malicious
content that is attached to a Mail Transfer Agent (MTA) implementing
option 2 above. An attacker could craft a message that includes this
malformation at a position above the property of interest, knowing
the MTA will not consider that content part of the header, and thus
the MTA will not feed it to the filter, thus avoiding detection.
Meanwhile, the Mail User Agent (MUA) which presents the content to an
end user, implements option 1 or 3, which has some undesirable
effect.
The preferred implementation is to apply the following heuristic when
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this malformation is detected:
1. Search forward for an empty line. If one is found, then apply
option 3 above to the anomalous line, and continue.
2. Search forward for another line that appears to be a new header
field, i.e., a name followed by a colon. If one is found, then
apply option 3 above to the anomalous line, and continue.
It should be noted that a few implementations choose option 4 above
since any reputable message generation program will get header
folding right, and thus anything so blatant as this malformation is
likely an error caused by a malefactor.
8.3. Unusual Spacing
The following message is valid per [MAIL]:
From: user@example.com {1}
To: userpal@example.net {2}
Subject: This is your reminder {3}
{4}
about the football game tonight {5}
Date: Wed, 20 Oct 2010 20:53:35 -0400 {6}
Don't forget to meet us for the tailgate party! {8}
Line {4} contains a single whitespace. The intended result is that
lines {3}, {4}, and {5} comprise a single continued header field.
However, some agents are aggressive at stripping trailing whitespace,
which will cause line {4} to be treated as an empty line, and thus
the separator line between header and body. This can affect header-
specific processing algorithms as described in the previous section.
Ideally, this case simply ought not to be generated.
Message handling agents receiving a message bearing this anomaly MUST
behave as if line {4} was not present on the message, and SHOULD emit
a version in which line {4} has been removed.
8.4. Header Malformations
There are various malformations that exist. A common one is
insertion of whitespace at unusual locations, such as:
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From: user@example.com {1}
To: userpal@example.net {2}
Subject: This is your reminder {3}
MIME-Version : 1.0 {4}
Content-Type: text/plain {5}
Date: Wed, 20 Oct 2010 20:53:35 -0400 {6}
Don't forget to meet us for the tailgate party! {8}
Note the addition of whitespace in line {4} after the header field
name but before the colon that separates the name from the value.
The acceptance grammar of [MAIL] permits that extra whitespace, so it
cannot be considered invalid. However, a consensus of
implementations prefers to remove that whitespace. There is no
perceived change to the semantics of the header field being altered
as the whitespace is itself semantically meaningless. Thus, a module
compliant with this memo MUST remove all whitespace after the field
name but before the colon, and MUST emit that version of that field
on output.
8.5. Header Field Counts
Section 3.6 of [MAIL] prescribes specific header field counts for a
valid message. Few agents actually enforce these in the sense that a
message whose header contents exceed one or more limits set there are
generally allowed to pass; they may add any required fields that are
missing, however.
Also, few agents that use messages as input, including Mail User
Agents (MUAs) that actually display messages to users, verify that
the input is valid before proceeding. Two popular open source
filtering programs and two popular Mailing List Management (MLM)
packages examined at the time this document was written select either
the first or last instance of a particular field name, such as From,
to decide who sent a message. Absent enforcement of [MAIL], an
attacker can craft a message with multiple fields if that attacker
knows the filter will make a decision based on one but the user will
be shown the other.
This situation is exacerbated when a claim of message validity is
inferred by something like a valid [DKIM] signature. Such a
signature might cover one instance of a constrained field but not
another, and a naive consumer of DKIM's output, not realizing which
one was covered by a valid signature, could presume the wrong one was
the "good" one. An MUA, for example could show the first of two From
fields as "good" or "safe" while the DKIM signature actually only
verified the second.
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Thus, an agent compliant with this specification MUST enact one of
the following:
1. reject outright or refuse to process further any input message
that does not conform to Section 3.6 of [MAIL];
2. remove or, in the case of an MUA, refuse to render any instances
of a header field whose presence exceeds a limit prescribed in
Section 3.6 of [MAIL] when generating its output;
3. alter the name of any header field whose presence exceeds a limit
prescribed in Section 3.6 of [MAIL] when generating its output so
that later agents can produce a consistent result.
8.6. Missing Header Fields
Similar to the previous section, there are messages seen in the wild
that lack certain required header fields. For example, [MAIL]
requires that a To field be present in all messages. When presented
with a message lacking a To field, some MTAs might do one of the
following:
1. Make no changes
2. Add a To field with a neutral value, such as "undisclosed-
recipients:;"
3. Add a non-standard header field, such as Apparently-To, that
contains the list of [SMTP] recipients
4. Add a To field that contains the list of [SMTP] recipients
Options 3 and 4 above risk exposing private information, such as
blind carbon copy (Bcc) recipients. Further, option 3 still leaves
the message in a non-compliant state.
Option 2 is RECOMMENDED for handling this case.
Other missing field anomalies include the absence of fields that are
required, most notably Date and From. Handling agents SHOULD add
these for internal hanlding if they are missing, but MUST NOT add
them to the external representation.
8.7. Eight-Bit Data
Standards-compliant mail messages do not contain any non-ASCII data
without indicating that such content is present by means of published
[SMTP] extensions. Absent that, [MIME] encodings are typically used
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to convert non-ASCII data to ASCII in a way that can be reversed by
other handling agents or end users.
Non-ASCII data otherwise found in messages can confound code that is
used to analyze content. For example, a null (ASCII 0x00) byte
inside a message can cause typical string processing functions to
mis-identify the end of a string, which can be exploited to hide
malicious content from analysis processes.
Handling agents MUST reject messages containing null bytes that are
not encoded in some standard way, and SHOULD reject other non-ASCII
bytes that are similarly not encoded.
9. MIME Anomalies
[MIME], et seq, define a mechanism of message extensions for
providing text in character sets other than ASCII, non-text
attachments to messages, multi-part message bodies, and similar
facilities.
Some anomalies with MIME-compliant generation are also common. This
section discusses some of those and presents preferred mitigations.
9.1. Header Field Names
[MAIL] permits header field names to begin with "--". This means
that a header field name can look like a [MIME] multipart boundary.
For example:
--foo:bar
This is a legal header field, whose name is "--foo" and whose value
is "bar". Thus, consider this header:
From: user@example.com {1}
To: userpal@example.net {2}
Subject: This is your reminder {3}
Date: Wed, 20 Oct 2010 20:53:35 -0400 {4}
MIME-Version: 1.0 {5}
Content-Type: multipart/mixed; boundary="foo:bar" {6}
--foo:bar {7}
Malicious-Content: muahaha {8}
One implementation could observe that line {7} is the beginning of
the first MIME part while another considers it a part of the
message's header.
If rejection of such messages cannot be done, agents SHOULD treat
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line {7} as part of the message's header block and not a MIME
boundary.
9.2. Missing MIME-Version Field
Any message that uses [MIME] constructs is required to have a MIME-
Version header field. Without them, the Content-Type and associated
fields have no semantic meaning.
It is often observed that a message has complete MIME structure, yet
lacks this header field.
As described at the end of Section 8.2, this is not expected from a
reputable content generator and is often an indication of mass-
produced spam or other undesirable messages.
Therefore, an agent compliant with this specification MUST internally
enact one or more of the following in the absence of a MIME-Version
header field:
1. Ignore all other MIME-specific fields, even if they are
syntactically valid, thus treating the entire message as a
single-part message of type text/plain;
2. Remove all other MIME-specific fields, even if they are
syntactically valid, both internally and when emitting the output
version of the message;
10. Body Anomalies
10.1. Oversized Lines
A message containing a line of content that exceeds 998 characters
plus the line terminator (1000 total) violates Section 2.1.1 of
[MAIL]. Some handling agents may not look at content in a single
line past the first 998 bytes, providing bad actors an opportunity to
hide malicious content.
There is no specified way to handle such messages, other than to
observe that they are non-compliant and reject them, or rewrite the
oversized line such that the message is compliant.
Handling agents MUST break such lines into multiple lines at a
position that does not change the semantics of the text being thus
altered. For example, breaking an oversized line such that a [URI]
then spans two lines could prevent the proper identification of that
URI.
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11. Security Considerations
The discussions of the anomalies above and their prescribed solutions
are themselves security considerations. The practises enumerated in
this memo are generally perceived to resolve security considerations
that already exist rather than introducing new ones.
12. IANA Considerations
This memo contains no actions for IANA.
[RFC Editor: Please remove this section prior to publication.]
13. References
13.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
13.2. Informative References
[DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message
Format for Delivery Status Notifications", RFC 3464,
January 2003.
[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[RFC822] Crocker, D., "Standard for the Format of Internet Text
Messages", RFC 822, August 1982.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
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Internet-Draft Safe Mail Handling May 2012
RFC 3986, January 2005.
Appendix A. Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: Tony Hansen, and Gregory
Shapiro
Author's Address
Murray S. Kucherawy
Cloudmark, Inc.
128 King St., 2nd Floor
San Francisco, CA 94107
US
Phone: +1 415 946 3800
EMail: msk@cloudmark.com
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