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Keywords: MTA, SMTP
Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 7103 G. Shapiro
Category: Informational N. Freed
ISSN: 2070-1721 January 2014
Advice for Safe Handling of Malformed Messages
Abstract
Although Internet message formats have been precisely defined since
the 1970s, authoring and handling software often shows only mild
conformance to the specifications. The malformed messages that
result are non-standard. Nonetheless, decades of experience have
shown that using some tolerance in the handling of the malformations
that result is often an acceptable approach and is better than
rejecting the messages outright as nonconformant. This document
includes a collection of the best advice available regarding a
variety of common malformed mail situations; it is to be used as
implementation guidance.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7103.
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Copyright Notice
Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The Purpose of This Work . . . . . . . . . . . . . . . . 3
1.2. Not the Purpose of This Work . . . . . . . . . . . . . . 4
1.3. General Considerations . . . . . . . . . . . . . . . . . 4
2. Document Conventions . . . . . . . . . . . . . . . . . . . . 5
2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Invariant Content . . . . . . . . . . . . . . . . . . . . . . 5
5. Mail Submission Agents . . . . . . . . . . . . . . . . . . . 6
6. Line Termination . . . . . . . . . . . . . . . . . . . . . . 7
7. Header Anomalies . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Converting Obsolete and Invalid Syntaxes . . . . . . . . 8
7.1.1. Host-Address Syntax . . . . . . . . . . . . . . . . . 8
7.1.2. Excessive Angle Brackets . . . . . . . . . . . . . . 8
7.1.3. Unbalanced Angle Brackets . . . . . . . . . . . . . . 8
7.1.4. Unbalanced Parentheses . . . . . . . . . . . . . . . 9
7.1.5. Commas in Address Lists . . . . . . . . . . . . . . . 9
7.1.6. Unbalanced Quotes . . . . . . . . . . . . . . . . . . 10
7.1.7. Naked Local-Parts . . . . . . . . . . . . . . . . . . 10
7.2. Non-Header Lines . . . . . . . . . . . . . . . . . . . . 10
7.3. Unusual Spacing . . . . . . . . . . . . . . . . . . . . . 12
7.4. Header Malformations . . . . . . . . . . . . . . . . . . 13
7.5. Header Field Counts . . . . . . . . . . . . . . . . . . . 13
7.5.1. Repeated Header Fields . . . . . . . . . . . . . . . 14
7.5.2. Missing Header Fields . . . . . . . . . . . . . . . . 15
7.5.3. Return-Path . . . . . . . . . . . . . . . . . . . . . 16
7.6. Missing or Incorrect Charset Information . . . . . . . . 16
7.7. Eight-Bit Data . . . . . . . . . . . . . . . . . . . . . 18
8. MIME Anomalies . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Missing MIME-Version Field . . . . . . . . . . . . . . . 19
8.2. Faulty Encodings . . . . . . . . . . . . . . . . . . . . 19
9. Body Anomalies . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Oversized Lines . . . . . . . . . . . . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 23
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1. Introduction
1.1. The Purpose of This Work
The history of email standards, going back to [RFC733] and beyond,
contains a fairly rigid evolution of specifications. However,
implementations within that culture have also long had an
undercurrent known formally as "the robustness principle", also known
informally as "Postel's Law": "Be liberal in what you accept, and
conservative in what you send" [RFC1122].
Jon Postel's directive is often interpreted to mean that any deviance
from a specification is acceptable. However, we believe it was
intended only to account for legitimate variations in interpretation
within specifications, as well as basic transit errors, like bit
errors. Taken to its unintended extreme, excessive tolerance would
imply that there are no limits to the liberties that a sender might
take, while presuming a burden on a receiver to guess "correctly" at
the meaning of any such variation. These matters are further
compounded by receiver software -- the end users' mail readers --
which are also sometimes flawed, leaving senders to craft messages
(sometimes bending the rules) to overcome those flaws.
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 the 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.
The distributed and non-interactive nature of email has often
prompted adjustments to receiving software, to handle these
variations, rather than trying to gain better conformance by senders,
since the receiving operator is primarily driven by complaints from
recipient users and has no authority over the sending side of the
system. Processing with such flexibility comes at some cost, since
mail software is faced with decisions about whether 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 reject them outright.
This document includes a collection of the best advice available
regarding a variety of common malformed mail situations; it is to be
used as implementation guidance. These malformations are typically
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based around loose interpretations or implementations of
specifications such as the Internet Message Format [MAIL] and
Multipurpose Internet Mail Extensions [MIME].
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 trade-offs 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.
1.3. General Considerations
Many deviations from message format standards are considered by some
receivers to be strong indications that the message is undesirable,
such as spam or something containing malware. These receivers
quickly decide that the best handling choice is simply to reject or
discard the message. This means malformations caused by innocent
misunderstandings or ignorance of proper syntax can cause messages
with no ill intent also to fail to be delivered.
Senders that want to ensure message delivery are best advised to
adhere strictly to the relevant standards (including, but not limited
to, [MAIL], [MIME], and [DKIM]), as well as observe other industry
best practices such as may be published from time to time by either
the IETF or independently.
Receivers that haven't the luxury of strict enforcement of the
standards on inbound messages are usually best served by observing
the following guidelines for handling of malformed messages:
1. Whenever possible, mitigation of syntactic malformations should
be guided by an assessment of the most likely semantic intent.
For example, it is reasonable to conclude that multiple sets of
angle brackets around an address are simply superfluous and can
be dropped.
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2. When the intent is unclear, or when it is clear but also
impractical to change the content to reflect that intent,
mitigation should be limited to cases where not taking any
corrective action would clearly lead to a worse outcome.
3. Security issues, when present, need to be addressed and may force
mitigation strategies that are otherwise suboptimal.
2. Document Conventions
2.1. 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 they 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 "message" should be
assumed to mean a block of text conforming to the Internet Message
Format.
4. Invariant Content
An agent handling a message could use several distinct
representations of the message. One is an internal representation,
such as separate blocks of storage for the header and body, some
header or body alterations, or tables indexed by header name, set up
to make particular kinds of processing easier. The other is the
representation passed along to the next agent in the handling chain.
This might be identical to the message input to the module, or it
might have some changes such as added or reordered header fields or
body elisions to remove malicious content.
Message handling is usually most effective when each in a sequence of
handling modules receives the same content for analysis. A module
that "fixes" or otherwise alters the content passed to later modules
can prevent the later modules from identifying malicious or other
content that exposes the end user to harm. It is important that all
processing modules can make consistent assertions about the content.
Modules that operate sequentially sometimes add private header fields
to relay information downstream for later filters to use (and
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possibly remove), or they may have out-of-band ways of doing so.
However, even the presence of private header fields can impact a
downstream handling agent unaware of its local semantics, so an out-
of-band method is always preferable.
The above is less of a concern when multiple analysis modules are
operated in parallel, independent of one another.
Often, abuse reporting systems can act effectively only when a
complaint or report contains the original message exactly as it was
generated. Messages that have been altered by handling modules might
render a complaint not actionable as the system receiving the report
may be unable to identify the original message as one of its own.
Some message changes alter syntax without changing semantics. For
example, Section 7.4 describes a situation where an agent removes
additional header whitespace. This is a syntax change without a
change in semantics, though some systems (such as DKIM) are sensitive
to such changes. Message system developers need to be aware of the
downstream impact of making either kind of change.
Where a change to content between modules is unavoidable, it is a
good idea to add standard trace data to indicate a "visible" handoff
between modules has occurred. The only advisable way to do this is
to prepend Received fields with the appropriate information, as
described in Section 3.6.7 of [MAIL].
There will always be local handling exceptions, but these guidelines
should be useful for developing integrated message processing
environments.
In most cases, this document only discusses techniques used on
internal representations. It is occasionally necessary to make
changes between the input and output versions; such cases will be
called out explicitly.
5. 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 Handling Service (MHS; see [EMAIL-ARCH]), which includes the
Mail Submission Agent (MSA). This is the component that is
essentially the interface between end users that create content and
the mail stream.
MHSs need to become more strict about enforcement of all relevant
email standards, especially [MAIL] and the [MIME] family of
documents.
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More strict conformance by relaying Mail Transfer Agents (MTAs) will
also be helpful. 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.
6. Line Termination
For interoperable Internet Mail messages, the only valid line
separation sequence during a typical SMTP session is ASCII 0x0D
("carriage return", or CR) followed by ASCII 0x0A ("line feed", or
LF), commonly referred to as "CRLF". This is not the case for binary
mode SMTP (see [BINARYSMTP]).
Common UNIX user tools, however, typically only use LF for internal
line termination. This means that a protocol engine that converts
between UNIX and Internet message formats has to convert between
these two end-of-line representations before transmitting a message
or after receiving it.
Non-compliant implementations can create messages with a mix of line
terminations, such as LF everywhere except CRLF only at the end of
the message. According to [SMTP] and [MAIL], this means the entire
message actually exists on a single line.
Within modern Internet Mail, it is highly unlikely that an isolated
CR or LF is valid in common ASCII text. Furthermore, when content
actually does need to contain such an unusual character sequence,
[MIME] provides mechanisms for encoding that content in an SMTP-safe
manner.
Thus, it will typically be safe and helpful to treat an isolated CR
or LF as equivalent to a CRLF when parsing a message.
Note that this advice pertains only to the raw SMTP data and not to
decoded MIME entities. As noted above, when MIME encoding mechanisms
are used, the unusual character sequences are not visible in the raw
SMTP stream.
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7. Header Anomalies
This section covers common syntactic and semantic anomalies found in
a message header and presents suggested methods of mitigation.
7.1. Converting Obsolete and Invalid Syntaxes
A message using an obsolete header syntax (see Section 4 of [MAIL])
might confound an agent that is attempting to be robust in its
handling of syntax variations. A bad actor could exploit such a
weakness in order to get abusive or malicious content through a
filter. This section presents some examples of such variations.
Messages including these variations ought to be rejected; where this
is not possible, recommended internal interpretations are provided.
7.1.1. Host-Address Syntax
The following obsolete syntax attempts to specify source routing:
To: <@example.net:fran@example.com>
This means "send to fran@example.com via the mail service at
example.net". It can safely be interpreted as:
To: <fran@example.com>
7.1.2. Excessive Angle Brackets
The following overuse of angle brackets:
To: <<<user2@example.org>>>
can safely be interpreted as:
To: <user2@example.org>
7.1.3. Unbalanced Angle Brackets
The following use of unbalanced angle brackets:
To: <another@example.net
can usually be treated as:
To: <another@example.net>
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The following:
To: second@example.org>
can usually be treated as:
To: second@example.org
7.1.4. Unbalanced Parentheses
The following use of unbalanced parentheses:
To: (Testing <fran@example.com>
can safely be interpreted as:
To: (Testing) <fran@example.com>
Likewise, this case:
To: Testing) <sam@example.com>
can safely be interpreted as:
To: "Testing)" <sam@example.com>
In both cases, it is obvious where the active email address in the
string can be found. The former case retains the active email
address in the string by completing what appears to be intended as a
comment; the intent in the latter case is less obvious, so the
leading string is interpreted as a display name.
7.1.5. Commas in Address Lists
This use of an errant comma:
To: <third@example.net, fourth@example.net>
can usually be interpreted as ending an address, so the above is
usually best interpreted as:
To: third@example.net, fourth@example.net
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7.1.6. Unbalanced Quotes
The following use of unbalanced quotation marks:
To: "Joe <joe@example.com>
leaves software with no unambiguous interpretation. One possible
interpretation is:
To: "Joe <joe@example.com>"@example.net
where "example.net" is the domain name or host name of the handling
agent making the interpretation. However, the more obvious and
likely best interpretation is simply:
To: "Joe" <joe@example.com>
7.1.7. Naked Local-Parts
[MAIL] defines a local-part as the user portion of an email address,
and the display-name as the "user-friendly" label that accompanies
the address specification.
Some broken submission agents might introduce messages with only a
local-part or only a display-name and no properly formed address.
For example:
To: Joe
A submission agent ought to reject this or, at a minimum, append "@"
followed by its own host name or some other valid name likely to
enable a reply to be delivered to the correct mailbox. Where this is
not done, an agent receiving such a message will probably be
successful by synthesizing a valid header field for evaluation using
the techniques described in Section 7.5.2.
7.2. Non-Header Lines
Some messages contain a line of text in the header that is not a
valid message header field of any kind. For example:
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}
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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 (that is, 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 or 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.
Consequently, the MTA will not feed it to the filter; thus, it avoids
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.
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.
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The preferred implementation if option 4 above is not employed is to
apply the following heuristic when 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 (a name followed by a colon). If one is found, then apply
option 3 above to the anomalous line, and continue.
7.3. 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.
This example was legal in earlier versions of the Internet message
format standard but was rendered obsolete as of [RFC2822] as line {4}
could be interpreted as the separator between the header and body.
The best handling of this example is for a message parsing engine to
behave as if line {4} were not present in the message and for a
message creation engine to emit the message with line {4} removed.
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7.4. Header Malformations
Among the many possible malformations, a common one is insertion of
whitespace at unusual locations, such as:
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 obsolete grammar of Section 4 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. Therefore, it
is best to remove all whitespace after the field name but before the
colon and to emit the field in this modified form.
7.5. 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 typically add any required fields
that are missing, however.
Also, few agents that use messages as input, including MUAs that
actually display messages to users, verify that the input is valid
before proceeding. Some popular open-source filtering programs and
some popular Mailing List Management (MLM) packages select either the
first or last instance of a particular field name, such as From, to
decide who sent a message. Absent strict enforcement of [MAIL], an
attacker can craft a message with multiple instances of the same
fields if that attacker knows the filter will make a decision based
on one, but the user will be shown the others.
This situation is exacerbated when message validity is assessed, such
as through enhanced authentication methods like DomainKeys Identified
Mail [DKIM]. Such methods might cover one instance of a constrained
field but not another, taking the wrong one as "good" or "safe". An
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MUA, for example, could show the first of two From fields to an end
user as "good" or "safe", while an authentication method actually
only verified the second.
In attempting to counter this exposure, one of the following
strategies can be used:
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. where a field can contain multiple distinct values (such as From)
or is free-form text (such as Subject), combine them into a
semantically identical, single header field of the same name (see
Section 7.5.1);
4. 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. Any
alteration likely to cause the field to be ignored by downstream
agents is acceptable. A common approach is to prefix the field
names with a string such as "BAD-".
When selecting a mitigation action (or some other action) from the
above list, an operator must consider its needs and the nature of its
user base.
7.5.1. Repeated Header Fields
There are some occasions where repeated fields are encountered where
only one is expected. Two examples are presented. First:
From: reminders@example.com {1}
To: jqpublic@example.com {2}
Subject: Automatic Meeting Reminder {3}
Subject: 4pm Today -- Staff Meeting {4}
Date: Wed, 20 Oct 2010 08:00:00 -0700 {5}
Reminder of the staff meeting today in the small {6}
auditorium. Come early! {7}
The message above has two Subject fields, which is in violation of
Section 3.6 of [MAIL]. A safe interpretation of this would be to
treat it as though the two Subject field values were concatenated, so
long as they are not identical, such as:
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From: reminders@example.com {1}
To: jqpublic@example.com {2}
Subject: Automatic Meeting Reminder {3}
4pm Today -- Staff Meeting {4}
Date: Wed, 20 Oct 2010 08:00:00 -0700 {5}
Reminder of the staff meeting today in the small {6}
auditorium. Come early! {7}
Second:
From: president@example.com {1}
From: vice-president@example.com {2}
To: jqpublic@example.com {3}
Subject: A note from the E-Team {4}
Date: Wed, 20 Oct 2010 08:00:00 -0700 {5}
This memo is to remind you of the corporate dress {6}
code. Attached you will find an updated copy of {7}
the policy. {8}
...
As with the first example, there is a violation in terms of the
number of instances of the From field. A likely safe interpretation
would be to combine these into a comma-separated address list in a
single From field:
From: president@example.com, {1}
vice-president@example.com {2}
To: jqpublic@example.com {3}
Subject: A note from the E-Team {4}
Date: Wed, 20 Oct 2010 08:00:00 -0700 {5}
This memo is to remind you of the corporate dress {6}
code. Attached you will find an updated copy of {7}
the policy. {8}
...
7.5.2. Missing Header Fields
Similar to the previous section, there are messages seen in the wild
that lack certain required header fields. In particular, [MAIL]
requires that a From and Date field be present in all messages.
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When presented with a message lacking these fields, the MTA might
perform one of the following:
1. Make no changes.
2. Add an instance of the missing field(s) using synthesized content
based on data provided in other parts of the protocol.
Option 2 is recommended for handling this case. Handling agents
should add these for internal handling if they are missing, but
should not add them to the external representation. The reason for
this advice is that there are some filter modules that would consider
the absence of such fields to be a condition warranting special
treatment (for example, rejection), and thus the effectiveness of
such modules would be stymied by an upstream filter adding them in a
way visible to other components.
The synthesized fields should contain a best guess as to what should
have been there; for From, the SMTP MAIL command's address can be
used (if not null) or a placeholder address followed by an address
literal (for example, unknown@[192.0.2.1]); for Date, a date
extracted from a Received field is a reasonable choice.
One other important case to consider is a missing Message-ID field.
An MTA that encounters a message missing this field should synthesize
a valid one and add it to the external representation, since many
deployed tools commonly use the content of that field as a unique
message reference, so its absence inhibits correlation of message
processing. Section 3.6.4 of [MAIL] describes advisable practice for
synthesizing the content of this field when it is absent, and
establishes a requirement that it be globally unique.
7.5.3. Return-Path
While legitimate messages can contain more than one Return-Path
header field, such usage is often an error rather that a valid
message containing multiple header field blocks as described in
Sections 3.6 of [MAIL]. Accordingly, when a message containing
multiple Return-Path header fields is encountered, all but the
topmost one is to be disregarded, as it is most likely to have been
added nearest to the mailbox that received that message.
7.6. Missing or Incorrect Charset Information
MIME provides the means to include textual material employing
character sets ("charsets") other than US-ASCII. Such material is
required to have an identified charset. Charset identification is
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done using a "charset" parameter in the Content-Type header field, a
charset label within the MIME entity itself, or the charset can be
implicitly specified by the Content-Type (see [CHARSET]).
Unfortunately, it is fairly common for required character set
information to be missing or incorrect in textual MIME entities. As
such, processing agents should perform basic sanity checks, such as:
o US-ASCII contains bytes between 1 and 127 inclusive only
(colloquially, "7-bit" data), so material including bytes outside
of that range ("8-bit" data) is necessarily not US-ASCII. (See
Section 2.1 of [MAIL].)
o [UTF-8] has a very specific syntactic structure that other 8-bit
charsets are unlikely to follow.
o Null bytes (ASCII 0x00) are not allowed in either 7-bit or 8-bit
data.
o Not all 7-bit material is US-ASCII. The presence of the various
escape sequences used for character switching can be used as an
indication of the various charsets based on ISO/IEC 2022
[ISO-2022], such as those defined in [ISO-2022-CN], [ISO-2022-JP],
and [ISO-2022-KR].
When a character set error is detected, processing agents should:
1. apply heuristics to determine the most likely character set and,
if successful, proceed using that information; or
2. refuse to process the malformed MIME entity.
A null byte inside a textual MIME entity can cause typical string
processing functions to misidentify the end of a string, which can be
exploited to hide malicious content from analysis processes.
Accordingly, null bytes require additional special handling.
A few null bytes in isolation is likely to be the result of poor
message construction practices. Such nulls should be silently
dropped.
Large numbers of null bytes are usually the result of binary material
that is improperly encoded, improperly labeled, or both. Such
material is likely to be damaged beyond the hope of recovery, so the
best course of action is to refuse to process it.
Finally, the presence of null bytes may be used as indication of
possible malicious intent.
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7.7. Eight-Bit Data
Standards-compliant email 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 to
convert non-ASCII data to ASCII in a way that can be reversed by
other handling agents or end users.
The best way to handle non-compliant 8-bit material depends on its
location.
Non-compliant 8-bit material in MIME entity content should simply be
processed as if the necessary SMTP extensions had been used to
transfer the message. Note that improperly labeled 8-bit material in
textual MIME entities may require treatment as described in
Section 7.6.
Non-compliant 8-bit material in message or MIME entity header fields
can be handled as follows:
1. Occurrences in unstructured text fields, comments, and phrases
can be converted into encoded-words (see [MIME3] if a likely
character set can be determined). Alternatively, 8-bit
characters can be removed or replaced with some other character.
2. Occurrences in header fields whose syntax is unknown may be
handled by dropping the field entirely or by removing/replacing
the 8-bit character as described above.
3. Occurrences in addresses are especially problematic. Agents
supporting [EAI] may, if the 8-bit material conforms to 8-bit
syntax, elect to treat the message as an EAI message and process
it accordingly. Otherwise, in most cases, it is best to exclude
the address from any sort of processing -- which may mean
dropping it entirely -- since any attempt to fix it definitively
is unlikely to be successful.
8. MIME Anomalies
The five-part set of MIME specifications includes a mechanism of
message extensions for providing text in character sets other than
ASCII, non-text attachments to messages, multipart message bodies,
and similar facilities.
Some anomalies with MIME-compliant generation are also common. This
section discusses some of those and presents preferred methods of
mitigation.
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8.1. Missing MIME-Version Field
Any message that uses [MIME] constructs is required to have a MIME-
Version header field. Without it, 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. It is prudent to disregard this absence and
conduct analysis of the message as if it were present, especially by
agents attempting to identify malicious material.
Further, the absence of MIME-Version might be an indication of
malicious intent, and extra scrutiny of the message may be warranted.
Such omissions are not expected from compliant message generators.
8.2. Faulty Encodings
There have been a few different specifications of base64 in the past.
The implementation defined in [MIME] instructs decoders to discard
characters that are not part of the base64 alphabet. Other
implementations consider an encoded body containing such characters
to be completely invalid. Very early specifications of base64 (see
[PEM89], for example, which was later obsoleted by [PEM93]) allowed
email-style comments within base64-encoded data.
The attack vector here involves constructing a base64 body whose
meaning varies given different possible decodings. If a security
analysis module wishes to be thorough, it should consider scanning
the possible outputs of the known decoding dialects in an attempt to
anticipate how the MUA will interpret the data.
9. Body Anomalies
9.1. 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.
To ensure long lines do not prevent analysis of potentially malicious
data, handling agents are strongly encouraged to take one of the
following actions:
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1. Break such lines into multiple lines at a position that does not
change the semantics of the text being thus altered. For
example, break an oversized line at a position such that a [URI]
does not span two lines (which could inhibit the proper
identification of the URI).
2. Rewrite the MIME part (or the entire message if not MIME) that
contains the excessively long line using a content encoding that
breaks the line in the transmission but would still result in the
line being intact on decoding for presentation to the user. Both
of the encodings declared in [MIME] can accomplish this.
10. Security Considerations
The discussions of the anomalies above and their prescribed solutions
are themselves security considerations. The practices enumerated in
this document are generally perceived as attempts to resolve security
considerations that already exist rather than introducing new ones.
However, some of the attacks described here may not have appeared in
previous email specifications.
11. References
11.1. Normative References
[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
11.2. Informative References
[BINARYSMTP] Vaudreuil, G., "SMTP Service Extensions for
Transmission of Large and Binary MIME Messages", RFC
3030, December 2000.
[CHARSET] Melnikov, A. and J. Reschke, "Update to MIME regarding
"charset" Parameter Handling in Textual Media Types",
RFC 6657, July 2012.
[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy,
Ed., "DomainKeys Identified Mail (DKIM) Signatures",
RFC 6376, September 2011.
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[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message
Format for Delivery Status Notifications", RFC 3464,
January 2003.
[EAI] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012.
[ISO-2022-CN] Zhu, HF., Hu, DY., Wang, ZG., Kao, TC., Chang, WCH.,
and M. Crispin, "Chinese Character Encoding for
Internet Messages", RFC 1922, March 1996.
[ISO-2022-JP] Murai, J., Crispin, M., and E. van der Poel, "Japanese
Character Encoding for Internet Messages", RFC 1468,
June 1993.
[ISO-2022-KR] Choi, U., Chon, K., and H. Park, "Korean Character
Encoding for Internet Messages", RFC 1557, December
1993.
[ISO-2022] ISO/IEC, "Information technology -- Character code
structure and extension techniques", ISO/IEC 2022,
1994, <http://www.iso.org/iso/
catalogue_detail.htm?csnumber=22747>.
[MIME3] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, November 1996.
[PEM89] Linn, J., "Privacy Enhancement for Internet Electronic
Mail: Part I -- Message Encipherment and Authentication
Procedures", RFC 1113, August 1989.
[PEM93] Linn, J., "Privacy Enhancement for Internet Electronic
Mail: Part I: Message Encryption and Authentication
Procedures", RFC 1421, February 1993.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts --
Communication Layers", RFC 1122, October 1989.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
April 2001.
[RFC733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
Jr., "Standard for the Format of Internet Text
Messages", RFC 733, November 1977.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
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[URI] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
RFC 3986, January 2005.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, 2003.
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Appendix A. Acknowledgements
The authors wish to acknowledge the following for their review and
constructive criticism of this proposal: Dave Cridland, Dave Crocker,
Jim Galvin, Tony Hansen, John Levine, Franck Martin, Alexey Melnikov,
and Timo Sirainen.
Authors' Addresses
Murray S. Kucherawy
EMail: superuser@gmail.com
Gregory N. Shapiro
EMail: gshapiro@proofpoint.com
Ned Freed
EMail: ned.freed@mrochek.com
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