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Internet Engineering Task Force (IETF)                      M. KucherawyRequest for Comments: 7103                                    G. ShapiroCategory: Informational                                         N. FreedISSN: 2070-1721                                             January 2014Advice for Safe Handling of Malformed MessagesAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7103.Kucherawy, et al.             Informational                     [Page 1]

RFC 7103                   Safe Mail Handling               January 2014Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Kucherawy, et al.             Informational                     [Page 2]

RFC 7103                   Safe Mail Handling               January 2014Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .31.1.  The Purpose of This Work  . . . . . . . . . . . . . . . .31.2.  Not the Purpose of This Work  . . . . . . . . . . . . . .41.3.  General Considerations  . . . . . . . . . . . . . . . . .42.  Document Conventions  . . . . . . . . . . . . . . . . . . . .52.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .53.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .54.  Invariant Content . . . . . . . . . . . . . . . . . . . . . .55.  Mail Submission Agents  . . . . . . . . . . . . . . . . . . .66.  Line Termination  . . . . . . . . . . . . . . . . . . . . . .77.  Header Anomalies  . . . . . . . . . . . . . . . . . . . . . .87.1.  Converting Obsolete and Invalid Syntaxes  . . . . . . . .87.1.1.  Host-Address Syntax . . . . . . . . . . . . . . . . .87.1.2.  Excessive Angle Brackets  . . . . . . . . . . . . . .87.1.3.  Unbalanced Angle Brackets . . . . . . . . . . . . . .87.1.4.  Unbalanced Parentheses  . . . . . . . . . . . . . . .97.1.5.  Commas in Address Lists . . . . . . . . . . . . . . .97.1.6.  Unbalanced Quotes . . . . . . . . . . . . . . . . . .107.1.7.  Naked Local-Parts . . . . . . . . . . . . . . . . . .107.2.  Non-Header Lines  . . . . . . . . . . . . . . . . . . . .107.3.  Unusual Spacing . . . . . . . . . . . . . . . . . . . . .127.4.  Header Malformations  . . . . . . . . . . . . . . . . . .137.5.  Header Field Counts . . . . . . . . . . . . . . . . . . .137.5.1.  Repeated Header Fields  . . . . . . . . . . . . . . .147.5.2.  Missing Header Fields . . . . . . . . . . . . . . . .157.5.3.  Return-Path . . . . . . . . . . . . . . . . . . . . .167.6.  Missing or Incorrect Charset Information  . . . . . . . .167.7.  Eight-Bit Data  . . . . . . . . . . . . . . . . . . . . .188.  MIME Anomalies  . . . . . . . . . . . . . . . . . . . . . . .188.1.  Missing MIME-Version Field  . . . . . . . . . . . . . . .198.2.  Faulty Encodings  . . . . . . . . . . . . . . . . . . . .199.  Body Anomalies  . . . . . . . . . . . . . . . . . . . . . . .199.1.  Oversized Lines . . . . . . . . . . . . . . . . . . . . .1910. Security Considerations . . . . . . . . . . . . . . . . . . .2011. References  . . . . . . . . . . . . . . . . . . . . . . . . .2011.1.  Normative References . . . . . . . . . . . . . . . . . .2011.2.  Informative References . . . . . . . . . . . . . . . . .20Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .23Kucherawy, et al.             Informational                     [Page 3]

RFC 7103                   Safe Mail Handling               January 20141.  Introduction1.1.  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 typicallyKucherawy, et al.             Informational                     [Page 4]

RFC 7103                   Safe Mail Handling               January 2014   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.Kucherawy, et al.             Informational                     [Page 5]

RFC 7103                   Safe Mail Handling               January 2014   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 Conventions2.1.  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 (andKucherawy, et al.             Informational                     [Page 6]

RFC 7103                   Safe Mail Handling               January 2014   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.Kucherawy, et al.             Informational                     [Page 7]

RFC 7103                   Safe Mail Handling               January 2014   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.Kucherawy, et al.             Informational                     [Page 8]

RFC 7103                   Safe Mail Handling               January 20147.  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>Kucherawy, et al.             Informational                     [Page 9]

RFC 7103                   Safe Mail Handling               January 2014   The following:       To: second@example.org>   can usually be treated as:       To: second@example.org7.1.4.  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.netKucherawy, et al.             Informational                    [Page 10]

RFC 7103                   Safe Mail Handling               January 20147.1.6.  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 inSection 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}Kucherawy, et al.             Informational                    [Page 11]

RFC 7103                   Safe Mail Handling               January 2014   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.Kucherawy, et al.             Informational                    [Page 12]

RFC 7103                   Safe Mail Handling               January 2014   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.Kucherawy, et al.             Informational                    [Page 13]

RFC 7103                   Safe Mail Handling               January 20147.4.  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".  AnKucherawy, et al.             Informational                    [Page 14]

RFC 7103                   Safe Mail Handling               January 2014   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 (seeSection 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:Kucherawy, et al.             Informational                    [Page 15]

RFC 7103                   Safe Mail Handling               January 2014       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.Kucherawy, et al.             Informational                    [Page 16]

RFC 7103                   Safe Mail Handling               January 2014   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   Sections3.6 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 isKucherawy, et al.             Informational                    [Page 17]

RFC 7103                   Safe Mail Handling               January 2014   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.Kucherawy, et al.             Informational                    [Page 18]

RFC 7103                   Safe Mail Handling               January 20147.7.  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 inSection 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.Kucherawy, et al.             Informational                    [Page 19]

RFC 7103                   Safe Mail Handling               January 20148.1.  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 Anomalies9.1.  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:Kucherawy, et al.             Informational                    [Page 20]

RFC 7103                   Safe Mail Handling               January 2014   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.  References11.1.  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",RFC3030, 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.Kucherawy, et al.             Informational                    [Page 21]

RFC 7103                   Safe Mail Handling               January 2014   [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.Kucherawy, et al.             Informational                    [Page 22]

RFC 7103                   Safe Mail Handling               January 2014   [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.Kucherawy, et al.             Informational                    [Page 23]

RFC 7103                   Safe Mail Handling               January 2014Appendix 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.comKucherawy, et al.             Informational                    [Page 24]