Common Format and MIME Type for Comma-Separated Values (CSV) Files
draft-shafranovich-rfc4180-bis-02

Network Working Group                                    Y. Shafranovich
Internet-Draft                                  Nightwatch Cybersecurity
Intended status: Informational                             19 March 2022
Expires: 20 September 2022

   Common Format and MIME Type for Comma-Separated Values (CSV) Files
                   draft-shafranovich-rfc4180-bis-02

Abstract

   This RFC documents the common format used for Comma-Separated Values
   (CSV) files and updates the associated MIME type "text/csv".

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Motivation For and Status of This Document
   2.  Definition of the CSV Format
     2.1.  High level description
     2.2.  Default charset and line break values
     2.3.  ABNF Grammar
   3.  Common implementation concerns
     3.1.  Null values
     3.2.  Empty files
     3.3.  Empty lines
     3.4.  Fields spanning multiple lines
     3.5.  Unique header names
     3.6.  Whitespace outside of quoted fields
     3.7.  Other field separators
     3.8.  Escaping double quotes
     3.9.  BOM header
   4.  Update to MIME Type Registration of text/csv
     4.1.  IANA Considerations
   5.  Security Considerations
   6.  Acknowledgments
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Major changes since RFC4180
   Appendix B.  Changes since the -00 draft
   Appendix C.  Changes since the -01 draft
   Appendix D.  Note to Readers
   Author's Address

1.  Introduction

   The comma separated values format (CSV) has been used as a common way
   to exchange data between disparate systems and applications for many
   years.  Surprisingly, while this format is very popular, it has never
   been formally documented and didn't have a media type registered.
   This was addressed in 2005 via publication of [RFC4180] and the
   concurrent registration of the "text/csv" media type.

   Since the publication of [RFC4180], the CSV format has evolved and
   this specification seeks to reflect these changes as well as update
   the "text/csv" media type registration.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Motivation For and Status of This Document

   The original motivation of [RFC4180] was to provide a reference in
   order to register the media type "text/csv".  It tried to document
   existing practices at the time based on the approaches used by most
   implementations.  This document continues to do the same, and updates
   the original document to reflect current practices for generating and
   consuming of CSV files.

   Both [RFC4180] and this document are published as informational RFC
   for the benefit of the Internet community and and not intended to be
   used as formal standards.  Implementers should consult [RFC1796] and
   [RFC2026] for crucial differences between IETF standards and
   informational RFCs.

2.  Definition of the CSV Format

   While there had been various specifications and implementations for
   the CSV format (for ex.  [CREATIVYST], [EDOCEO], [CSVW] and [ART])),
   prior to publication of [RFC4180] there is no attempt to provide a
   common specification.  This section documents the format that seems
   to be followed by most implementations (incorporating changes since
   the publication of [RFC4180]).

2.1.  High level description

   1.  Each record is located on a separate line, ended by a line break
       (CR, LF or CRLF).  For example:

       aaa,bbb,cccCRLF
       zzz,yyy,xxxCRLF

   2.  The last record in the file MUST have an ending line break.  For
       example:

       aaa,bbb,cccCRLF
       zzz,yyy,xxxCRLF

   3.  The first record in the file MAY be an optional header with the
       same format as normal records.  This header will contain names
       corresponding to the fields in the file and SHOULD contain the
       same number of fields as the records in the rest of the file.
       For example:

       field_name_1,field_name_2,field_name_3CRLF
       aaa,bbb,cccCRLF
       zzz,yyy,xxxCRLF

   4.  Within each record, there MAY be one or more fields, separated by
       commas.  Each record SHOULD contain the same number of fields
       throughout the file.  Spaces are considered part of a field and
       SHOULD NOT be ignored.  The last field in the record MUST NOT be
       followed by a comma.  For example:

       aaa,bbb,cccCRLF

   5.  Each field MAY be enclosed in double quotes (however some
       programs, do not use double quotes at all).  If fields are not
       enclosed with double quotes, then double quotes MUST NOT appear
       inside the fields.  For example:

       "aaa","bbb","ccc"CRLF
       zzz,yyy,xxxCRLF

   6.  Fields containing line breaks (CR, LF or CRLF), double quotes, or
       commas MUST be enclosed in double-quotes.  The same applies for
       the first field of a record that starts with a hash to avoid the
       field from being parsed as a comment.  For example:

       "aaa","b CRLF
       bb","ccc"CRLF
       zzz,yyy,xxxCRLF
       "#aaa",#bbb,cccCRLF

   7.  A double-quote appearing inside a field MUST be escaped by
       preceding it with another double quote.  For example:

       "aaa","b""bb","ccc"CRLF

   8.  A hash sign MAY be used to mark lines that are meant to be
       commented lines.  A commented line can contain any whitespace or
       visible character until it is terminated by a line break (CR, LF
       or CRLF).  A comment line MAY appear in any line of the file
       (before or after an OPTIONAL header) but MUST NOT be mistaken
       with a subsequent line of a multi-line field.  Subsequent lines
       of multi-line fields can start with a hash sign and MUST NOT
       interpreted as comments.  For example:

       #commentCRLF
       aaa,bbb,cccCRLF
       #comment 2CRLF
       "aaa","this is CRLF
       # not a comment","ccc"CRLF

2.2.  Default charset and line break values

   Since the initial publication of [RFC4180], the default charset for
   "text/*" media types has been changed to UTF-8 (as per [RFC6657]) and
   [RFC7111].  This document reflects this change and the default
   charset for CSV files is now UTF-8.

   Although section 4.1.1. of [RFC2046] defines CRLF to denote line
   breaks, implementers MAY recognize a single CR or LF as a line break
   (similar to section 3.1.1.3 of [RFC7231]).  However, some
   implementations MAY use other values.

2.3.  ABNF Grammar

   The ABNF grammar (as per [RFC5234]) appears as follows:

file = *((comment / record) linebreak)

comment = hash *comment-data

record = first-field *(comma field)

linebreak = CR / LF / CRLF

first-field = (escaped / first-non-escaped)

field = (escaped / non-escaped)

escaped = DQUOTE *(data-with-hash / comma / CR / LF / 2DQUOTE) DQUOTE

first-non-escaped = [data *data-with-hash]

non-escaped = *data-with-hash

comma = %x2C

hash = %x23

comment-data = WSP / %x21-7E / UTF8-data
         ; characters without control characters

data = WSP / %x21 / %x24-2B / %x2D-7E / UTF8-data
         ; characters without control characters, comma, hash and DQUOTE

data-with-hash = data / hash

CR = %x0D ; as per section B.1 of [RFC5234]

DQUOTE = %x22 ; as per section B.1 of [RFC5234]

LF = %x0A ; as per section B.1 of [RFC5234]

CRLF = CR LF ; as per section B.1 of [RFC5234]

HTAB = %x09 ; as per section B.1 of [RFC5234]

SP = %x20 ; as per section B.1 of [RFC5234]

WSP = SP / HTAB ; as per section B.1 of [RFC5234]

UTF8-data = UTF8-2 / UTF8-3 / UTF8-4 ; as per section 4 of [RFC3629]

   Note that the authoritative definition of UTF-8 is in [UNICODE].

3.  Common implementation concerns

   This section describes some common concerns that may arise when
   producing or parsing CSV files.  All of these remain out of scope for
   this document and are included for awareness.  Implementers may also
   use other means to handle these use cases such as [CSVW].

3.1.  Null values

   Some implementations (such as databases) treat empty fields and null
   values differently.  For these implementations, there is a need to
   define a special value representing a null.

   Example of a CSV file with nulls (if "NULL" is used to mark nulls):

   field_name_1,field_name_2,field_name_3CRLF
   aaa,bbb,cccCRLF
   zzz,NULL,xxxCRLF

3.2.  Empty files

   Implementers should be aware that in accordance to this specification
   a file does not need to contain any comments or records (empty file
   with zero bytes).

3.3.  Empty lines

   This specification recommends but doesn't require having the same
   number of fields in every line.  This allows CSV files to have empty
   lines without any fields at all.  Some implementations can be
   configured to skip empty lines instead of parsing them.

   Example of a CSV file with empty lines:

   field_name_1,field_name_2,field_name_3CRLF
   aaa,bbb,cccCRLF
   CRLF
   zzz,yyy,xxxCRLF

   However, if the records are only made up of one field it is not
   possible to differentiate between an empty line, and an empty and
   unquoted field.  This differentiation might play an important role in
   some implementations such as database exports/imports.

   Example of a CSV file with empty lines and only one field per record:

   aaa
   CRLF
   bbbCRLF

3.4.  Fields spanning multiple lines

   When quoted fields are used, it is possible for a field to span
   multiple lines, even when line breaks appear within such field.

3.5.  Unique header names

   Implementers should be aware that some applications may treat header
   values as unique (either case-sensitive or case-insensitive).

3.6.  Whitespace outside of quoted fields

   When quoted fields are used, this document does not allow whitespace
   between double quotes and commas.  Implementers should be aware that
   some applications may be more lenient and allow whitespace outside
   the double quotes.

3.7.  Other field separators

   This document defines a comma as a field separator but implementers
   should be aware that some applications may use different values,
   especially with non-English languages.  Those are outside the scope
   of this document and implementers should consult other efforts such
   as [CSVW].

3.8.  Escaping double quotes

   This document prescribes that a double-quote appearing inside a field
   must be escaped by preceding it with another double quote.
   Implementers should be aware that some applications may choose to use
   a different escaping mechanism.

3.9.  BOM header

   Applications that create text files with unicode character encoding
   might write a BOM (byte order mark) header in order to support
   multiple unicode encodings (like UTF-16 and UTF-32).  Some
   applications might be able to read and properly interpret such a
   header, others could break.  Implementors should review section 6 of
   [RFC3629] and section 23.8 of [UNICODE].

4.  Update to MIME Type Registration of text/csv

   The media type registration of "text/csv" should be updated as per
   specific fields below:

   Encoding considerations:

      CSV MIME entities can consist of binary data as per section 4.8 of
      [RFC6838].  Although section 4.1.1. of [RFC2046] defines CRLF to
      denote line breaks, implementers MAY recognize a single CR or LF
      as a line break (similar to section 3.1.1.3 of [RFC7231]).
      However, some implementations may use other values.

   Published specification:

      While numerous private specifications exist for various programs
      and systems, there is no single "master" specification for this
      format.  An attempt at a common definition can be found in
      [RFC4180] and this document.  Implementers should note that both
      documents are informational in nature and are not standards.

   Optional parameters: charset

      The "charset" parameter specifies the charset employed by the CSV
      content.  In accordance with [RFC6657], the charset parameter
      SHOULD be used, and if it is not present, UTF-8 SHOULD be assumed
      as the default (this implies that US- ASCII CSV will work, even
      when not specifying the "charset" parameter).  Any charset defined
      by IANA for the "text" tree may be used in conjunction with the
      "charset" parameter.

   Security considerations:

      Text/csv consists of nothing but passive text data that should not
      pose any direct risks.  However, it is possible that malicious
      data may be included in order to exploit buffer overruns or other
      bugs in the program processing the text/csv data.

      Implementers and users should also be aware that some software
      applications may interpret certain characters in the beginning of
      CSV fields as referring to code or formulas, thus resulting in
      malicious code execution.  This is known as "CSV injection" and
      users consuming CSV files should filter out such characters.

      The text/csv format provides no confidentiality or integrity
      protection, so if such protections are needed they must be
      supplied externally.

      The fact that software implementing fragment identifiers for CSV
      and software not implementing them differs in behavior, and the
      fact that different software may show documents or fragments to
      users in different ways, can lead to misunderstandings on the part
      of users.  Such misunderstandings might be exploited in a way
      similar to spoofing or phishing.

      Implementers and users of fragment identifiers for CSV text should
      also be aware of the security considerations in RFC 3986 [RFC3986]
      and RFC 3987 [RFC3987].

   Interoperability considerations:

      Due to lack of a single specification, there are considerable
      differences among implementations.  Implementers should "be
      conservative in what you do, be liberal in what you accept from
      others" ([RFC0793]) when processing CSV files.  An attempt at a
      common definition can be found in Section 2.

4.1.  IANA Considerations

   IANA is directed to update the MIME type registration for "text/csv"
   as per instructions provided in Section 4 of this document and
   include a reference to this document within the registration.

5.  Security Considerations

   All security considerations discussed in Section 4 still apply.

6.  Acknowledgments

   In addition to everyone thanked previously in [RFC4180], the author
   would like to thank acknowledge the contributions of the following
   people to this document: Alperen Belgic, Abed BenBrahim, Damon Koach,
   Barry Leiba, Oliver Siegmar, Marco Diniz Sousa and Greg Skinner.

   A special thank you to L.T.S.

7.  References

7.1.  Normative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4180]  Shafranovich, Y., "Common Format and MIME Type for Comma-
              Separated Values (CSV) Files", RFC 4180,
              DOI 10.17487/RFC4180, October 2005,
              <https://www.rfc-editor.org/info/rfc4180>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6657]  Melnikov, A. and J. Reschke, "Update to MIME regarding
              "charset" Parameter Handling in Textual Media Types",
              RFC 6657, DOI 10.17487/RFC6657, July 2012,
              <https://www.rfc-editor.org/info/rfc6657>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC7111]  Hausenblas, M., Wilde, E., and J. Tennison, "URI Fragment
              Identifiers for the text/csv Media Type", RFC 7111,
              DOI 10.17487/RFC7111, January 2014,
              <https://www.rfc-editor.org/info/rfc7111>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [ART]      Raymond, E., "The Art of Unix Programming, Chapter 5",
              September 2003,
              <http://www.catb.org/~esr/writings/taoup/html/
              ch05s02.html>.

   [CREATIVYST]
              Repici, J., "HOW-TO: The Comma Separated Value (CSV) File
              Format", 2010,
              <http://www.creativyst.com/Doc/Articles/CSV/CSV01.htm>.

   [CSVW]     W3C, "CSV on the Web Working Group", 2016,
              <https://www.w3.org/2013/csvw/wiki/Main_Page>.

   [EDOCEO]   Edoceo, Inc., "Comma Separated Values (CSV) Standard File
              Format", 2020, <https://edoceo.com/dev/csv-file-format>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1796]  Huitema, C., Postel, J., and S. Crocker, "Not All RFCs are
              Standards", RFC 1796, DOI 10.17487/RFC1796, April 1995,
              <https://www.rfc-editor.org/info/rfc1796>.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
              <https://www.rfc-editor.org/info/rfc2026>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <https://www.rfc-editor.org/info/rfc3987>.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
              13.0.0", March 2020,
              <https://www.unicode.org/versions/Unicode13.0.0/>.

Appendix A.  Major changes since [RFC4180]

   *  Added a section clarifying motivation for this document and
      standards status

   *  Changing default encoding to UTF-8 and adding Unicode to the ABNF
      grammar

   *  Allowing CR, LF and CRLF for line breaks

   *  Allowing HTAB in text data

   *  Mandating a line break at the end of the last line in the file

   *  Making records and headers optional, thus allowing for an empty
      file

   *  Adding definition of commented lines

   *  Adding a section on common implementation concerns

   *  Removed "header" parameter for the MIME type since it is not used

Appendix B.  Changes since the -00 draft

   *  Added CSV injection to security considerations (#30

   *  Added a reference to RFC 7111 (#27)

Appendix C.  Changes since the -01 draft

   *  No changes yet, refreshed to keep draft alive

Appendix D.  Note to Readers

      *Note to the RFC Editor:* Please remove this section prior to
      publication.

   Development of this draft takes place on Github at:
   https://github.com/nightwatchcybersecurity/rfc4180-bis

   Comments can also be sent to the ART mailing list at:
   https://www.ietf.org/mailman/listinfo/art

   Full list of changes can be viewed via the IETF document tracker:
   https://tools.ietf.org/html/draft-shafranovich-rfc4180-bis

Author's Address

   Yakov Shafranovich
   Nightwatch Cybersecurity

   Email: yakov+ietf@nightwatchcybersecurity.com