Network Working Group                                         S. Leonard
Internet-Draft                                             Penango, Inc.
Intended Status: Informational                         September 4, 2015
Expires:                             March 6, 2016
Expires: September 7, 2016

                       Windows Image Media Types
                     draft-seantek-windows-image-01
                     draft-seantek-windows-image-02

Abstract

   This document registers media types for certain image formats
   promulgated in Microsoft Windows, namely image/wmf, image/x-wmf,
   image/emf, image/x-emf, and image/bmp for use with Windows Metafile,
   Enhanced Metafile, and Windows Bitmap formats. Originally designed
   for Microsoft Windows 2.0 and 3.0, these image files are intended to
   be portable between applications and devices, and may contain both
   vector and raster graphics.

Status of this Memo

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1. Introduction

1.1. Windows Metafiles

   Long before the invention of Scalable Vector Graphics, Microsoft
   Corporation recognized the value of recording images in a format that
   its applications and operating systems could easily render
   irrespective of the output device. With the release of Windows 3.0,
   Microsoft released its Windows Metafile (WMF) format, which can
   contain vector and raster graphics in one package. As a binary format
   that needed to work on 16-bit machines, WMF is comprised of a
   sequence of record structures. Each record contains drawing commands,
   object definitions, and configuration settings. When a metafile is
   processed, the image can be rendered on a display, output to a
   printer or plotter, stored in memory, or saved to some persistent
   storage. Reflecting the relationship to the Windows Graphics Device
   Interface (GDI) API, WMF metafiles are "played" to a playback device
   context in the same manner that PostScript content is treated as an
   executable program that results in the output of the original
   document.

   As Microsoft's first 32-bit operating system, Windows NT 3.1
   introduced an overhaul to the Windows API ("Win32") and the in-memory
   formats upon which those APIs relied. The Enhanced Metafile (EMF)
   format was created at this time, using 32-bit values instead of WMF's
   16-bit values. In Windows XP, Microsoft extended EMF with "EMF+",
   adding support for Windows GDI+.

   Many implementations of WMF and EMF were created because of Windows'
   commercial success in the 1990s. A large body of free and
   commercially available clip art and other artwork exists in this
   format. Furthermore, WMF content appears non-normatively in certain
   standards (e.g., [OOXML]); the usage is common enough that an
   implementer would almost certainly need to support it for basic
   interoperability.

   Microsoft publicly documented the WMF format as early as the 1992
   Windows 3.1 SDK. Since 2007 Microsoft has released the format
   specifications [MS-WMF] [MS-WMF], [MS-EMF], and [MS-EMF] [MS-EMF+] under its Open
   Specification Promise [MS-OSP].

1.2. Windows Bitmaps

   Long before the invention of Portable Network Graphics (PNG),
   Microsoft Corporation and IBM Corporation needed to record images in
   a format that their applications and operating systems could easily
   render on low-end machines (Intel 80286). The resulting "BMP" format
   contains a single raster graphic with basic header fields that can be
   easily mapped (or "blitted") to locations in memory. As computing
   moved from 16-bit to 32-bit, BMP evolved to contain 32-bit
   structures. As the 90s wore on, the venerable BMP got boosts with
   support for additional color spaces, color profiles, and compression
   formats. The same basic format can be used to convey 2-bit black-and-
   white bitmaps with a 1-bit alpha mask from the '80s, and full-color
   Ultra HD images on leading-edge displays. BMP is a building block of
   other formats, including Windows Metafiles, Windows Icons, and
   Windows Cursors.

   Many implementations of BMP were created because of Windows'
   commercial success in the 1990s. Usage of the format for interchange
   has declined since the advent of PNG (for lossless raster graphics)
   and JPEG (for lossy raster graphics); however, a large body of free
   and commercially available BMP artwork exists. Since Windows Icons
   are a building block of "favicon.ico" Web technology, an implementer
   would almost certainly need to support this format for basic
   interoperability.

   Microsoft publicly documented the BMP format as early as the 1992
   Windows 3.1 SDK (in the Windows Metafile documentation). Since 2007
   Microsoft has released the format specification [MS-WMF], which
   includes most components of the Windows Bitmap format, under its Open
   Specification Promise [MS-OSP]. See Section 2.2.2.9 of [MS-WMF]
   (DeviceIndependentBitmap Object). BMP data begins with a
   BITMAPFILEHEADER and is followed by one of the bitmap headers
   (BITMAPINFOHEADER, BITMAPV4HEADER, or BITMAPV5HEADER), optional color
   table data, bitmap data, and optional profile data, in that order
   [BMPSTOR].

1.3. Definitions

   The key word "SHOULD" in this document is to be interpreted as
   described in [RFC2119].

2. Windows Metafile Media Type Registration Application

   Type name: image

   Subtype name: wmf

   Required parameters: None.

   Optional parameters:

    DEFAULT_CHARSET: The character set intended when the CharacterSet
     Enumeration (see [MS-WMF]) specifies DEFAULT_CHARSET. The value of
     this parameter is a charset name defined in accordance to the
     procedures laid out in [RFC2978]. When this parameter is not
     specified, DEFAULT_CHARSET has the following meaning in [MS-WMF]:
     "a character set based on the current system locale; for example,
     when the system locale is United States English, the default
     character set is ANSI_CHARSET" (which is Windows-1252, more-or-
     less). I.e., when not specified, the default character set is
     system-dependent. As this optional parameter is novel, content
     producers embedding text SHOULD use EMF instead of WMF (or if
     absolutely necessary, SHOULD embed EMF within WMF).

   Encoding considerations: Binary.

   Security considerations:

     The Windows Metafile format's security history is punctuated in
     2005-2006 with the disclosure of the Metafile Image Code Execution
     vulnerability, codenamed MICE. MICE won the 2007 Pwnie Award for
     "Mass 0wnage" and "Breaking the Internet" [PWNIES07]. The official
     Microsoft security bulletin [MICE] describes that the flaw occurs
     because Windows Metafiles can set the SETABORTPROC value of the
     MetafileEscapes enumeration (accessible via the META_ESCAPE
     record), allowing for arbitrary code execution.

     Windows Metafiles can contain Enhanced Metafiles using the
     META_ESCAPE_ENHANCED_METAFILE record; thus, the security
     considerations of EMF apply to WMF.

     Windows Metafiles are historically very buggy. As the original
     intent was to replicate Windows GDI calls, flaws in GDI, or in a
     display or printer driver implementing the back-end to GDI, could
     be exploitable. WMF implementations not backed by Windows GDI have
     different risks: namely, while a malicious WMF author may not
     consider the non-Windows GDI implementation as a primary target,
     WMF has many "corner case" records for which an implementation's
     processing may not have received the same level of scrutiny as the
     Windows implementation. "Fuzzing" the implementation is
     appropriate.

   Interoperability considerations:

     Windows Metafile is the original 16-bit metafile format; it was
     released in 1990 at what some computer historians might consider
     the "zenith" of the desktop publishing revolution. Accordingly,
     there is a large body of free and commercially available clip art
     that is still in use, either independently or embedded in
     productivity documents (word processing documents, desktop
     publishing documents, slideshows and presentations, and
     spreadsheets and workbooks). For example, references to WMF content
     appear (non-normatively) in Office Open XML [OOXML]. To say that
     support for this format is necessary for interoperability would not
     be an understatement.

     Accommodations for comments or arbitrary data storage in Windows
     Metafiles are virtually non-existent. However, Windows Metafiles
     can contain Enhanced Metafiles using the
     META_ESCAPE_ENHANCED_METAFILE record; an implementation SHOULD be
     able to handle both types. Windows Metafiles can store and output
     text strings (see META_TEXTOUT and META_EXTTEXTOUT records), but
     the encodings of the strings may be ambiguous. Unicode encodings
     are not possible without the DEFAULT_CHARSET parameter defined in
     this registration.

     The previously unregistered type "image/x-wmf" is also in wide use.
     Accordingly, it is registered as a deprecated alias. See Appendix A
     and Section 4.2.9 of [RFC6838].

   Published specification: [MS-WMF].

   Applications that use this media type:

     Office productivity applications; clip art applications; desktop
     publishing applications; some Web browsers (e.g., Internet
     Explorer).

   Fragment identifier considerations: None.

   Additional information:

     Deprecated alias names for this type: image/x-wmf
     Magic number(s): D7 CD C6 9A (little-endian DWORD 0x9AC6CDD7)
     File extension(s): .wmf
     Macintosh file type code(s):
       None. A uniform type identifier (UTI) of "com.microsoft.wmf" is
       RECOMMENDED.

   Person & email address to contact for further information:

     Sean Leonard <dev+ietf@seantek.com>

   Restrictions on usage: None.

   Author/Change controller: Sean Leonard <dev+ietf@seantek.com>

   Intended usage: COMMON

   Provisional registration? No

3. Enhanced Metafile Media Type Registration Application

   Type name: image

   Subtype name: emf

   Required parameters: None.

   Optional parameters: None.

   Encoding considerations: Binary.

   Security considerations:

     Enhanced Metafiles are not afflicted with [MICE]. There has been no
     public disclosure of vulnerabilities specific to EMF or EMF+ to
     date. Nonetheless:

     Enhanced Metafiles can contain Encapsulated PostScript (EPS) data;
     thus the security considerations of PostScript processing may also
     apply to EMF.

     As the original intent was to replicate Windows GDI calls, flaws in
     GDI, or in a display or printer driver implementing the back-end to
     GDI, could be exploitable with maliciously crafted EMF content. EMF
     implementations not backed by Windows GDI have different risks:
     namely, while a malicious EMF author may not consider the non-
     Windows GDI implementation as a primary target, EMF has many
     "corner case" records for which an implementation's processing may
     not have received the same level of scrutiny as the Windows
     implementation. "Fuzzing" the implementation is appropriate. It is
     also possible that EMF+ data is "safe" while EMF data contains an
     exploit (or vice-versa); the EMF+-aware implementation (such as an
     application designed for GDI+ on Windows XP or above) would skip
     the "unsafe" data while another implementation would fall prey to
     the exploit.

   Interoperability considerations:

     Enhanced Metafile is the 32-bit metafile format; it was released in
     1992 along with Windows NT 3.1. There is a large body of free and
     commercially available clip art that is still in use, either
     independently or embedded in productivity documents (word
     processing documents, desktop publishing documents, slideshows and
     presentations, and spreadsheets and workbooks). To say that support
     for this format is necessary for interoperability would not be an
     understatement.

     Enhanced Metafiles have extensive accommodations for comments and
     arbitrary data storage. Enhanced Metafiles can store and output
     text strings. Mercifully, the encodings of these strings are well-
     defined. Record examples include EMR_EXTTEXTOUTA (US-ASCII),
     EMR_EXTTEXTOUTW (UTF16-LE), EMR_POLYTEXTOUTA (US-ASCII),
     EMR_POLYTEXTOUTW (UTF16-LE), and EMR_SMALLTEXTOUT (UTF16-LE or the
     low-order 8 bits of UTF16-LE--effectively ISO-8859-1--depending on
     ETO_SMALL_CHARS).

     Enhanced Metafiles can contain Encapsulated PostScript (EPS) data
     in the EpsData object [MS-EMF]. The FormatSignature EPS_SIGNATURE
     (0x46535045, in little-endian) is used instead of ENHMETA_SIGNAUTRE
     (0x464D4520, in little-endian) in such a case.

     Windows XP introduced the GDI+ API, along with EMF+ [MS-EMF+]. EMF+
     is actually an embedded format in which GDI+ commands are stored as
     EMF comment records (EMR_COMMENT_EMFPLUS record type). Content
     containing EMF+ data can be identified as "EMF+ Only" (only EMF+;
     the EMF records are not sufficient to reconstitute the drawing) or
     "EMF+ Dual" (both EMF records alone or EMF+ records alone, when
     played back, are sufficient to reconstitute the drawing) [MS-EMF+].
     Support for EMF+ records may not be as extensive as support for the
     original EMF records.

     The previously unregistered type "image/x-emf" is also in wide use.
     Accordingly, it is registered as a deprecated alias. See Appendix A
     and Section 4.2.9 of [RFC6838].

   Published specification: [MS-EMF] and [MS-EMF+].

   Applications that use this media type:

     Office productivity applications; clip art applications; desktop
     publishing applications; some Web browsers (e.g., Internet
     Explorer).

   Fragment identifier considerations: None.

   Additional information:

     Deprecated alias names for this type: image/x-emf
     Magic number(s): 01 00 00 00 (little-endian DWORD 0x00000001),
                      corresponding to the EMR_HEADER Type field.
                      The next field (EMR_HEADER Size) should be
                      at least 88 (little-endian DWORD 0x00000050).
     File extension(s): .emf
                        (for both EMF and EMF+ content)
     Macintosh file type code(s):

       None. A uniform type identifier (UTI) of "com.microsoft.emf" is
       RECOMMENDED.

   Person & email address to contact for further information:

     Sean Leonard <dev+ietf@seantek.com>

   Restrictions on usage: None.

   Author/Change controller: Sean Leonard <dev+ietf@seantek.com>

   Intended usage: COMMON

   Provisional registration? No

4. Windows Bitmap Media Type Registration Application

   Type name: image

   Subtype name: bmp

   Required parameters: None.

   Optional parameters: None.

   Encoding considerations: Binary.

   Security considerations:

     Bitmaps have a mostly unremarkable security history.

     Because BMP data can encapsulate JPEG or PNG data (BI_JPEG, BI_PNG
     values of the Compression enumeration in Section 2.1.1.7 of [MS-
     WMF]), the security considerations of JPEG and PNG processing may
     also apply to BMP.

   Interoperability considerations:

     Uncompressed Windows Bitmaps can be rather large. If there is a
     need to compress an image, modern applications SHOULD consider
     emitting JPEG or PNG data instead of embedding them in BMP
     payloads.

   Published specification: [MS-WMF] and [BMPSTOR].

   Applications that use this media type:

     Office productivity applications; clip art applications; desktop
     publishing applications; Web browsers; graphics processing
     applications.

   Fragment identifier considerations: None.

   Additional information:

     Magic number(s): 42 4D ("BM"), meaning "bitmap". The next
                      field (BITMAPFILEHEADER bfSize) is a
                      little-endian DWORD indicating the size
                      of the bitmap content in bytes.
     File extension(s): .bmp, .dib
     Macintosh file type code(s):
       "BMP ", "BMPf", or "BMPp". Apple has promulgated a
       uniform type identifier (UTI) of "com.microsoft.bmp".

   Person & email address to contact for further information:

     Sean Leonard <dev+ietf@seantek.com>

   Restrictions on usage: None.

   Author/Change controller: Sean Leonard <dev+ietf@seantek.com>

   Intended usage: COMMON

   Provisional registration? No

5.  IANA Considerations

   IANA is asked to register the media types image/wmf, image/x-wmf,
   image/emf, image/x-emf, and image/bmp in the Standards tree using the
   applications provided in Sections 2, 3, and 4 of this document.

5. Security Considerations

   See the registration templates for their respective security
   considerations.

6. References

6.1. Normative References

   [BMPSTOR]  Microsoft Corporation, "Bitmap Storage",
              MSDN ID dd183391, 2014,
              <http://msdn.microsoft.com/library/dd183391>.

   [MS-WMF]   Microsoft Corporation, "Windows Metafile Format",
              [MS-WMF], v20140502 (Rev 11.1), May 2014,
              <http://msdn.microsoft.com/library/cc250370>.

   [MS-EMF]   Microsoft Corporation, "Enhanced Metafile Format",
              [MS-EMF], v20140502 (Rev 10.0), May 2014,
              <http://msdn.microsoft.com/library/cc230514>.

   [MS-EMF+]  Microsoft Corporation, "Enhanced Metafile Format Plus
              Extensions", [MS-EMFPLUS], v20140502 (Rev 13.0), May 2014,
              <http://msdn.microsoft.com/library/cc230724>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, October 2000.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, January 2013.

6.2. Informative References

   [MICE]     Microsoft Corporation, "Vulnerability in Graphics
              Rendering Engine Could Allow Remote Code Execution
              (912919)", MS06-001, V1.0, January 2006,
              <https://technet.microsoft.com/library/security/ms06-001>.

   [MS-OSP]   Microsoft Corporation, "Open Specification Promise",
              February 2007,
              <http://www.microsoft.com/interop/osp/default.mspx>.

   [OOXML]    Ecma International, "Office Open XML File Formats",
              Standard ECMA-376, Fourth Edition, ISO/IEC 29500, December
              2012, <http://www.ecma-international.org/publications/
              standards/Ecma-376.htm>.

   [PWNIES07] Pwnie Awards LLC, "Pwnie Awards 2007", 2007,
              <http://pwnies.com/archive/2007/winners/>.

Author's Address

   Sean Leonard
   Penango, Inc.
   5900 Wilshire Boulevard
   21st Floor
   Los Angeles, CA  90036
   USA

   EMail: dev+ietf@seantek.com
   URI:   http://www.penango.com/