Internet DRAFT - draft-seantek-image-wmf-emf
draft-seantek-image-wmf-emf
Network Working Group S. Leonard
Internet-Draft Penango, Inc.
Intended Status: Informational October 17, 2014
Expires: April 20, 2015
The Windows Metafile and Enhanced Metafile Media Types
draft-seantek-image-wmf-emf-00
Abstract
This document registers the image/wmf and image/emf media types for
use with Windows Metafile and Enhanced Metafile formats. Originally
designed for Microsoft Windows 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
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] and [MS-EMF] under its Open Specification
Promise [MS-OSP].
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
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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
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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):
????. A uniform type identifier (UTI) of "????" is RECOMMENDED.
Person & email address to contact for further information:
Sean Leonard <dev+ietf@seantek.com>
Restrictions on usage: None.
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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
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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),
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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):
????. A uniform type identifier (UTI) of "????" 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. IANA Considerations
IANA is asked to register the media types image/wmf and image/emf in
the Standards tree using the applications provided in Sections 2 and
3 of this document.
5. Security Considerations
See the image/wmf and image/emf registration templates for their
respective security considerations.
6. References
6.1. Normative References
[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
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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/
Leonard Exp. April 20, 2015 [Page 8]
