Network Working Group B. Hoehrmann
Request for Comments: 4329 April 2006
Category: Informational
Scripting Media Types
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes the registration of media types for the
ECMAScript and JavaScript programming languages and conformance
requirements for implementations of these types.
Table of Contents
1. Introduction ....................................................2
2. Conformance and Document Conventions ............................2
3. Deployed Scripting Media Types and Compatibility ................2
4. Character Encoding Scheme Handling ..............................4
4.1. Charset Parameter ..........................................4
4.2. Character Encoding Scheme Detection ........................4
4.3. Character Encoding Scheme Error Handling ...................6
5. Security Considerations .........................................6
6. IANA Considerations .............................................8
7. JavaScript Media Types ..........................................9
7.1. text/javascript (obsolete) .................................9
7.2. application/javascript ....................................10
8. ECMAScript Media Types .........................................11
8.1. text/ecmascript (obsolete) ................................11
8.2. application/ecmascript ....................................12
9. References .....................................................13
9.1. Normative References ......................................13
9.2. Informative References ....................................13
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1. Introduction
This memo describes media types for the JavaScript and ECMAScript
programming languages. Refer to "Brief History" and "Overview" in
[ECMA] for background information on these languages.
Programs written in these programming languages have historically
been interchanged using inapplicable, experimental, and unregistered
media types. This document defines four of the most commonly used
media types for such programs to reflect this usage in the IANA media
type registry, to foster interoperability by defining underspecified
aspects, and to provide general security considerations.
2. Conformance and Document Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, [RFC2119] and
indicate requirement levels for compliant implementations.
Requirements apply to all implementations unless otherwise stated.
An implementation is a software module that supports one of the media
types defined in this document. Software modules may support
multiple media types but conformance is considered individually for
each type.
Implementations that fail to satisfy one or more "MUST" requirements
are considered non-compliant. Implementations that satisfy all
"MUST" requirements, but fail to satisfy one or more "SHOULD"
requirements, are said to be "conditionally compliant". All other
implementations are "unconditionally compliant".
3. Deployed Scripting Media Types and Compatibility
Various unregistered media types have been used in an ad-hoc fashion
to label and exchange programs written in ECMAScript and JavaScript.
These include:
+-----------------------------------------------------+
| text/javascript | text/ecmascript |
| text/javascript1.0 | text/javascript1.1 |
| text/javascript1.2 | text/javascript1.3 |
| text/javascript1.4 | text/javascript1.5 |
| text/jscript | text/livescript |
| text/x-javascript | text/x-ecmascript |
| application/x-javascript | application/x-ecmascript |
| application/javascript | application/ecmascript |
+-----------------------------------------------------+
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Use of the "text" top-level type for this kind of content is known to
be problematic. This document thus defines text/javascript and text/
ecmascript but marks them as "obsolete". Use of experimental and
unregistered media types, as listed in part above, is discouraged.
The media types,
* application/javascript
* application/ecmascript
which are also defined in this document, are intended for common use
and should be used instead.
This document defines equivalent processing requirements for the
types text/javascript, text/ecmascript, and application/javascript.
Use of and support for the media type application/ecmascript is
considerably less widespread than for other media types defined in
this document. Using that to its advantage, this document defines
stricter processing rules for this type to foster more interoperable
processing.
The types defined in this document are applicable to scripts written
in [JS15] and [ECMA], respectively, as well as to scripts written in
a compatible language or profile such as [EcmaCompact].
This document does not address scripts written in other languages.
In particular, future versions of JavaScript, future editions of
[ECMA], and extensions to [ECMA], such as [E4X], are not directly
addressed. This document may be updated to take other content into
account.
Updates of this document may introduce new optional parameters;
implementations MUST consider the impact of such an update. For the
application/ecmascript media type, implementations MUST NOT process
content labeled with a "version" parameter as if no such parameter
had been specified; this is typically achieved by treating the
content as unsupported. This error handling behavior allows
extending the definition of the media type for content that cannot be
processed by implementations of [ECMA].
The programming languages defined in [JS15] and [ECMA] share a common
subset. Choice of a type for scripts compatible with both languages
is out of the scope of this document.
This document does not define how fragment identifiers in resource
identifiers ([RFC3986], [RFC3987]) for documents labeled with one of
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the media types defined in this document are resolved. An update of
this document may define processing of fragment identifiers.
4. Character Encoding Scheme Handling
Refer to [RFC3536] for a discussion of terminology used in this
section. Source text (as defined in [ECMA], section 6) can be binary
source text. Binary source text is a textual data object that
represents source text encoded using a character encoding scheme. A
textual data object is a whole text protocol message or a whole text
document, or a part of it, that is treated separately for purposes of
external storage and retrieval. An implementation's internal
representation of source text and source text are not considered
binary source text.
Implementations need to determine a character encoding scheme in
order to decode binary source text to source text. The media types
defined in this document allow an optional charset parameter to
explicitly specify the character encoding scheme used to encode the
source text.
How implementations determine the character encoding scheme can be
subject to processing rules that are out of the scope of this
document. For example, transport protocols can require that a
specific character encoding scheme is to be assumed if the optional
charset parameter is not specified, or they can require that the
charset parameter is used in certain cases. Such requirements are
not considered part of this document.
Implementations that support binary source text MUST support binary
source text encoded using the UTF-8 [RFC3629] character encoding
scheme. Other character encoding schemes MAY be supported. Use of
UTF-8 to encode binary source text is encouraged but not required.
4.1. Charset Parameter
The charset parameter provides a means to specify the character
encoding scheme of binary source text. Its value MUST match the
mime-charset production defined in [RFC2978], section 2.3, and SHOULD
be a registered charset [CHARSETS]. An illegal value is a value that
does not match that production.
4.2. Character Encoding Scheme Detection
It is possible that implementations cannot interoperably determine a
single character encoding scheme simply by complying with all
requirements of the applicable specifications. To foster
interoperability in such cases, the following algorithm is defined.
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Implementations apply this algorithm until a single character
encoding scheme is determined.
1. If a charset parameter with a legal value is specified, the value
determines the character encoding scheme.
2. If the binary source text starts with a Unicode encoding form
signature, the signature determines the encoding. The following
octet sequences, at the very beginning of the binary source text,
are considered with their corresponding character encoding
schemes:
+------------------+----------+
| Leading sequence | Encoding |
+------------------+----------+
| FF FE 00 00 | UTF-32LE |
| 00 00 FE FF | UTF-32BE |
| FF FE | UTF-16LE |
| FE FF | UTF-16BE |
| EF BB BF | UTF-8 |
+------------------+----------+
The longest matching octet sequence determines the encoding.
Implementations of this step MUST use these octet sequences to
determine the character encoding scheme, even if the determined
scheme is not supported. If this step determines the character
encoding scheme, the octet sequence representing the Unicode
encoding form signature MUST be ignored when decoding the binary
source text to source text.
3. The character encoding scheme is determined to be UTF-8.
If the character encoding scheme is determined to be UTF-8 through
any means other than step 2 as defined above and the binary source
text starts with the octet sequence EF BB BF, the octet sequence is
ignored when decoding the binary source text to source text. (The
sequence will also be ignored if step 2 determines the character
encoding scheme per the requirements in step 2).
In the cited case, implementations of the types text/javascript,
text/ecmascript, and application/javascript SHOULD and
implementations of the type application/ecmascript MUST implement the
requirements defined in this section.
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4.3. Character Encoding Scheme Error Handling
The following error processing behavior is RECOMMENDED for the media
types text/javascript, text/ecmascript, and application/javascript,
and REQUIRED for the media type application/ecmascript.
o If the value of a charset parameter is illegal, implementations
MUST either recover from the error by ignoring the parameter or
consider the character encoding scheme unsupported.
o If binary source text is determined to have been encoded using a
certain character encoding scheme that the implementation is
unable to process, implementations MUST consider the resource
unsupported (i.e., they MUST NOT decode the binary source text
using a different character encoding scheme).
o Binary source text can be determined to have been encoded using a
certain character encoding scheme but contain octet sequences that
are not legal according to that scheme. This is typically caused
by a lack of proper character encoding scheme information; such
errors can pose a security risk, as discussed in section 5.
Implementations SHOULD detect such errors as early as possible; in
particular, they SHOULD detect them before interpreting any of the
source text. Implementations MUST detect such errors and MUST NOT
interpret any source text after detecting such an error. Such
errors MAY be reported, e.g., as syntax errors as defined in
[ECMA], section 16.
This document does not define facilities that allow specification of
the character encoding scheme used to encode binary source text in a
conflicting manner. There are only two sources for character
encoding scheme information: the charset parameter and the Unicode
encoding form signature. If a charset parameter is specified, binary
source text is processed as defined for that character encoding
scheme.
5. Security Considerations
Refer to [RFC3552] for a discussion of terminology used in this
section. Examples in this section and discussions of interactions of
host environments with scripts and extensions to [ECMA] are to be
understood as non-exhaustive and of a purely illustrative nature.
The programming language defined in [ECMA] is not intended to be
computationally self-sufficient, rather it is expected that the
computational environment provides facilities to programs to enable
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specific functionality. Such facilities constitute unknown factors
and are thus considered out of the scope of this document.
Derived programming languages are permitted to include additional
functionality that is not described in [ECMA]; such functionality
constitutes an unknown factor and is thus considered out of the scope
of this document. In particular, extensions to [ECMA] defined for
the JavaScript programming language are not discussed in this
document.
Uncontrolled execution of scripts can be exceedingly dangerous.
Implementations that execute scripts MUST give consideration to their
application's threat models and those of the individual features they
implement; in particular, they MUST ensure that untrusted content is
not executed in an unprotected environment.
Specifications for host environment facilities and for derived
programming languages should include security considerations. If an
implementation supports such facilities, the respective security
considerations apply. In particular, if scripts can be referenced
from or included in specific document formats, the considerations for
the embedding or referencing document format apply.
For example, scripts embedded in application/xhtml+xml [RFC3236]
documents could be enabled through the host environment to manipulate
the document instance, which could cause the retrieval of remote
resources; security considerations regarding retrieval of remote
resources of the embedding document would apply in this case.
This circumstance can further be used to make information, that is
normally only available to the script, available to a web server by
encoding the information in the resource identifier of the resource,
which can further enable eavesdropping attacks. Implementation of
such facilities is subject to the security considerations of the host
environment, as discussed above.
The facilities defined in [ECMA] do not include provisions for input
of external data, output of computed results, or modification of
aspects of the host environment. An implementation of only the
facilities defined in [ECMA] is not considered to support dangerous
operations.
The programming language defined in [ECMA] does include facilities to
loop, cause computationally complex operations, or consume large
amounts of memory; this includes, but is not limited to, facilities
that allow dynamically generated source text to be executed (e.g.,
the eval() function); uncontrolled execution of such features can
cause denial of service, which implementations MUST protect against.
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A host environment can provide facilities to access external input.
Scripts that pass such input to the eval() function or similar
language features can be vulnerable to code injection attacks.
Scripts are expected to protect against such attacks.
A host environment can provide facilities to output computed results
in a user-visible manner. For example, host environments supporting
a graphical user interface can provide facilities that enable scripts
to present certain messages to the user. Implementations MUST take
steps to avoid confusion of the origin of such messages. In general,
the security considerations for the host environment apply in such a
case as discussed above.
Implementations are required to support the UTF-8 character encoding
scheme; the security considerations of [RFC3629] apply. Additional
character encoding schemes may be supported; support for such schemes
is subject to the security considerations of those schemes.
Source text is expected to be in Unicode Normalization Form C.
Scripts and implementations MUST consider security implications of
unnormalized source text and data. For a detailed discussion of such
implications refer to the security considerations in [RFC3629].
Scripts can be executed in an environment that is vulnerable to code
injection attacks. For example, a CGI script [RFC3875] echoing user
input could allow the inclusion of untrusted scripts that could be
executed in an otherwise trusted environment. This threat scenario
is subject to security considerations that are out of the scope of
this document.
The "data" resource identifier scheme [RFC2397], in combination with
the types defined in this document, could be used to cause execution
of untrusted scripts through the inclusion of untrusted resource
identifiers in otherwise trusted content. Security considerations of
[RFC2397] apply.
Implementations can fail to implement a specific security model or
other means to prevent possibly dangerous operations. Such failure
could possibly be exploited to gain unauthorized access to a system
or sensitive information; such failure constitutes an unknown factor
and is thus considered out of the scope of this document.
6. IANA Considerations
This document registers four new media types as defined in the
following sections.
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7. JavaScript Media Types
7.1. text/javascript (obsolete)
Type name: text
Subtype name: javascript
Required parameters: none
Optional parameters: charset, see section 4.1.
Encoding considerations:
The same as the considerations in section 3.1 of [RFC3023].
Security considerations: See section 5.
Interoperability considerations:
None, except as noted in other sections of this document.
Published specification: [JS15]
Applications which use this media type:
Script interpreters as discussed in this document.
Additional information:
Magic number(s): n/a
File extension(s): .js
Macintosh File Type Code(s): TEXT
Person & email address to contact for further information:
See Author's Address section.
Intended usage: OBSOLETE
Restrictions on usage: n/a
Author: See Author's Address section.
Change controller: The IESG.
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7.2. application/javascript
Type name: application
Subtype name: javascript
Required parameters: none
Optional parameters: charset, see section 4.1.
Encoding considerations:
The same as the considerations in section 3.2 of [RFC3023].
Security considerations: See section 5.
Interoperability considerations:
None, except as noted in other sections of this document.
Published specification: [JS15]
Applications which use this media type:
Script interpreters as discussed in this document.
Additional information:
Magic number(s): n/a
File extension(s): .js
Macintosh File Type Code(s): TEXT
Person & email address to contact for further information:
See Author's Address section.
Intended usage: COMMON
Restrictions on usage: n/a
Author: See Author's Address section.
Change controller: The IESG.
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8. ECMAScript Media Types
8.1. text/ecmascript (obsolete)
Type name: text
Subtype name: ecmascript
Required parameters: none
Optional parameters: charset, see section 4.1.
Encoding considerations:
The same as the considerations in section 3.1 of [RFC3023].
Security considerations: See section 5.
Interoperability considerations:
None, except as noted in other sections of this document.
Published specification: [ECMA]
Applications which use this media type:
Script interpreters as discussed in this document.
Additional information:
Magic number(s): n/a
File extension(s): .es
Macintosh File Type Code(s): TEXT
Person & email address to contact for further information:
See Author's Address section.
Intended usage: OBSOLETE
Restrictions on usage: n/a
Author: See Author's Address section.
Change controller: The IESG.
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8.2. application/ecmascript
Type name: application
Subtype name: ecmascript
Required parameters: none
Optional parameters: charset, see section 4.1.
Note: Section 3 defines error handling behavior for content
labeled with a "version" parameter.
Encoding considerations:
The same as the considerations in section 3.2 of [RFC3023].
Security considerations: See section 5.
Interoperability considerations:
None, except as noted in other sections of this document.
Published specification: [ECMA]
Applications which use this media type:
Script interpreters as discussed in this document.
Additional information:
Magic number(s): n/a
File extension(s): .es
Macintosh File Type Code(s): TEXT
Person & email address to contact for further information:
See Author's Address section.
Intended usage: COMMON
Restrictions on usage: n/a
Author: See Author's Address section.
Change controller: The IESG.
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9. References
9.1. Normative References
[CHARSETS] IANA, "Assigned character sets",
.
[ECMA] European Computer Manufacturers Association,
"ECMAScript Language Specification 3rd Edition",
December 1999,
[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.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC3536] Hoffman, P., "Terminology Used in Internationalization
in the IETF", RFC 3536, May 2003.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing
RFC Text on Security Considerations", BCP 72, RFC
3552, July 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
9.2. Informative References
[E4X] European Computer Manufacturers Association,
"ECMAScript for XML (E4X)", June 2004,
[EcmaCompact] European Computer Manufacturers Association,
"ECMAScript 3rd Edition Compact Profile", June 2001,
[JS15] Netscape Communications Corp., "Core JavaScript
Reference 1.5", September 2000,
RFC, FYI, BCP