types under the "text" top-level type.

   It is noteworthy that the label "UTF-8" does not contain a version
   identification, referring generically to ISO/IEC 10646.  This is
   intentional, the rationale being as follows:

   A MIME charset label is designed to give just the information needed
   to interpret a sequence of bytes received on the wire into a sequence
   of characters, nothing more (see RFC 2045, section 2.2, in [MIME]).
   As long as a character set standard does not change incompatibly,
   version numbers serve no purpose, because one gains nothing by
   learning from the tag that newly assigned characters may be received
   that one doesn't know about.  The tag itself doesn't teach anything
   about the new characters, which are going to be received anyway.

   Hence, as long as the standards evolve compatibly, the apparent
   advantage of having labels that identify the versions is only that,
   apparent.  But there is a disadvantage to such version-dependent
   labels: when an older application receives data accompanied by a
   newer, unknown label, it may fail to recognize the label and be
   completely unable to deal with the data, whereas a generic, known
   label would have triggered mostly correct processing of the data,
   which may well not contain any new characters.

   Now the "Korean mess" (ISO/IEC 10646 amendment 5) is an incompatible
   change, in principle contradicting the appropriateness of a version
   independent MIME charset label as described above.  But the
   compatibility problem can only appear with data containing Korean
   Hangul characters encoded according to Unicode 1.1 (or equivalently
   ISO/IEC 10646 before amendment 5), and there is arguably no such data
   to worry about, this being the very reason the incompatible change
   was deemed acceptable.






 
RFC 2279                         UTF-8                      January 1998


   In practice, then, a version-independent label is warranted, provided
   the label is understood to refer to all versions after Amendment 5,
   and provided no incompatible change actually occurs.  Should
   incompatible changes occur in a later version of ISO/IEC 10646, the
   MIME charset label defined here will stay aligned with the previous
   version until and unless the IETF specifically decides otherwise.

   It is also proposed to register the charset parameter value
   "UNICODE-1-1-UTF-8", for the exclusive purpose of labelling text data
   containing Hangul syllables encoded to UTF-8 without taking into
   account Amendment 5 of ISO/IEC 10646 (i.e. using the pre-amendment 5
   code point assignments).  Any other UTF-8 data SHOULD NOT use this
   label, in particular data not containing any Hangul syllables, and it
   is felt important to strongly recommend against creating any new
   Hangul-containing data without taking Amendment 5 of ISO/IEC 10646
   into account.

6.  Security Considerations

   Implementors of UTF-8 need to consider the security aspects of how
   they handle illegal UTF-8 sequences.  It is conceivable that in some
   circumstances an attacker would be able to exploit an incautious
   UTF-8 parser by sending it an octet sequence that is not permitted by
   the UTF-8 syntax.

   A particularly subtle form of this attack could be carried out
   against a parser which performs security-critical validity checks
   against the UTF-8 encoded form of its input, but interprets certain
   illegal octet sequences as characters.  For example, a parser might
   prohibit the NUL character when encoded as the single-octet sequence
   00, but allow the illegal two-octet sequence C0 80 and interpret it
   as a NUL character.  Another example might be a parser which
   prohibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the
   illegal octet sequence 2F C0 AE 2E 2F.

Acknowledgments

   The following have participated in the drafting and discussion of
   this memo:

   James E. Agenbroad    Andries Brouwer
   Martin J. D|rst       Ned Freed
   David Goldsmith       Edwin F. Hart
   Kent Karlsson         Markus Kuhn
   Michael Kung          Alain LaBonte
   John Gardiner Myers   Murray Sargent
   Keld Simonsen         Arnold Winkler





 
RFC 2279                         UTF-8                      January 1998


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