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Re: [whatwg] [mimesniff] Complete MIME type parsing algorithm for section 5

From: Peter Occil <poccil14@gmail.com>
Date: Tue, 28 May 2013 16:25:12 -0400
Message-ID: <492803336E0B41ED945CE86452049D29@PeterPC>
To: "Gordon P. Hemsley" <gphemsley@gmail.com>
Cc: WHATWG <whatwg@whatwg.org>

I see you've updated the MIME sniffing algorithm in response to my feedback. 
Here
I'll go over the difference and I want you to comment on these.

1. I assume the term "whitespace character" means the same as a "whitespace 
byte" under
the MIME Sniffing spec.  As such the use of that term is inadequate for the 
following reasons.

   * A whitespace character includes 0x0C, form feed (FF), which is not 
considered whitespace
      in either HTTP or the Internet Message Format (IMF, RFC5322).

      For example, the following would not be well-formed under HTTP or IMF:

      text/plain{FF}; charset=utf-8

      But the current algorithm would consider that string well-formed 
anyway.

   * All steps in the document that are the same as step 7 skip all 
whitespace characters, even
      if the whitespace isn't well formed under HTTP or IMF.  For example, a 
bare carriage
      return (CR) or line feed character (LF) is not allowed, and a CR-LF 
pair not followed by either
      SPACE or TAB is also not allowed. IMF also allows comments within 
whitespace.

      For example, the following would not be well-formed under HTTP or IMF:

      text/plain;{CR} charset=utf-8
      text/plain;{LF} charset=utf-8
      text/plain;{CR}{LF}charset=utf-8

      (Note the lack of space in the last example. Note also that folding 
whitespace is deprecated
      under the current HTTP draft.)

      And the following examples would be allowed under IMF, but not HTTP:

      (comment) text/plain; charset=utf-8
      text/plain; (comment) charset=utf-8
      text/plain; (comment (nested)) charset=utf-8
      text/plain; charset=utf-8 (comment)
      text/plain; {CR}{LF} (comment) charset=utf-8

2. While the type, subtype, and parameter name are checked for their length, 
the other rules
  for wellformedness are not checked in your version, namely, that they must 
not be empty,
  contain a byte that isn't a MIME type byte (see my original message), or 
begin with a byte that
  isn't an ASCII alphanumeric.

  For example, the following would not be well-formed under RFC6838:

  te*xt/plain;charset=utf-8
  text/pl*ain;charset=utf-8
  text/plain;ch*arset=utf-8
  text/plain;=utf-8
  text/;charset=utf-8
  /plain;charset=utf-8

  The first three examples are because "*" isn't a MIME type byte.


3. Unquoted parameter values are not checked to ensure that they are not 
empty and do
  not contain a byte that isn't a parameter value byte (see my original 
message).

  For example, the following would not be well-formed under HTTP or MIME:

  text/plain;charset=ut?f-8
  text/plain;charset=utf=8

4. Quoted parameter values are not checked to ensure that they do not 
contain a 0x7F byte
  or a byte other than TAB (0x09) that is less than 0x20.

  For example, the following would not be well-formed under HTTP or MIME:

  text/plain;charset="utf{LF}-8"
  text/plain;charset="utf{0x7F}-8"
  text/plain;charset="utf\{LF}-8"
  text/plain;charset="utf\{0x7F}-8"

Please give your comments.

--Peter

-----Original Message----- 
From: Gordon P. Hemsley
Sent: Saturday, May 25, 2013 1:26 PM
To: Peter Occil
Cc: WHATWG
Subject: Re: [whatwg] [mimesniff] Complete MIME type parsing algorithm for 
section 5

On Sat, May 25, 2013 at 12:46 PM, Peter Occil <poccil14@gmail.com> wrote:
> My algorithm skips only SPACE and TAB instead of all whitespace characters
> because it assumes that the field value was already extracted from
> Content-Type according to the HTTP/HTTPbis spec (0x0C, form feed, is never
> considered whitespace in HTTP headers). In particular, it assumes that
> folding whitespace (obs-fold) was replaced with spaces (or the message 
> with
> obs-fold rejected) before the Content-Type value was interpreted.

Thanks for your detailed explanation.

It'll take me a little while to evaluate what you've proposed here,
but in the meantime: Keep in mind that the Content-Type header is not
the only source for a MIME type. This algorithm needs to consider MIME
types from all possible sources.

--
Gordon P. Hemsley
me@gphemsley.org
http://gphemsley.org/http://gphemsley.org/blog/ 
Received on Tuesday, 28 May 2013 20:25:45 UTC

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