If a malformed XML file (containing non unicode
characters) is parsed with pyexpat, python crashes.

Most details on request.

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Maybe security related, as it can lead to denial of service:
it crashes a Zope server using the ParsedXML product simply
by uploading a malformed XML file.

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STRING_CONV_FUNC returns NULL if the string is contains
non-ascii and non-unicode characters.

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I realize this has sat too long; sorry.

Can you send an example XML file for which this crashes for you?

Do you let Expat determine and handle the encoding itself,
or do you override the detected encoding when you create the
parser?

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Sorry, I don't have the malformed XML file anymore. I've
tried and failed to reproduce the problem by hand.

The ParsedXML product lets expat determine the encoding
itself : 
    def createParser(self):
        """Create a new parser object."""
        return expat.ParserCreate()

If you have a look at my patch, it's a real simple one-liner
(ok two ;) that checks the return value of the
STRING_CONV_FUNC is not NULL before using it.

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A similar patch was commited to pyexpat.c in 39631 from
Patch #1309009.  Thanks.