Christian Heimes <li...@cheimes.de> added the comment:

> * only Windows and macOS will get the fix

Modules/expat can be used on all platforms. A downstream patch is only a 
problem for platforms that compile Python with "./configure 
--with-system-expat".

The security fixes for entity expansion blowup and external entity loading are 
backwards incompatible fixes. Technically they also violate XML standards. In 
practice the vast majority of users will never run into the issue, because 
external entities are scarcely used. The expat parser is a non-validating XML 
parser, so DTDs aren't useful at all. I'd rather break a handful of users than 
to keep the majority of users vulnerable.

To fix billion laughs and quadratic blowup once and for all, we also have to 
break backwards compatibility and require expat >= 2.3.0. For now the modules 
still work with old versions of expat. IMO it's fine. Vendors either have to 
update their libraries or use our copy of expat.

Ultimately it's Benjamin's, Larry's, and Ned's decision. They are release 
managers.

----------

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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue17239>
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