[Django] #36733: Fix unescape attributes in Stylesheet.__str__

[Django]

#36733: Fix unescape attributes in Stylesheet.__str__
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Reporter: Baptiste Mispelon | Type: Bug
Status: new | Component:
| contrib.syndication
Version: 5.2 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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,,This was originally reported by Mustafa Barakat as a security issue but
was deemed low-risk enough to be tracked publicly.,,

The `django.utils.feedgenerator.Stylesheet` class (introduced in #12978)
has a `__str__` method which is used when outputting a `<?xml-stylesheet
... ?>`. The method uses f-strings with three different attributes: `url`,
`mimetype`, and `media`.

However these attributes are not escaped, which could potentially lead to
invalid markup if any of those attributes were to contain a quote for
example.

Escaping using Django's `escape` (or even `format_html`) should work even
though those functions are meant for HTML and not XML.
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Ticket URL: <https://code.djangoproject.com/ticket/36733>
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