[naaz234]

Description

The codehilite extension's non-Pygments output builds the <code> element's class attribute from the language name without escaping it. In CodeHilite.hilite, the fallback branch (reached when Pygments is not installed or use_pygments is False) does classes.append('{}{}'.format(self.lang_prefix, self.lang)) and then ' class="{}"'.

When codehilite is used together with attr_list, the language is taken from a fenced-block class token. The attr_list scanner allows any character in a class token except space, = and }, which includes ", < and >. So an input such as:

```{.a"><script>alert(1)</script>}
code
```

produces class="language-a"><script>alert(1)</script>", which closes the attribute and injects a live <script> element (stored XSS for anything rendering untrusted Markdown in that configuration). The language can also be set directly, e.g. CodeHilite('x', lang='"><script>', use_pygments=False).

The fenced_code extension already escapes the language in its own non-Pygments branch with _escape_attrib_html; this applies the same escaping at the one site in codehilite that missed it. Valid language names are unchanged. Added a regression test (runs without Pygments via use_pygments=False) and a changelog entry.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.

• AI tools were used and I have disclosed which ones, and have fully reviewed and verified their output.

  AI tools were use to assist with the following:

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  • Other: ______________

  Claude (Anthropic) was used to help locate the issue, draft the patch, the test, the changelog entry, the commit message, and this description. I reviewed and verified all of it, and can explain the change.

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the Documentation Style Guide.
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.