Patched code sample

import html
from html.parser import HTMLParser


def patched_markdown_html_parser(html_chunk: str) -> str:
    """
    A patched parser function that safely handles malformed HTML-like
    sequences during Markdown processing.

    This function represents the fix for a vulnerability where an unhandled
    AssertionError from html.parser.HTMLParser could crash an application.
    The fix is to wrap the parsing call in a try...except block to prevent
    a Denial of Service.
    """
    # In a real application, a more complex parser state would be managed.
    # For this demonstration, a new parser is instantiated for each chunk.
    parser = HTMLParser()
    try:
        # The vulnerable operation: feeding potentially malformed data to the parser.
        # In a vulnerable implementation, certain inputs (e.g., "<!->") could
        # cause an uncaught AssertionError, crashing the process.
        parser.feed(html_chunk)

        # If parsing is successful, the original HTML is returned.
        return html_chunk
    except AssertionError:
        # THE FIX: Catch the AssertionError from the underlying parser.
        # Instead of crashing, the application can now handle the error
        # gracefully. Here, we prevent the DoS and mitigate potential
        # issues by escaping the problematic chunk and returning it as plain text.
        return html.escape(html_chunk)