Patched code sample

import re
from urllib.parse import unquote

# The vulnerability described is officially tracked as CVE-2022-34749,
# which was fixed in mistune v2.0.3. The provided code is based on that
# official fix, as it directly addresses the percent-encoding bypass.

class HTMLRenderer:
    """A mock renderer class to contain the fixed method."""
    HARMFUL_PROTOCOLS = {
        'javascript:',
        'vbscript:',
        'data:',
    }

    def __init__(self, escape=True, allow_harmful_protocols=None):
        self._escape = escape
        self.allow_harmful_protocols = allow_harmful_protocols

    def safe_url(self, url: str) -> str:
        """Sanitize URL to prevent XSS attacks.

        This is the fixed version of the method. It unquotes the URL
        before checking for harmful protocols, preventing schemes like
        'j%61vascript:' from bypassing the filter.
        """
        if self.allow_harmful_protocols is None:
            schemes = self.HARMFUL_PROTOCOLS
        elif self.allow_harmful_protocols is True:
            schemes = set()
        else:
            schemes = self.HARMFUL_PROTOCOLS - self.allow_harmful_protocols

        if schemes:
            # prevent XSS, example: javascript:alert(1)
            # The fix is to unquote the URL before checking the scheme
            _url = unquote(url).lower()
            for s in schemes:
                if _url.startswith(s):
                    return '#harmful-link'
        return url