Patched code sample

# This is a simplified example and may not be a complete or accurate representation of the actual fix.
# It aims to demonstrate the general idea of restricting attribute access.

from jinja2 import Environment, BaseLoader

class SafeEnvironment(Environment):
    def getattr(self, obj, attribute):
        """Override getattr to restrict access to potentially dangerous attributes."""
        if attribute in ['__class__', '__mro__', '__subclasses__', '__globals__', '__code__', 'format', 'func_globals']: # added 'format'
            raise AttributeError(f"Access to attribute '{attribute}' is not allowed.")
        return super().getattr(obj, attribute)


def render_template(template_string, context):
    """Renders a Jinja2 template with a safe environment."""
    env = SafeEnvironment(loader=BaseLoader())
    template = env.from_string(template_string)
    return template.render(context)


if __name__ == '__main__':
    # Example Usage (attempting to exploit the vulnerability, but now prevented)
    template_string = "{{ ''.__class__.__base__.__subclasses__() | attr('__getitem__')(132) | attr('__init__') | attr('__globals__') | attr('__builtins__') | attr('eval')('__import__(\'os\').system(\'echo vulnerable\')') }}" #Example exploitation

    #Example usage of the 'format' to exploit the vulnerability
    template_string2 = "{{ '' | attr('format') }}"


    context = {}

    try:
        result = render_template(template_string, context)
        print(f"Rendered template: {result}")
    except AttributeError as e:
        print(f"Caught expected AttributeError: {e}")

    try:
        result2 = render_template(template_string2, context)
        print(f"Rendered template: {result2}")
    except AttributeError as e:
        print(f"Caught expected AttributeError: {e}")