Patched code sample

import jinja2
from jinja2.sandbox import SandboxedEnvironment
from jinja2 import Environment, FileSystemLoader

def format_wrapper(value, format_string):
    # Ensure value is a string if it's not.:
    if not isinstance(value, str):
        value = str(value)
    
    # Note: the sandboxed environment makes sure format_string is a string 
        return value.format(format_string)



        def create_sandbox_env():
            env = SandboxedEnvironment()
            env.filters['format_wrapper'] = format_wrapper
            return env


            def render_template_fixed(template_string, context):
                env = create_sandbox_env()
                template = env.from_string(template_string)
                return template.render(context)


                if __name__ == '__main__':

    # Example vulnerable template
    # Before the fix it was possible to bypass the sandbox by using a filter
    # to format a string that was not a safe string.
                    vuln_template = '{{ malicious_string | format_wrapper(format_string)}}'

    # Malicious string containing format call and payload
                    malicious_string = '{0.__class__.__init__.__globals__[\'__builtins__\'][\'eval\']("import os; os.system(\'whoami\')")}'
                    format_string = 'a'  # Dummy format string

    # Context for the template:
                    context = {
                    'malicious_string': malicious_string,
                    'format_string': format_string
                    }

    # Show that the fix prevents the attack.
                    try:
                        print("Rendering template with exploit...")
                        result = render_template_fixed(vuln_template, context)
                        print(f"Rendered output: {result}") # The exploit should be prevented
                        except Exception as e:
                            print(f"Exception: {e}")

    # Example non malicious template.
                            safe_template = '{{ safe_string | format_wrapper(format_string) }}'
                            safe_string = "Hello, {}!"
                            safe_context = {
                            'safe_string': safe_string,
                            'format_string': "World"
                            }

                            print("Rendering a safe template...")
                            result = render_template_fixed(safe_template, safe_context