Patched code sample

import sys
import io

def evaluate_user_template(template_string: str) -> str:
    """
    Safely evaluates a template string from user metadata.

    The fix for the vulnerability is to explicitly disallow templates
    that start with 'python:', preventing them from being passed to exec().
    """
    # --- The Fix ---
    # The vulnerability was caused by executing code when the 'python:' prefix
    # was found. The fix is to detect this prefix and explicitly refuse
    # to process it, treating it as an invalid/disallowed template.
    if template_string.strip().startswith('python:'):
        # Instead of executing, raise an error or return an empty/error string.
        # This prevents the arbitrary code from ever reaching an unsafe function.
        raise ValueError("Python-based templates are not allowed from this source.")
    # --- End of Fix ---

    # For demonstration, a placeholder for safe template processing.
    # In a real application, this would use a safe templating engine.
    safe_output = f"Safely processed template: '{template_string}'"
    return safe_output

if __name__ == '__main__':
    # This represents a malicious template embedded in an e-book's metadata.
    malicious_template = "python:import os; os.system('echo Vulnerability Exploited!')"
    
    # This represents a normal, safe template.
    safe_template = "{title} by {author}"

    print("--- Attempting to process malicious template with the FIXED function ---")
    try:
        evaluate_user_template(malicious_template)
    except ValueError as e:
        print(f"SUCCESS: The code was NOT executed. The function raised an error as expected:")
        print(f"  {e}")

    print("\n--- Attempting to process a safe template with the FIXED function ---")
    try:
        result = evaluate_user_template(safe_template)
        print("SUCCESS: The safe template was processed correctly.")
        print(f"  Output: {result}")
    except ValueError as e:
        print(f"FAILURE: An unexpected error occurred: {e}")