Read the error message:
Security Exception
Description: The application attempted to perform an operation not allowed by the security policy. To grant this application the required permission please contact your system administrator or change the application's trust level in the configuration file.
The do exactly what it says: your web page app is trying to perform an IO operation that needs it to have specific permissions. Only your admin can change the permissions your app runs under, or the permissions on the folder or file it is trying to access.