You’re reviewing a Python script that processes user input from a web form. The script uses `eval()` to dynamically execute a configuration string provided by the user. What is the most significant security risk associated with this approach, and which alternative method would be the most secure for this use case?

During a code review, you notice a colleague’s script uses `pickle` to deserialize session data loaded from a file provided by an external user. They argue it’s safe because they validate the file’s MD5 hash first. Why is this defense insufficient?

Consider a Flask web application that uses templates to render user profiles. To prevent Cross-Site Scripting (XSS) in this context, what is the most effective and maintainable strategy a developer should implement?