Why PasswordBox is Sealed in Silverlight?

A simple question, but google has no answer on th开发者_运维知识库at!

I'm hitting a wall today, because the PasswordBox in Silverlight is Sealed. I have no idea why they do that. Is somebody have an idea on that?

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Because if you can subclass it, you can introduce vulnerabilities into it.

Some vulnerabilities that might be introduced:

1. Accessing protected members.
2. Creating special accessors to protected members.
3. 3rd Party Injection - someone else subclasses your class to get at the password.

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PasswordBox isn't just sealed in Silverlight, but also WPF. According to the Silverlight team it is sealed because of security concerns, but they don't mention what those concerns are.

I don't see how any of the vulnerabilities mentioned here are valid since as a developer you have full access to the typed in Password which you can do whatever you want with.