The user is submiting a secure form with authentication details. The username & password sent in via form should be modified according to the system without user's knowledge. What pattern should be used?

The Intercepting Filter pattern can intercept and modify authentication data in requests before they reach the authentication mechanism. Filters can transform, encode, or process credentials without the user's awareness, which is exactly what this scenario requires. Facade simplifies interfaces, Specialization isn't relevant, and Modifer isn't a standard pattern.