(This is also known as glass box, clear box, and open box testing.) In white box testing, you create test cases by looking at the code to detect any potential failure scenarios. You determine the suitable input data for testing various APIs and the special code paths that need to be tested by analyzing the source code for the application block. Therefore, the test plans need to be updated before starting white box testing and only after a stable build of the code is available.
A failure of a white box test may result in a change that requires all black box testing to be repeated and white box testing paths to be reviewed and possibly changed.