"Legal muster" refers to the minimum level of legal requirements or qualifications that need to be met. It means making sure that everything is done correctly and in accordance with the law. Full definition
This type of program... has passed legal muster in many states. (foxbusiness.com)
It's unclear if Mainframe's literal «air drop» would pass legal muster in the United States, or if regulators in other countries will agree the gimmick does not violate securities laws. (fortune.com)
Coal - state support floods in — while environmental law groups say the controversial proposal doesn't pass first legal muster. (greentechmedia.com)