From the Document: "In recent months, President Trump has issued several presidential proclamations that provide for the exclusion of broad categories of non-U.S. nationals (aliens) from the United States. [...] For statutory authority, all of these recent proclamations rely principally upon § 212(f) of the Immigration and Nationality Act (INA). That provision authorizes the President 'to suspend the entry of all aliens or any class of aliens' whose entry he 'finds ... would be detrimental to the interests of the United States.' The provision has been in the INA since the INA's original enactment in 1952."
CRS Legal Sidebar, LSB10458
Congressional Research Service: https://crsreports.congress.gov/