Are F-1 or J-1 students working for a US company subject to federal withholding?

It depends on residency status, treaty elections, and whether the work is considered US-source. Many students are nonresident aliens for US tax purposes initially, but rules change if you become a resident alien or if treaty benefits apply. Employers often withhold by default, your Form 1040 or 1040-NR position may differ. Get facts reviewed before assuming zero US tax.