How Do Foreigners Obtain A Social Security Card And Why Do They Need Them?

A social security card is for three kinds of people: Citizens, permanent residents and temporary "working" residents. In practice, the social security number has become a national identity number although its basic purpose is to track people for taxation. Note that there are three kinds of social security cards: One for US citizens and US permanent residents, one reads "NOT VALID FOR EMPLOYMENT"(not acceptable as a List C document on the I-9 form) and one reads "VALID FOR WORK ONLY WITH DHS AUTHORISATION" (can satisfy the I-9 requirement along with the work authorisation card). The I-9 form is checked as social security cards can also be issued to foreigners who are temporary allowed to work in the US.

I-9 form