HelloCustomerand welcome to Just Answer.

Yes, it is legal. Your employer needs this information to verify your eligibility to work in the United States, and must document that you provided proof on your I-9 form. Your I-9 must be complete before you are allowed to begin work.

Click here for more information on the I-9 form

There are other forms of ID which are acceptable for verifying your right to work if you feel uncomfortable allowing your employer to photocopy your social security card. Speak with your employer about your alternatives.

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This is the list of acceptable documents: (Reference)

• Example of an appropriate request for documentation: "If you do not have one document from list "A," then you must provide one document from list "B" and one from list "C" (note: list A documents prove identity and eligibility to work so no further documentation is necessary unless the document indicates that there are employment restrictions. List "B" documents only establish proof of identity and list "C" documents only establish proof of eligibility to work in the United States, so a list "B" document must always be accompanied by a document from list "C").
• Human resource representatives and any other hiring personnel cannot refuse to honor documents that, on their face, reasonably appear to be genuine and relate to the employee in question.
  

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List of Acceptable Documents

In order to prove identity and eligibility for employment in the United States, the Act requires employees to allow examination of one of the following documents:

Documents that Establish Both Identity and Employment Eligibility: "List A"
1.	United States Passport (expired or unexpired)
2.	Certificate of US Citizenship (INS Form N-560 or N-561
3.	Certificate of Naturalization (INS Form N-550 or N-570)
4.	Unexpired foreign passport, with I-551 stamp or attached INS Form I-94 indicating unexpired employment auth.
5.	Permanent Resident Card or Alien Registration Receipt Card with photograph (INS Form I-151 or I-551)
6.	Unexpired Temporary Resident Card (INS Form I-668)
7.	Unexpired Employment Authorization Card (INS Form I-688A)
8.	Unexpired Reentry Perrmit (INS Form I-327)
9.	Unexpired Refugee Travel Document (INS Form I-571)
10.	Unexpired Employment Authorization Document issued by the INS which contains a photo (INS Form I-688B)

If you do not have one of the documents listed above, the Act requires that you provide two documents: one from "List B" and one from "List C" (below) for examination and verification of identity and employment eligibility:

Documents that Establish Identity: "List B"
1.	Driver's license or I.D. card issued by a state or outlying possession of the United States provided it contains a photograph or information such as, name sex, date of birth, height, eye color, and address
2.	ID Card issued by federal, state or local government agencies or entities provided it contains a photograph or information such as name, gender, date of birth, height, eye color, and address
3.	School Identification card with a photograph
4.	Voter's registration card
5.	U.S. Military card or draft record
6.	Military dependent's identification card
7.	US Coast Guard Merchant Mariner card
8.	Native American tribal documents
9.	Driver's license issued by a Canadian government authority
	For persons under age 18 who are unable to present a document listed above:
10.	School record or report card
11.	Clinic, doctor, or hospital record
12.	Day-care or nursery school record

Documents that Establish Employment Eligibility: "List C"
1.	US social security card issued by the Social Security Administration (other than a card stating it is not valid for employment)
2.	Certification of Birth Abroad issued by the Department of State (Form FS-545 or Form DS-1350)
3.	Original or certified copy of a birth certificate issued by a state, county, municipal authority or outlying possession of the United States bearing an official seal
4.	Native American tribal document
5.	US Citizen ID Card (INS Form I-197)
6.	ID Card for use of Resident Citizen in the United (INS Form I-179)
7.	Unexpired employment authorization document issued by the INS (other than those listed under List A)