Illegal immigrants are not authorized to work in the US and are not entitled to all the benefits of a citizen or legal resident, However, if they are here they are expected to pay taxes as any citizen would. If they choose to work, they are expected to pay income taxes.If they use a fake Social Security number to obtain work, it usually belongs to another person. This is illegal and could result in being ineligible to get a green card because any reported wages would be tracked to the account of the actual owner of the Social Security number along with any FICA/ Medicare taxes that the employer withholds.In order to isolate the tax reporting for the immigrant from that of the actual Social Security holder, the IRS issues an Individual Tax Identification Number (ITIN) to the immigrant under which he/she may report their earnings. When the copies of any W-2 or other withholding documents with fake SSN are included on the return, then the IRS now has the ability to segregate the wages that are incorrectly reported and notify Social Security Administration to segregate any reported FICA/ Medicare from the actual owneru2019s account.If the immigrant works u201cunder the tableu201d in either a cash-based transaction or self-employed status, it does not remove the obiligation to correctly report earnings. It just changes the forms required on the tax return and may incur penalties to worker and/or the employer.Disclaimer: Since you are not my client, the above message is not intended to constitute written tax advice,but general information for discussion purposes only. You should not, therefore, interpret the statements to be written tax advice or rely on the statements for any purpose.