From the Wall Street Journal:
Sen. Charles E. Grassley, ranking Republican on the Senate Finance Committee, is raising questions about a housekeeper who worked briefly for Treasury Secretary-nominee Timothy Geithner without proper immigration papers, and multiple years when Mr. Geithner didn’t pay Social Security and Medicare taxes for himself….
The paper says Geithner had a housekeeper whose papers had expired, and:
The second issue involved taxes due while Mr. Geithner worked for the International Monetary Fund between 2001 and 2004. As an employee, Mr. Geithner was technically considered self-employed and was required to pay Social Security and Medicare taxes for himself as both an employer and an employee.
He apparently failed to do so, resulting in Internal Revenue Service audits his last two years at the IMF. As soon as the IRS brought the issue to his attention, he paid the taxes with interest, these people said.