From a Social Security website:
... The Help America Vote Act of 2002, P.L. 107-252 (HAVA) requires states to verify the information of newly registered voters for federal elections. Each state must establish a computerized state-wide voter registration list and verify new voter information with the Motor Vehicle Administration (MVA).
States are required to verify the driver’s license number against the MVA’s database. Only in situations where no driver’s license exists may the state verify the last four digits of the new voter’s Social Security Number (SSN). The state submits the last four digits of the SSN, name, and date of birth to the MVA for verification with Social Security Administration (SSA). In addition, SSA is required to report whether its records indicate that the person is deceased.
The information submitted through the Help America Vote Verification (HAVV) system is kept confidential and must be used only for voter registration. ...