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. ...