Board spokesman James Allen said a mistake allowed a database containing applicants' names, addresses, driver license numbers and the last four digits of social security numbers to be accessed, the Chicago Tribune reported Wednesday.
A computer security firm called Forensicon uncovered the breach while researching voting patterns on the website of the Chicago Board of Election Commissioners.
The firm reported that the personal information of up to 1.7 million registered voters had been exposed.
Allen said the problem was corrected 15 minutes after Forensicon notified the board of elections and the 1,200 applicants will be informed that their information was vulnerable for about a week.
There is no indication that anyone other than the computer security firm accessed the information, he said.