ABC News has the story:

A nefarious hacker could have changed every voter’s status from active to inactive, which would have prevented them from voting, [Bob Wilson, an official with the Illinois Ballot Integrity Project] said.

“Or we could’ve changed the information on what precinct you were in or what polling place you were supposed to go to,” he said. “So there were ways that we could potentially change the entire online data base and disenfranchise voters throughout the entire city of Chicago.”

“If we’d wanted to, we could’ve wiped the entire database out,” Wilson claimed.

Tom Leach, a spokesman for the Chicago Election Board, tells ABC News that the problem seems to have arisen because the city’s database allowing voters to locate their voting precinct once asked voters for detailed information such as Social Security numbers.

The Chicago Tribune adds some perspective on the magnitude of the risk:

Officials acknowledged that for the last five or six years it would have only taken a few keystrokes for a knowledgeable computer user to obtain the personal information for more than half of the 1.3 million identities in the system.

More information available here, from the Illinois Ballot Integrity Project.