More than 930,000 free rides were given out from October through Dec. 19, the CTA reported.
Those free rides make up about half of the CTA's average weekday ridership, the transit system said.
The CTA said it will ask Cubic Transportation Systems Inc., the vendor awarded a contract of about $454 million to develop Ventra, to cover the cost of those rides. Cubic hasn't been paid a cent of its contract yet, according to CTA spokeswoman Tammy Chase.
During that period, travelers were given 909,000 free bus rides and about 21,000 free rail rides when Ventra card readers malfunctioned.
During one memorable rush-hour in November, the CTA handed out 15,000 free trips at rail stations when 165 Ventra "L" readers went down.
Despite the lost cash, the CTA said things have improved with Ventra since it laid out new performance guidelines for Cubic in November. Improvements include shorter tap times (cards on Ventra readers), reduced customer service waits and better system availability, according to the transit authority.
The CTA said that since Ventra's summer launch, people have used the system to take more than 56 million rides.