The company expected the process to take "several days and not weeks," the report added, citing an automated message sent to customers on Sunday.
CDK did not immediately respond to request for comment outside regular business hours.
Last week, a dealer who received a letter from CDK said the company informed him it could take several more days to get the systems up and running.
The company, which provides software to car dealerships, briefly shut down all its systems on Wednesday, saying it was investigating a cyber incident.
U.S. auto retailers Sonic Automotive and Penske Automotive flagged a hit to their operations on Friday, as CDK experienced a third consecutive day of outage.
Bloomberg had previously reported that a group of hackers claiming responsibility for a cyberattack on CDK's software systems has demanded millions of dollars in ransom to put an end to the hack.
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