Spokesman Luc Chatel told France's Radio-J an investigation is under way and insists the incident "proves that this system of checking (bank accounts) via the Internet isn't infallible." He did not elaborate.
Weekly Journal du Dimanche reported Sunday that thieves seized Sarkozy's bank account information and swiped small sums of money.
The newspaper said Sarkozy reported the theft last month and that those responsible haven't been found.
The report cited an unnamed official close to the investigation for its information.
The press service for Sarkozy's office declined comment.