The update, issued on July 19 through Apple's iTunes App Store, addresses a programming design flaw that stored personal banking data in a hidden but accessible file on the user's iPhone.
Citibank notified its mobile banking customers in the U.S. about the problem on July 20, according to The Wall Street Journal.
Citibank did not immediately respond to a request for comment, but a company spokesperson told The Wall Street Journal that Citibank had no reason to believe that any personal information was compromised as a result of the flaw.
It's not clear how long Apple took to approve the update or when Citibank first learned of the flaw.
The app, Citi Mobile, was developed in part using code provided by mFoundry, a mobile banking technology company based in Larkspur, Calif.
Drew Sievers, CEO of mFoundry, said in a phone interview that none of his company's 150 clients is affected by this incident.
Citibank's situation is unique, he said, because the Citibank app is a combination of some custom code provided by mFoundry and some of Citibank's own custom code. He described it as a unique app that combines and co-mingles code.
Sievers said he could not comment on how Citibank handles the security review for its app, but stressed that Citibank's situation isn't comparable to other companies using mFoundry's mobile banking platform. He also said that Citibank's mobile banking software for other platforms, such as RIM's BlackBerry, isn't affected.
Andrew Storms, director of security nCircle, a network security firm, suggested that companies may be rushing too quickly to deploy mobile banking technology.
"The good news is that as soon as Citi became aware of the problem, they worked quickly to address the issue," he said in an e-mailed statement. "Still, this should be a wake-up call for all financial institutions deploying applications for wireless devices. The disclosure of banking and personal information can have serious repercussions, and security on these applications is still a relative unknown."