The bank has no plans to rip out its mainframe -- which, along with Windows, makes up about a third of its back-end system -- and replace it with Linux servers, said Seager.

Key is one of the earliest American banks to use Linux widely, according to Alenka Grealish, an analyst at San Francisco-based Celent Group. "The economics of banks are hard. IT projects tend to hit a glass ceiling in terms of size and also extend out in their cycle time," she said. Moreover, "the exposure to brand and regulatory risk is huge. So until it's tried and really true, banks are not going to go there."

The largest banks, such as Citigroup Inc., Bank of America Corp. and Wells Fargo & Co., have all "done a lot of beta testing" of Linux, according to Grealish. "They're a lot more cautious about moving forward, because they see a bigger risk of failure than a super-regional bank [such as Key], and a lot more competitors will feed off their carcass if they do."

Also slowing any moves to Linux has been the number of mergers that have taken place in the banking industry in the past several years. Rather than using that as an opportunity to make a platform change, merging banks are generally too preoccupied with ensuring IT integration goes smoothly and cheaply to consider anything else, Grealish said. That has often meant "in the long run, they often don't do anything," Grealish said.