Data breaches are some of the scariest — and most inevitable — risks facing bankers today, a Citigroup executive said Monday, as several big banks faced a wave of cyber attacks.

Julie Churylo Pukas, the global head of consumer integrated payments for Citi's global enterprise payment group, said worries about data breaches wake her up at night.

"All of us have probably [experienced] one way or the other some type of data breach, and … it's probably what I wake up in the middle of the night thinking about, because it's really to some degree out of your control," Pukas said during a panel discussion in New York on Monday evening.

Citigroup (NYSE: C) is one of several banks facing renewed concerns about cyber attacks, amid reports that Iranian hackers have targeted the three largest U.S. banks. The websites of JPMorgan Chase (JPM) and Bank of America (BAC) have both experienced delays in the past week, and other big banks have also come under cyber fire. This week, Wells Fargo (WFC), U.S. Bank (USB) and PNC all saw their websites suffer outages.

The increasing complexity of the U.S. payment system offers hackers more opportunities for attacks, Pukas said. As more nonbanks get into the payments business, "it's even more complex now, and the vulnerabilities for people to come in at various stages of the ecosystem" increase, she said. "It's just very, very difficult for all of us to control."

She spoke at a panel discussion about "emerging risks" to the payments industry, hosted by the New York industry group NYPAY. Pukas was joined on the panel by Anita Boomstein of law firm Hughes Hubbard & Reed, Steve Kietz of digital marketing company edo Interactive and Lillie Platko, group head of U.S. debit at MasterCard (MA).