The banks plan to upgrade to a newer operating system in due course, but the lack of updates in the meantime could leave bank details vulnerable to theft, experts warn.
"It is absolutely a concern," Ernest Hilbert, former FBI agent and head of cyber investigations for EMEA at risk consultancy Kroll, told CNBC in a phone interview.
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"The bad guys are going to continue to look for a way into the system but there is going to be nobody on the back end to fix it. And if there is nobody fixing it and the problems are such that they are opening up a hole, then these bad guys can walk right in."
Microsoft did not respond to CNBC's request for comment.
About 95 percent of ATMs around the world run Windows XP, according to NCR, the largest provider of cash machines globally. The company predicted that only a third will upgrade their systems to a higher version of Windows before Microsoft's deadline.
The United States Computer Emergency Readiness Team, a part of the Department for Homeland Security, also warned last week that running unsupported software causes an "elevated risk to cybersecurity dangers" and certain organizations under specific regulatory control may "no longer (be) able to satisfy compliance requirements".
(Read more: Microsoft reports may aid hack attacks on businesses)