Challenger bank Monzo has asked up to half a million of its customers to change their PINs after discovering a bug that meant they were visible to engineers.
The security cock-up saw customer PINs inadvertently stored in two distinct files in the company's architecture, one of which was open to engineers as part of their job.
"No one outside Monzo had access to these PINs," the bank said. "We’ve checked all the accounts that have been affected by this bug thoroughly, and confirmed the information hasn’t been used to commit fraud.
"Just in case, we’ve messaged everyone that’s been affected to let them know they should change their PIN by going to a cash machine."