Monzo, the U.K.-based challenger bank, alerted customers that some of their PIN codes were mistakenly left exposed inside the company and they needed to update their mobile apps and change the PIN codes at a cash machine, in a blogpost. 

Monzo officials said it discovered on Aug. 2 that some of its PIN codes were incorrectly stored in a separate internal location where company engineers had access to the log files. Monzo officials said that nobody outside the company had access to the information and none of the codes, which need to be used to authenticate identity during purchases, had been fraudulently used. 

Monzo told customers to update their apps to iOS 2.59.0 on the App Store or Android 2.59.1 on Google Play to make sure they have the most up to date version, which were updated on Saturday after the PIN code issue was discovered. 

Monzo, which has about 2.5 million users, said the bug impacted less than 20% of its customers. 

Monzo in June announced plans to enter the U.S. and later that month said it raised $144 million led by Y Combinator Continuity and Latitude. The company also began testing business accounts. 

Photo courtesy of Monzo