UK-based digital mobile-only bank Monzo asked roughly 480,000 of its customers to change their debit card PINs (personal identification numbers) after discovering that they were stored in encrypted log files.

While Monzo stores customer PINs on a regular basis, the records are kept in a hardened happened part of Monzo’s systems that is particularly secure and under tight control when it comes to who has access to them according to the bank.

However, “On Friday 2nd August, we discovered that we’d also been recording some people’s PINs in a different part of our internal systems (in encrypted log files). Engineers at Monzo have access to these log files as part of their job,” the company says.

No information’s been exposed outside Monzo, and this data hasn’t been used for fraud.

You should update your app, and we’re emailing everyone that’s been affected to let them know they should change their PIN as a precaution.

Read our full update https://t.co/cKf5p5I87w

— Monzo (@monzo) August 5, 2019

Monzo states that the exposed customers’ information was immediately deleted upon discovery and measures were taken to make it impossible for any of the employees to access it.

Monzo apps updated over the weekend

“By 5:25am on Saturday morning, we had released updates to the Monzo apps. Over the weekend, we then worked to delete the information that we’d stored incorrectly, which we finished on Monday morning,” also says the bank’s statement.

Customers should update their Monzo apps for Android and iOS to the latest versions, 2.59.0 for iOS and 2.59.1 for Android

The company adds that all the affected accounts that were impacted by the bug were checked after discovering the issue and it can confirm that “the information hasn’t been used to commit fraud.”

All customers affected by the security flaw were contacted by Monzo and they were urged to go to the closest cash machine and change their PIN numbers as a precautionary measure.

Changing the PIN for a Monzo debit card can be done by inserting it in an ATM, “entering your old PIN and choosing ‘PIN services’. Then choose ‘Select a new PIN’ and change it to a new number. “

PINs need to be reset for both joint and current accounts

The clients who observe any out of the ordinary activity on their Monzo accounts should get in touch with their bank immediately either via the support phone number available on their debit cards or through the in-app chat feature.

Monzo also said that customers affected by this bug should change PINs for both joint and current accounts, and that having the PIN only would not allow a third party to do any damage.

They wouldn’t be able to do any damage with just your PIN.

If somebody got access to your PIN and wanted to use it, they’d either have to steal your Monzo card, get access to your unlocked phone, or they would need to have access to your email account (to log into the app).

— Monzo (@monzo) August 5, 2019

Monzo is not the only company that stored sensitive customer information like passwords in plaintext and it joins a long list of higher-profile companies who made the same mistake over the years, with Facebook [1, 2], Google, Twitter, and GitHub being the most prominent examples.

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