The bank has emailed 480,000 customers (around a fifth of its total user base) to tell them to update their mobile app and change PIN number after it discovered the flaw over the weekend.
There's no suggestion that any customer accounts were compromised or criminal activity has occured, but if you're one of those affected, here's our guide on how to reset your Monzo PIN.
Reset Monzo PIN
Luckily, resetting your Monzo PIN is fairly straightforward - all you'll need is your Monzo card, and access to a high-street cash machine.
Simply pop your Monzo card into the ATM, and after keying in your existing PIN, select the 'PIN services' option. From there, you can choose 'Select a new PIN' and change it to a new number.
Monzo says that most high-street bank ATMs will be able to carry out this service, including the likes of Barclays, HSBC and Lloyds, and has a link on its site to help you find the nearest one.
You can also change your PIN on some overseas cash machines, but Monzo warns this may not work every time, so it's better to do it in the UK.
If you are overseas for a long period of time or can't access an ATM, Monzo says you can contact it through the chat feature in its Android or iOS mobile app to request a new card. The company will then send this out with a different PIN that you will be able to access through the app.
Finally, the company is also recommending you update your Monzo app to the latest version, to make sure you have the most up-to-date edition - new editions of the iOS and Android apps can be downloaded now.
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