Personal data belonging to over 31 million customers of a popular virtual keyboard app has leaked online, after the app's developer failed to secure the database's server.

The server is owned by Eitan Fitusi, co-founder of AI.type, a customizable and personalizable on-screen keyboard, which boasts more than 40 million users across the world.

But the server wasn't protected with a password, allowing anyone to access the company's database of user records, totaling more than 577 gigabytes of sensitive data.

The database appears to only contain records on the app's Android users.