A malfunctioning log-in system affected millions of people's ability to access a variety of Google applications last week, including Gmail and Drive.

The problem, which lasted for about three hours on the morning of April 17, occurred when the main user authentication system for Google applications was misconfigured.

The improper configuration, introduced a day earlier, caused log-in requests to be funnelled to a small number of servers, which in turn ran out of capacity, and the overload caused them to malfunction.

"At the peak of the outage, this issue affected 50% of the Admin panel and 60% of Google Sync login requests. The percentages of affected users for other services were lower, such as 0.18% users for Gmail," according to a Google incident report.

Google fixed the configuration problem and will take other steps to help prevent the issue from affecting its systems again, including improving monitoring and alerts for server overloads caused by authentication-system misconfiguration. Google also plans to alter the log-in retry behaviour of large systems like Gmail, so that they don't aggravate outages caused by server overloads.