The names and license plate numbers of about 50,000 Uber drivers were compromised in a security breach last year, the company revealed Friday.

Uber discovered a possible breach of its systems in September, and a subsequent investigation revealed an unauthorized third party had accessed one of its databases four months earlier, the company said.

The files accessed held the names and license plate numbers of about 50,000 current and former drivers, which Uber described as a “small percentage” of the total. About 21,000 of the affected drivers are in California. The company has several hundred thousand drivers altogether.

It’s in the process of notifying the affected drivers and advised them to monitor their credit reports for fraudulent transactions and accounts. It said it hadn’t received any reports yet of actual misuse of the data.

Uber will provide a year of free identity protection service to the affected drivers, it said, which has become fairly standard for such breaches.

The company said it had filed a “John Doe” lawsuit Friday to help it confirm the identity of the party responsible for the breach.