Yahoo said Thursday that some of its customers’ email accounts were compromised in a recent cyber-attack.

TIME:

The Internet company did not disclose how many accounts were impacted by the security breach, but said in a blog post that its security team took “immediate action to protect our users.” Yahoo has the world’s second-largest email service after Google, the Associated Press reports, with 273 million users worldwide and 81 million in the United States.

The company said affected users have been prompted to reset their passwords by email notification or text message.

The attackers likely targeted third-party databases to obtain customer usernames and passwords, Yahoo said, adding that it has “implemented additional measures” to prevent future attacks. The attack is the second problem for Yahoo mail users in as many months. In December, Yahoo mail experienced an outage that left many users without email access for days.