Yahoo said that some of its customers’ email accounts were compromised in a recent cyberattack.

The 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, says AP, 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, Time said. Yahoo said that it has “implemented additional measures” to prevent future attacks.