The Heartbleed bug is already being called one of the biggest security threats the Internet has ever seen.

The bug has affected a bunch of popular websites and services — and ones you probably use every day, like Gmail and Facebook — and could have quietly exposed your sensitive account information (such as passwords and credit card numbers) over the past two years.

Some Internet companies that were vulnerable to the bug have already updated their servers with a security patch to fix the issue. This means you'll need to go in and change your passwords immediately for these sites, just to be safe.