A very nasty bug called the Heartbleed bug could very well be one of the biggest security threats the Internet has ever experienced. This bug has affected tons of popular websites and services such as facebook, gmail.
I suggest you read this article from mashable that explains it all:
Mashable contacted various companies to find out they had taken measures against this bug. Read the article if you’d like to find out more.
And.. change your passwords 🙂