In the last few days a serious bug, nicknamed Heartbleed, has been found in the cryptographic software library OpenSSL.
This bug allows any ill-intentioned hacker to read the memory of the systems with the vulnerable versions of the OpenSSL software installed; with this kind of access an attacker can read the secret keys used to encrypt the traffic to the server including the usernames and passwords of the users and the actual content.
Fortunately only the 1.0.1 series (up to 1.0.1f) and 1.0.2-beta series (up to 1.0.2-beta1) of OpenSSL are affected by the Heartbleed bug.