Security researchers discovered a gaping hole in OpenSSL that may have left roughly two-thirds of the web vulnerable to eavesdropping for the past two years! By exploiting the bug, attackers are able to intercept communications, steal data directly from the services and users and to impersonate services and users. The bug affects "heartbeat", an OpenSSL extension, and is therefore called Heartbleed.
In effect, the bug allows a malicious users to request data from a Web server's memory—data that could include the site's SSL encryption keys, user passwords and other sensitive information. According to Heartbleed, a website established by researchers at Codenomicon who identified the bug (as did a Google engineer):
The Heartbleed bug allows anyone on the Internet to read the memory of the systems protected by the vulnerable versions of the OpenSSL software. This compromises the secret keys used to identify the service providers and to encrypt the traffic, the names and passwords of the users and the actual content. This allows attackers to eavesdrop on communications, steal data directly from the services and users and to impersonate services and users.