THE HEARTBLEED BUG

The Heartbleed Bug is a serious vulnerability in the popular OpenSSL cryptographic software library. This weakness allows stealing the information protected, under normal conditions, by the SSL/TLS encryption used to secure the Internet. SSL/TLS provides communication security and privacy over the Internet for applications such as web, email, instant messaging (IM) and some virtual private networks (VPNs).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.

For More Details Please visit below link
http://heartbleed.com/
Interesting Video to explain the HeartBleed Bug
http://vimeo.com/91425662

Now most important how to find your server is vulnerable or not

1. Check the openssl version using the command - yum list installed openssl* .  If you're running version 1.0.1e and the last numbers are lower than 5.7, then you're affected.

Eg. You will have to update the openssl in the below case :

[root@abhishek ~]# yum list installed openssl*

Loaded plugins: fastestmirror, priorities, security, update-motd, upgrade-helper
Loading mirror speeds from cached hostfile
* amzn-main: packages.us-east-1.amazonaws.com
* amzn-updates: packages.us-east-1.amazonaws.com
   amzn-main                                                                                                                         | 2.1 kB    
  amzn-updates                                                                                                                     | 2.3 kB    
 2 packages excluded due to repository priority protections
  Installed Packages
  openssl.i686                                                            1:1.0.1e-4.53.amzn1                                                     @amzn-updates
 openssl.x86_64                                                        1:1.0.1e-4.53.amzn1                                                     @amzn-updates
 openssl-devel.x86_64                                               1:1.0.1e-4.53.amzn1                                                     @amzn-updates

2. Use the command yum update -y openssl  to update openssl on the server.
3. Restart crond service.  This step is very important.  service crond restart
4. Verify the version of openssl again (yum list installed openssl* ).  Also verify cron logs in the file /var/log/cron