On September 24th, 2014 an error has been found in the GNU Bash program. Not patched GNU Bash 4.3 and earlier contains a command injection vulnerability that may allow remote code execution.

In most used distributions like Debian and Centos there is already available upgrade package. For install this update follow these steps:
  1. after login via SSH
    enter command

     yum update bash

    if system not contains yum, download package bash from CentOS repozitory and update by command

     rpm -Uvh packagename.rpm

    Official repozitory you can find here  CentOS 5, CentOS 6

  2. In the Power Panel software center:
    please see article Correction of SSL critical error. The procedure is identical.

For dedicated servers with Debian/Ubuntu installations upgrade an application by command

 apt-get install --only-upgrade bash

For more infromations see for example Wikipedia article  Shellshock (software bug)