The most new and dangerous bug around it’s the one called “shellshock”, this is a security bug in the widely used Unix Bash shell which was disclosed on 24 September 2014. Many Internet daemons, such as web servers, use Bash to process certain commands, allowing an attacker to cause vulnerable versions of Bash to execute arbitrary commands. This can allow an attacker to gain unauthorized access to a computer system.
The bug causes Bash to unintentionally execute commands when they are stored in specially crafted environment variables. Within days, a series of further related vulnerabilities in Bash were found, leading to the need for further patches.
By 25 September, botnets based on computers compromised with this exploit were being used by attackers for distributed denial-of-service attacks and vulnerability scanning, source wikipedia
Let’s see how to check if your computer or server is vulnerable.