Shellshock (software bug) - Definition from the Hotspot Shield Glossary
This is a family of security bugs also known as Bashdoor and are used on Unix systems using the Bash Shell command language. There are many Unix devices, particularly web servers that use the Bash command language to process requests; attackers can exploit weaknesses within Bash when these processes are executed to gain access to a user’s computer system.
The vulnerabilities in Bash have existed since 1989 when the initial version was released and have been documented. Shellshock uses these vulnerabilities to invoke a form of privilege escalation where the attacker is granted permissions that should not be available to them. The bug has been reported extensively and working patches were not created until September 2014, with final fixes released in October 2014.