ShellShock is a vulnerability that affects the well known Unix/Linux shell environment called Bash (Bourne-again shell). Bash is distributed widely in the GNU operating system and is a default shell on some operating systems. Bash is a command processor, typically in a command text window, where a user can type commands resulting or execute instructions from command script files. The ShellShock vulnerability causes Bash to unintentionally or intentionally execute commands when commands are appended to the of function definitions stored in the values of environment variables. This vulnerability was discovered around mid September in 2014 but had most likely existed since the early days of Bash (1992).
The ShellShock vulnerability has the potential to compromise many servers and other systems and has been compared to the Heartbleed vulnerability in terms of serverity. After the disclosure of the ShellShock vulnerability, attackers within hours of disclosure had already exploited this by creating botnets to implement distributed denial-of-service (DDoS) attacks and vulnerability scanning attacks. In the days after the disclosure of ShellShock, millions of probes and attacks related to ShellShock was discovered by various security companies.
The National Vulnerability Database is tracking ShellShock with (CVE-2014-6271), dated 09/24/2014.
"GNU Bash through 4.3 processes trailing strings after function definitions in the values of environment variables, which allows remote attackers to execute arbitrary code via a crafted environment, as demonstrated by vectors involving the ForceCommand feature in OpenSSH sshd, the mod_cgi and mod_cgid modules in the Apache HTTP Server, scripts executed by unspecified DHCP clients, and other situations in which setting the environment occurs across a privilege boundary from Bash execution, aka "ShellShock." NOTE: the original fix for this issue was incorrect; CVE-2014-7169 has been assigned to cover the vulnerability that is still present after the incorrect fix."
The National Vulnerability Database has a list of vulnerabilities under the ShellShock umbrella and more maybe added: