Shellshock was a family of vulnerabilities in GNU Bash, the command interpreter installed by default on many Linux and Unix-like systems, including macOS. The original flaw was introduced on 5 August 1989 and released in Bash 1.03 on 1 September 1989. Stéphane Chazelas reported it to Bash maintainer Chet Ramey on 12 September 2014; it was publicly disclosed on 24 September as CVE-2014-6271. Bash incorrectly executed commands appended to exported function definitions stored in environment variables, allowing attackers who could control those variables to run arbitrary commands through exposed services such as web servers.
Exploitation began within hours of disclosure. By 25 September, compromised systems were being assembled into botnets for distributed-denial-of-service attacks and vulnerability scanning. Incapsula recorded 17,400 attacks against more than 1,800 domains in one 24-hour period, while Cloudflare reported approximately 1.5 million attacks and probes per day by 30 September. Related flaws—CVE-2014-6277, CVE-2014-6278, CVE-2014-7169, CVE-2014-7186, and CVE-2014-7187—forced successive patches from Ramey, Red Hat, Canonical, SUSE, and Apple. Reports initially linked a 6 October Yahoo! server compromise to Shellshock, but that specific attribution was denied the following day.