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CVE-2014-6271 is nearly a decade old. It affects GNU Bash up to version 4.3. And I still find it. IoT devices. Legacy embedded systems. Forgotten Ubuntu 14.04 servers running CGI scripts. If you see /cgi-bin/ in a directory listing, test for Shellshock.
User-Agent header becomes HTTP_USER_AGENT. Vulnerable Bash versions interpret function definitions in environment variables and execute trailing commands. You control the User-Agent. You control the command.The () { :; }; defines an empty function. The trailing command after the semicolon executes because vulnerable Bash doesn't stop parsing at the function definition's end. The two echo statements create a blank line separating headers from body.
Patched systems won't execute trailing commands. Bash no longer interprets y=(){...} as function definitions—environment variable functions must be prefixed with BASH_FUNC_ now. But unpatched IoT and legacy systems don't get updates. Shellshock isn't dead. It's just hiding in devices nobody remembers to patch.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.