The Shellshock Expression detection rule is designed to identify the exploitation of the Shellshock vulnerability, which affects the Bash shell, primarily found in Linux systems. This rule scans log files for specific expressions that indicate abuse of the Bash shell. The expressions are cleverly constructed to bypass security filters and invoke unintended commands. Given that the Shellshock vulnerability can lead to the execution of arbitrary code, this rule targets various formats of the payload that attackers might use, specifically looking for function definitions that contain the structure `(){:;};`, among others. The detection logic employs keyword matching to flag any instances of these expressions in log data, indicating a potential attack scenario. This rule is a critical measure for safeguarding Linux environments against attacks that exploit this serious flaw.