Which of the following should be used to best mitigate this type of attack?

Secure cookies are primarily focused on enhancing security in the context of session management by ensuring that cookies are only transmitted over secure HTTPS connections. While they contribute to overall security, they do not directly address the risk of command injection or other types of malicious input that can be exploited through user-supplied data.

Static code analysis involves reviewing code for potential vulnerabilities before it is deployed. Although this practice can help identify security flaws, it does not directly prevent the exploitation of vulnerabilities through malicious input. It is a preventative measure but does not mitigate attacks that exploit existing vulnerabilities during runtime.

Sandboxing creates isolated environments where code can be executed without affecting the host system. While it provides a layer of protection against malicious activities, it does not prevent the injection of harmful commands through user inputs. Sandboxing is more of a containment strategy rather than a direct solution to the problem of unvalidated input.