None of these options are secure for storing sensitive data. localStorage and sessionStorage are not encrypted and accessible by any JavaScript code running on the page. Regular cookies can be vulnerable to XSS attacks. Sensitive data should be: 1) Minimized - store only what's necessary, 2) Encrypted before storage if absolutely needed, 3) Preferably stored server-side, 4) If cookies are used, they should be HTTP-only and secure flagged.