Client-side code is logic that runs in the user’s browser rather than on a server. In web apps, this usually means JavaScript that updates the page, validates forms, handles buttons, or processes data after the page loads. Because it executes on the endpoint the user actually sees, it can change what is displayed, copied, or submitted.
This matters in cyber security because client-side code is often highly trusted and can reach sensitive workflows such as payments, authentication, and account settings. If an attacker tampers with a script, they may be able to rewrite a cryptocurrency address, steal form input, or redirect users without changing the visible website itself. Defenses include minimizing third-party scripts, using integrity checks, enforcing content security policies, and reviewing code that touches security-critical data in the browser.