Server-side security protects your infrastructure with tools like firewalls, a WAF protecting the perimeter against malicious inbound requests, and vulnerability scanners, building a wall around your critical infrastructure. Client-side security protects where your applications actually execute: in your users' browsers. Think of it this way, server-side security protects your kitchen, but client-side security protects the meal after it's served to your customers. You need both layers because attackers increasingly target the client-side, where they can steal data directly from users without touching your servers.
Client-side attacks typically occur when a malicious actor compromises a third-party service your website uses.
Traditional security tools are designed for server infrastructure and can't see what's executing in users' browsers.
Server-side security protects your infrastructure, while client-side security protects where your applications actually execute in users' browsers.
Client-side security is a critical subset of AppSec that focuses on protecting applications where they actually execute--in users' browsers.