Client-side security is about protecting your web applications right where they're being used, which is in your customers' web browsers. Traditional security tools may do a great job monitoring your servers, but they fail to protect the real attack surface where your applications run. Modern websites rely on dozens of third-party scripts for analytics, payments, ads, chat, and more. Just one of these gets compromised, and attackers can quietly steal valuable information without you or your users knowing. Client-side security gives you visibility and control over what is actually happening in the browser.
Compromising a third-party service your website relies on is one common way attackers get in.
Firewalls, WAFs, and vulnerability scanners are traditional security tools to protect your servers, but they can't see what's happening in your users' browsers.
Server-side security protects your infrastructure, while client-side security focuses on where your application actually runs, inside your users' browsers.
Client-side security is a critical subset of AppSec that focuses on protecting applications where they actually execute--in users' browsers.