Firewalls, WAFs, and vulnerability scanners are traditional security tools used to protect your server, but they cannot see what's happening in your users' browsers. They monitor sanitized data, can slow down your site, or completely miss threats that change based on user location, device, or timing. Similar limitations are also encountered by Content Security Policies and JavaScript agents. CSP evasion, shadow-DOM tricks, or obfuscated code are techniques that can bypass them.
Compromising a third-party service your website relies on is one common way attackers get in.
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.
Protecting your website visitors from malicious JavaScript attacks that happen in their browsers is the goal of client-side security.