Firewalls, WAFs, and vulnerability scanners are traditional security tools to protect your servers, but they can't see what's actually happening in your users' browsers. They rely on filtered data, may slow down your site, and often miss threats that change based on a user's 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.