The most common client-side attacks include credit card skimming (like Magecart attacks). But theft of session tokens through client-side scripts, malicious redirects, or general sensitive high-value data exfiltration are on the rise. These attacks have affected major companies, like British Airways and Ticketmaster with over 380,000 documented attacks in 2025 alone so far. Client-side attacks are often highly dynamic and targeted to prevent detection. Flying below the radar by only injecting malicious payloads under certain circumstances. They only fire at specific times, request locations, or user agents, making them nearly impossible to detect with traditional security tools.
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.