Client-side security focuses on protecting web applications where they run in the user's browser. This includes securing JavaScript execution, preventing data theft, protecting against XSS attacks, and ensuring safe resource loading. It encompasses everything from input validation to secure storage practices and proper implementation of security headers. As web applications grow more complex and process more sensitive data in the browser, client-side security matters more than ever.
Client-side security protects the part of a web application that runs in the user's browser: the HTML, JavaScript, and third-party scripts that execute after the page loads. It complements server-side and network security, which cannot see what happens once code reaches the browser. The discipline covers script integrity, data-entry protection, and monitoring of everything the page executes on a real user's device.
For years security focused on the server and the network perimeter. But attacks moved to where the data is entered: the browser. Magecart skimming, formjacking, malicious browser extensions, and compromised third-party scripts all operate client-side, out of reach of firewalls and server logs. Regulation followed, and PCI DSS 4.0 now explicitly requires merchants to manage and monitor the scripts running on payment pages.
cside analyses the third-party services your pages depend on, from inside the visitor's own session. It analyses the actual code of every script in real time, blocks malicious behaviour before it reaches the browser, and captures forensic evidence of what ran. That combination, detection, prevention, and an auditable record, is designed to satisfy both the security need and the PCI DSS 6.4.3 and 11.6.1 compliance requirements.
A WAF inspects requests before they reach your server and knows nothing about what executes in the visitor's browser afterwards. Client-side security operates in that blind spot, watching the scripts, page, and form interactions that happen on the user's device, where skimmers and injected code do their work.
They help but don't cover it. HTTPS encrypts data in transit; a CSP restricts which domains can load scripts. Neither inspects what an allow-listed script actually does once it runs. A compromised but trusted script passes both checks, which is why dedicated monitoring is needed.
We answer client-side security questions every day. Bring yours.