# Client-Side Processing Does Not Move the Server Trust Boundary Moving media processing into the browser does not move the server's validation boundary. ## Scope & conditions - This applies when WordPress generates image sub-sizes in the browser before upload. - The browser may reduce server CPU and memory work, but it remains an untrusted client. The server must still validate file type, dimensions, permissions, and metadata. - The claim does not require the server to repeat every transformation. It requires the server to validate every security-relevant property it accepts. ## Source anchor > "Client-side processing is a **performance optimization, not a trust boundary**." <!-- atomicity: 1 idea ✓ --> ## Links - [[3. Resources/Permanent Notes/Client-Supplied Hints Are Not Security Boundaries]] — extends: browser-generated files and metadata are claims from a client, not server policy. - [[3. Resources/Permanent Notes/Defense in Depth Concedes Perfect Safety Is Impossible and Optimizes for Narrow Costly Monitored Failure]] — applies-in: client processing can optimize performance while server validation remains an independent control. - [[WordPress Hosting Security Stack MOC]] — belongs-in: media handling is part of the host's application-layer security boundary. ## Source - [[3. Resources/Readwise/Articles/Client-Side Media Processing in WordPress 7.1]] — states that client-side work is a performance optimization while the server retains validation.