self.crossOriginIsolated
The Modern Web Security Boundary & High-Performance API Vault
Why does crossOriginIsolated exist?
To defend against hardware CPU timing attacks like Spectre, web browsers stripped high-resolution timers and memory sharing. self.crossOriginIsolated is the official standard seal: when active, your page proves it won't allow untrusted cross-origin leaks, unlocking sub-microsecond timers, shared multi-threading memory, profiling, and deep heap measurement!
Without COOP & COEP headers, malicious cross-origin frames could probe CPU L1/L3 caches via nanosecond timing loop side-channels.
The Unlocked API Vault
Interactive, real-time live demonstrations of every feature unlocked by isolation.
Zero-Copy Multi-Worker Physics Simulator
SharedArrayBuffer allows main thread and Web Workers to read & write directly to the exact same raw byte buffer in memory without expensive serialization (structuredClone).
Non-Blocking Lock & Thread Mutex Sync
Atomics.waitAsync() lets the main UI thread wait asynchronously for a worker thread lock notification on a Int32Array without blocking the UI rendering frame loop!
Sub-Microsecond Jitter Oscilloscope
Without isolation, performance.now() resolution is intentionally truncated (e.g. 100µs steps) and added noise to prevent cache timing leaks. With isolation, sub-microsecond precision is unlocked!
Browser Heap Memory Measurement
performance.measureUserAgentSpecificMemory() returns the exact byte size of the JavaScript engine heap. Strictly locked behind crossOriginIsolated to prevent memory footprint leaks across frames!
Execution Flame Profiler (new Profiler())
Collects sampling JS stack traces in production at micro-second intervals to generate flame charts without external dev tools. Requires crossOriginIsolated security guarantees!
Multithreaded WebAssembly Memory
Instantiation of WebAssembly.Memory({ shared: true }) allows C++/Rust Wasm binaries compiled with pthreads to run parallel code on web threads.
How to enable crossOriginIsolated on your website
Isolation requires serving two HTTP headers on your root response document, plus ensuring cross-origin assets send proper CORS/CORP headers.
app.use((req, res, next) => {
// COOP: Restricts main window context
res.setHeader('Cross-Origin-Opener-Policy', 'same-origin');
// COEP: Demands cross-origin resources load with CORP or CORS
res.setHeader('Cross-Origin-Embedder-Policy', 'require-corp');
next();
});The COEP Gotcha: Cross-Origin Asset Pitfall Diagnostic
When COEP (require-corp) is enabled, external images, audio, or scripts without Cross-Origin-Resource-Policy: cross-origin will be BLOCKED BY THE BROWSER!
<img src="https://other.com/cat.jpg"> <img src="..." crossorigin="anonymous"> How does the "SW Magic Shim" button work?
If you cannot control the web server headers (e.g. GitHub Pages, JSFiddle, CodePen), a Service Worker can intercept the document fetch request and dynamically inject Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp on the fly!
"Do. Or do not. There is no try."
— Yoda