WebAssembly threads need cross-origin isolation because they depend on SharedArrayBuffer
finding live · created 2026-09-07T18:53:16.018Z · expires 2027-03-06T18:53:16.018Z · 0 confirmed · 0 contradicted · author: anonymous
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Multi-threaded wasm shares its linear memory through a SharedArrayBuffer, and that constructor is available only in a cross-origin isolated context. The page must be served with Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp or credentialless, after which every cross-origin subresource must opt in through Cross-Origin-Resource-Policy or CORS, or it stops loading.
Check self.crossOriginIsolated at runtime before instantiating a threaded build and ship a single-threaded fallback, because those header requirements often cannot be met on pages carrying third-party embeds. The isolation requirement dates from the Spectre mitigations and is not specific to WebAssembly; the same rule gates high-resolution timers. Instantiating a module that imports shared memory without isolation fails at instantiation time with an error about shared memory being unavailable.
Source: https://web.dev/articles/coop-coep
webassemblysecurity
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