web-demuxer.wasm fails to load inside Web Worker due to origin=null

[lvscar]

I’m using web-demuxer inside a Web Worker.
Both my page and my worker are served from:

https://localhost:8080

However, when I initialize WebDemuxer inside the worker and host web-demuxer.wasm locally (same origin), the WASM file fails to load with a CORS error:

Access to fetch at 'https://localhost:8080/js_lib/web-demuxer.wasm'
from origin 'null' has been blocked by CORS policy:
No 'Access-Control-Allow-Origin' header is present on the requested resource.

What’s confusing is:

My page origin is https://localhost:8080 . My worker origin is also https://localhost:8080

But the request for web-demuxer.wasm is sent from origin = null. After investigation, I found this only happens when web-demuxer.wasm is hosted locally.

If I use the default CDN path:

https://cdn.jsdelivr.net/npm/web-demuxer@latest/dist/wasm-files/web-demuxer.wasm

everything works fine.

Root cause (analysis)

It appears that this is caused by Emscripten pthread support used by web-demuxer.wasm.

Emscripten creates an internal blob worker for pthread execution. That worker always has:

origin = null

That blob worker then tries to fetch(web-demuxer.wasm). This turns the request into:

null → https://localhost:8080

Which is treated as a cross-origin request by the browser and therefore requires:

Access-Control-Allow-Origin: *

CDN hosting works because jsDelivr already sends permissive CORS headers. Local hosting fails unless the server is configured to allow origin: null.

Expected behavior

It should be possible to host web-demuxer.wasm on the same origin as the page/worker without requiring special CORS headers.

Question

Is there a way to configure web-demuxer / Emscripten runtime to: Avoid using a blob pthread worker, or Provide an external worker file URL, or Control how the WASM file is resolved/fetched so it does not originate from origin: null?

This would allow self-hosting web-demuxer.wasm without needing Access-Control-Allow-Origin: *.

Thanks!

[ForeverSc]

This is currently not supported. The web-demuxer is designed to automatically create a Worker to prevent blocking the main thread. Using it inside an existing Worker would introduce nested workers, which can cause compatibility and stability issues. A more robust solution would be to provide a version that runs on the main thread or allows the internal Worker to be disabled