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When you load a with a nonce, that script can await import() additional sources from any hostname. The nonce is propagated automatically for the one purpose of importing more scripts. This is not related to strict-dynamic, which propagates nonces for any propose not limited to imports (e.g. inserting elements). Example We have a restrictive CSP that only allows nonces: Content-Security-Policy: default-src 'none'; script-src 'nonce-secret123' Our HTML loads script.js using that nonce: Our script.js imports other.js without a nonce: let other = await import('other.js') console.log("Look, script.js has imported %o", other) The import succeeds without a nonce, due to implicit nonce propagation. Why this is useful In modern build pipelines, code splitting (chunking) is implemented using dynamic imports. Nonce propagation allows us to use automatic chunking with restrictive, nonce-based CSPs without using strict-dynamic. E.g. esbuild automatically groups dynamically imported modules into chunks, and writes that chunk to disk. The compiled build has an await import('assets/chunk-NAXSMFJV.js'). There's no way to inject a nonce into that import(), but implicit nonce propagation still allows the request. Should I worry about this? It would require some truly strange code for user input to make it into an import() argument. I wouldn't lose sleep over this. Is this a browser bug? It is by design. Here are some sources: HTML Spec Section 8 (Web Application APIs) (search for "descendant script fetch options") Chromium test ensuring none propagation Firefox bug implementing nonce propagation CSP issue: Someone concerned about propagation being a vulnerability CSP issue: Proposal for import-src that went nowhere Are other CSP sources also propagated? No, only nonces. In particular host-based CSPs do not propagate trust. For example, you only allow scripts from our own host (no nonces): Content-Security-Policy: default-src 'none'; script-src 'self' Our HTML loads script.js from our own host: Our script.js imports other.js from a different host: let other = await import('https://other-host.com/other.js') This fails with a CSP violation: Executing inline script violates the following Content Security Policy directive 'script-src 'self''

Below is a strict, but still workable Content Security Policy for your Ruby on Rails project. Use this CSP if...