Failed to execute compile on WebAssembly - expected magic word 00 61 73 6d

Environment

Product	UI for Blazor
Application type	WebAssembly
Hosting	cloudflare.com integration with smarterasp.net, may affect others

Description

Publishing a client-side (WASM) Blazor app may fail with messages similar to the error below.

A particular environment is an ASP.NET Core hosted Blazor project hosted on smarterAsp.net with the automatic config which enabled caching and compacting with cloudflare.com.

Error Message

• WASM: wasm streaming compile failed: Failed to execute 'compile' on 'WebAssembly': Incorrect response MIME type. Expected 'application/wasm'
• WASM: failed to asynchronously prepare wasm: CompileError: WebAssembly.instantiate(): expected magic word 00 61 73 6d, found ef bb bf 3c @+0
• CompileError: WebAssembly.instantiate(): expected magic word 00 61 73 6d, found ef bb bf 3c @+0

Possible Cause

The errors indicate the following issues:

• The MIME type for the wasm files is wrong - it must be application/wasm but the hosting environment used the wrong one
• The wasm file is broken (for example, archived, compressed twice or otherwise truncated/modified).

Solution

The general solution is to make sure that the application files are not tampered with and that you have the correct server settings as per Microsoft's documentation: https://docs.microsoft.com/en-us/aspnet/core/host-and-deploy/blazor/webassembly?view=aspnetcore-3.1

In the particular smarterasp.net hosting case, disabling the cloudflare integration resolved the issue.