Host and deploy ASP.NET Core Blazor WebAssembly

This article explains how to host and deploy Blazor WebAssembly using ASP.NET Core, Content Delivery Networks (CDN), file servers, and GitHub Pages.

With the Blazor WebAssembly hosting model:

• The Blazor app, its dependencies, and the .NET runtime are downloaded to the browser in parallel.
• The app is executed directly on the browser UI thread.

This article pertains to the deployment scenario where the Blazor app is placed on a static hosting web server or service, .NET isn't used to serve the Blazor app. This strategy is covered in the Standalone deployment section, which includes information on hosting a Blazor WebAssembly app as an IIS sub-app.

Subdomain and IIS sub-application hosting

Subdomain hosting doesn't require special configuration of the app. You don't need to configure the app base path (the <base> tag in wwwroot/index.html) to host the app at a subdomain.

IIS sub-application hosting does require you to set the app base path. For more information and cross-links to further guidance on IIS sub-application hosting, see Host and deploy ASP.NET Core Blazor.

Decrease maximum heap size for some mobile device browsers

When building a Blazor app that runs on the client (.Client project of a Blazor Web App or standalone Blazor WebAssembly app) and targets mobile device browsers, especially Safari on iOS, decreasing the maximum memory for the app with the MSBuild property EmccMaximumHeapSize may be required. The default value is 2,147,483,648 bytes, which may be too large and result in the app crashing if the app attempts to allocate more memory with the browser failing to grant it. The following example sets the value to 268,435,456 bytes in the Program file:

<EmccMaximumHeapSize>268435456</EmccMaximumHeapSize>

For more information on Mono/WebAssembly MSBuild properties and targets, see WasmApp.Common.targets (dotnet/runtime GitHub repository).

Webcil packaging format for .NET assemblies

Webcil is a web-friendly packaging format for .NET assemblies designed to enable using Blazor WebAssembly in restrictive network environments. Webcil files use a standard WebAssembly wrapper, where the assemblies are deployed as WebAssembly files that use the standard .wasm file extension.

Webcil is the default packaging format when you publish a Blazor WebAssembly app. To disable the use of Webcil, set the following MSBuild property in the app's project file:

<PropertyGroup>
  <WasmEnableWebcil>false</WasmEnableWebcil>
</PropertyGroup>

Customize how boot resources are loaded

Customize how boot resources are loaded using the loadBootResource API. For more information, see ASP.NET Core Blazor startup.

Compression

When a Blazor WebAssembly app is published, the output is statically compressed during publish to reduce the app's size and remove the overhead for runtime compression. The following compression algorithms are used:

• Brotli (highest level)
• Gzip

Blazor relies on the host to serve the appropriate compressed files. When hosting a Blazor WebAssembly standalone app, additional work might be required to ensure that statically-compressed files are served:

• For IIS web.config compression configuration, see the IIS: Brotli and Gzip compression section.
• When hosting on static hosting solutions that don't support statically-compressed file content negotiation, consider configuring the app to fetch and decode Brotli compressed files:

Obtain the JavaScript Brotli decoder from the google/brotli GitHub repository. The minified decoder file is named decode.min.js and found in the repository's js folder.

Note

If the minified version of the decode.js script (decode.min.js) fails, try using the unminified version (decode.js) instead.

Update the app to use the decoder.

In the wwwroot/index.html file, set autostart to false on Blazor's <script> tag:

<script src="_framework/blazor.webassembly.js" autostart="false"></script>

After Blazor's <script> tag and before the closing </body> tag, add the following JavaScript code <script> block.

Blazor Web App:

<script type="module">
  import { BrotliDecode } from './decode.min.js';
  Blazor.start({
    webAssembly: {
      loadBootResource: function (type, name, defaultUri, integrity) {
        if (type !== 'dotnetjs' && location.hostname !== 'localhost' && type !== 'configuration' && type !== 'manifest') {
          return (async function () {
            const response = await fetch(defaultUri + '.br', { cache: 'no-cache' });
            if (!response.ok) {
              throw new Error(response.statusText);
            }
            const originalResponseBuffer = await response.arrayBuffer();
            const originalResponseArray = new Int8Array(originalResponseBuffer);
            const decompressedResponseArray = BrotliDecode(originalResponseArray);
            const contentType = type === 
              'dotnetwasm' ? 'application/wasm' : 'application/octet-stream';
            return new Response(decompressedResponseArray, 
              { headers: { 'content-type': contentType } });
          })();
        }
      }
    }
  });
</script>

Standalone Blazor WebAssembly:

<script type="module">
  import { BrotliDecode } from './decode.min.js';
  Blazor.start({
    loadBootResource: function (type, name, defaultUri, integrity) {
      if (type !== 'dotnetjs' && location.hostname !== 'localhost' && type !== 'configuration') {
        return (async function () {
          const response = await fetch(defaultUri + '.br', { cache: 'no-cache' });
          if (!response.ok) {
            throw new Error(response.statusText);
          }
          const originalResponseBuffer = await response.arrayBuffer();
          const originalResponseArray = new Int8Array(originalResponseBuffer);
          const decompressedResponseArray = BrotliDecode(originalResponseArray);
          const contentType = type === 
            'dotnetwasm' ? 'application/wasm' : 'application/octet-stream';
          return new Response(decompressedResponseArray, 
            { headers: { 'content-type': contentType } });
        })();
      }
    }
  });
</script>

For more information on loading boot resources, see ASP.NET Core Blazor startup.

To disable compression, add the CompressionEnabled MSBuild property to the app's project file and set the value to false:

<PropertyGroup>
  <CompressionEnabled>false</CompressionEnabled>
</PropertyGroup>

The CompressionEnabled property can be passed to the dotnet publish command with the following syntax in a command shell:

dotnet publish -p:CompressionEnabled=false

Rewrite URLs for correct routing

Routing requests for page components in a Blazor WebAssembly app isn't as straightforward as routing requests in a Blazor Server app. Consider a Blazor WebAssembly app with two components:

• Main.razor: Loads at the root of the app and contains a link to the About component (href="About").
• About.razor: About component.

When the app's default document is requested using the browser's address bar (for example, https://www.contoso.com/):

1. The browser makes a request.
2. The default page is returned, which is usually index.html.
3. index.html bootstraps the app.
4. Router component loads, and the Razor Main component is rendered.

In the Main page, selecting the link to the About component works on the client because the Blazor router stops the browser from making a request on the Internet to www.contoso.com for About and serves the rendered About component itself. All of the requests for internal endpoints within the Blazor WebAssembly app work the same way: Requests don't trigger browser-based requests to server-hosted resources on the Internet. The router handles the requests internally.

If a request is made using the browser's address bar for www.contoso.com/About, the request fails. No such resource exists on the app's Internet host, so a 404 - Not Found response is returned.

Because browsers make requests to Internet-based hosts for client-side pages, web servers and hosting services must rewrite all requests for resources not physically on the server to the index.html page. When index.html is returned, the app's Blazor router takes over and responds with the correct resource.

When deploying to an IIS server, you can use the URL Rewrite Module with the app's published web.config file. For more information, see the IIS section.

Standalone deployment

A standalone deployment serves the Blazor WebAssembly app as a set of static files that are requested directly by clients. Any static file server is able to serve the Blazor app.

Standalone deployment assets are published into either the /bin/Release/{TARGET FRAMEWORK}/publish/wwwroot or bin\Release\{TARGET FRAMEWORK}\browser-wasm\publish\ folder (depending on the version of the .NET SDK in use), where the {TARGET FRAMEWORK} placeholder is the target framework.

Azure App Service

Blazor WebAssembly apps can be deployed to Azure App Services on Windows, which hosts the app on IIS.

Deploying a standalone Blazor WebAssembly app to Azure App Service for Linux isn't currently supported. We recommend hosting a standalone Blazor WebAssembly app using Azure Static Web Apps, which supports this scenario.

Azure Static Web Apps

Use one of the following approaches to deploy a Blazor WebAssembly app to Azure Static Web Apps:

• Deploy from Visual Studio
• Deploy from Visual Studio Code
• Deploy from GitHub

Deploy from Visual Studio

To deploy from Visual Studio, create a publish profile for Azure Static Web Apps:

1. Save any unsaved work in the project, as a Visual Studio restart might be required during the process.

2. In Visual Studio's Publish UI, select Target > Azure > Specific Target > Azure Static Web Apps to create a publish profile.

3. If the Azure WebJobs Tools component for Visual Studio isn't installed, a prompt appears to install the ASP.NET and web development component. Follow the prompts to install the tools using the Visual Studio Installer. Visual Studio closes and reopens automatically while installing the tools. After the tools are installed, start over at the first step to create the publish profile.

4. In the publish profile configuration, provide the Subscription name. Select an existing instance, or select Create a new instance. When creating a new instance in the Azure portal's Create Static Web App UI, set the Deployment details > Source to Other. Wait for the deployment to complete in the Azure portal before proceeding.

5. In the publish profile configuration, select the Azure Static Web Apps instance from the instance's resource group. Select Finish to create the publish profile. If Visual Studio prompts to install the Static Web Apps (SWA) CLI, install the CLI by following the prompts. The SWA CLI requires NPM/Node.js (Visual Studio documentation).

After the publish profile is created, deploy the app to the Azure Static Web Apps instance using the publish profile by selecting the Publish button.

Deploy from Visual Studio Code

To deploy from Visual Studio Code, see Quickstart: Build your first static site with Azure Static Web Apps.

Deploy from GitHub

To deploy from a GitHub repository, see Tutorial: Building a static web app with Blazor in Azure Static Web Apps.

IIS

IIS is a capable static file server for Blazor apps. To configure IIS to host Blazor, see Build a Static Website on IIS.

Published assets are created in the /bin/Release/{TARGET FRAMEWORK}/publish or bin\Release\{TARGET FRAMEWORK}\browser-wasm\publish folder, depending on which version of the SDK is used and where the {TARGET FRAMEWORK} placeholder is the target framework. Host the contents of the publish folder on the web server or hosting service.

web.config

When a Blazor project is published, a web.config file is created with the following IIS configuration:

• MIME types
• HTTP compression is enabled for the following MIME types:
  • application/octet-stream
  • application/wasm
• URL Rewrite Module rules are established:
  • Serve the sub-directory where the app's static assets reside (wwwroot/{PATH REQUESTED}).
  • Create SPA fallback routing so that requests for non-file assets are redirected to the app's default document in its static assets folder (wwwroot/index.html).

Use a custom web.config

To use a custom web.config file:

1. Place the custom web.config file in the project's root folder.
2. Publish the project. For more information, see Host and deploy ASP.NET Core Blazor.

If the SDK's web.config generation or transformation during publish either doesn't move the file to published assets in the publish folder or modifies the custom configuration in your custom web.config file, use any of the following approaches as needed to take full control of the process:

• If the SDK doesn't generate the file, for example, in a standalone Blazor WebAssembly app at /bin/Release/{TARGET FRAMEWORK}/publish/wwwroot or bin\Release\{TARGET FRAMEWORK}\browser-wasm\publish, depending on which version of the SDK is used and where the {TARGET FRAMEWORK} placeholder is the target framework, set the <PublishIISAssets> property to true in the project file (.csproj). Usually for standalone WebAssembly apps, this is the only required setting to move a custom web.config file and prevent transformation of the file by the SDK.

  <PropertyGroup>
    <PublishIISAssets>true</PublishIISAssets>
  </PropertyGroup>
  

• Disable the SDK's web.config transformation in the project file (.csproj):

  <PropertyGroup>
    <IsTransformWebConfigDisabled>true</IsTransformWebConfigDisabled>
  </PropertyGroup>
  

• Add a custom target to the project file (.csproj) to move a custom web.config file. In the following example, the custom web.config file is placed by the developer at the root of the project. If the web.config file resides elsewhere, specify the path to the file in SourceFiles. The following example specifies the publish folder with $(PublishDir), but provide a path to DestinationFolder for a custom output location.

  <Target Name="CopyWebConfig" AfterTargets="Publish">
    <Copy SourceFiles="web.config" DestinationFolder="$(PublishDir)" />
  </Target>
  

Install the URL Rewrite Module

The URL Rewrite Module is required to rewrite URLs. The module isn't installed by default, and it isn't available for install as a Web Server (IIS) role service feature. The module must be downloaded from the IIS website. Use the Web Platform Installer to install the module:

1. Locally, navigate to the URL Rewrite Module downloads page. For the English version, select WebPI to download the WebPI installer. For other languages, select the appropriate architecture for the server (x86/x64) to download the installer.
2. Copy the installer to the server. Run the installer. Select the Install button and accept the license terms. A server restart isn't required after the install completes.

Configure the website

Set the website's Physical path to the app's folder. The folder contains:

• The web.config file that IIS uses to configure the website, including the required redirect rules and file content types.
• The app's static asset folder.

Host as an IIS sub-app

If a standalone app is hosted as an IIS sub-app, perform either of the following:

• Disable the inherited ASP.NET Core Module handler.

  Remove the handler in the Blazor app's published web.config file by adding a <handlers> section to the <system.webServer> section of the file:

  <handlers>
    <remove name="aspNetCore" />
  </handlers>
  

• Disable inheritance of the root (parent) app's <system.webServer> section using a <location> element with inheritInChildApplications set to false:

  <?xml version="1.0" encoding="utf-8"?>
  <configuration>
    <location path="." inheritInChildApplications="false">
      <system.webServer>
        <handlers>
          <add name="aspNetCore" ... />
        </handlers>
        <aspNetCore ... />
      </system.webServer>
    </location>
  </configuration>
  

  Note

  Disabling inheritance of the root (parent) app's <system.webServer> section is the default configuration for published apps using the .NET SDK.

Removing the handler or disabling inheritance is performed in addition to configuring the app's base path. Set the app base path in the app's index.html file to the IIS alias used when configuring the sub-app in IIS.

Configure the app's base path by following the guidance in the Host and deploy ASP.NET Core Blazor article.

Brotli and Gzip compression

This section only applies to standalone Blazor WebAssembly apps.

IIS can be configured via web.config to serve Brotli or Gzip compressed Blazor assets for standalone Blazor WebAssembly apps. For an example configuration file, see web.config.

Additional configuration of the example web.config file might be required in the following scenarios:

• The app's specification calls for either of the following:
  • Serving compressed files that aren't configured by the example web.config file.
  • Serving compressed files configured by the example web.config file in an uncompressed format.
• The server's IIS configuration (for example, applicationHost.config) provides server-level IIS defaults. Depending on the server-level configuration, the app might require a different IIS configuration than what the example web.config file contains.

For more information on custom web.config files, see the Use a custom web.config section.

Troubleshooting

If a 500 - Internal Server Error is received and IIS Manager throws errors when attempting to access the website's configuration, confirm that the URL Rewrite Module is installed. When the module isn't installed, the web.config file can't be parsed by IIS. This prevents the IIS Manager from loading the website's configuration and the website from serving Blazor's static files.

For more information on troubleshooting deployments to IIS, see Troubleshoot ASP.NET Core on Azure App Service and IIS.

Azure Storage

Azure Storage static file hosting allows serverless Blazor app hosting. Custom domain names, the Azure Content Delivery Network (CDN), and HTTPS are supported.

When the blob service is enabled for static website hosting on a storage account:

• Set the Index document name to index.html.
• Set the Error document path to index.html. Razor components and other non-file endpoints don't reside at physical paths in the static content stored by the blob service. When a request for one of these resources is received that the Blazor router should handle, the 404 - Not Found error generated by the blob service routes the request to the Error document path. The index.html blob is returned, and the Blazor router loads and processes the path.

If files aren't loaded at runtime due to inappropriate MIME types in the files' Content-Type headers, take either of the following actions:

• Configure your tooling to set the correct MIME types (Content-Type headers) when the files are deployed.

• Change the MIME types (Content-Type headers) for the files after the app is deployed.

  In Storage Explorer (Azure portal) for each file:

  1. Right-click the file and select Properties.
  2. Set the ContentType and select the Save button.

For more information, see Static website hosting in Azure Storage.

Nginx

The following nginx.conf file is simplified to show how to configure Nginx to send the index.html file whenever it can't find a corresponding file on disk.

events { }
http {
    server {
        listen 80;

        location / {
            root      /usr/share/nginx/html;
            try_files $uri $uri/ /index.html =404;
        }
    }
}

When setting the NGINX burst rate limit with limit_req, Blazor WebAssembly apps may require a large burst parameter value to accommodate the relatively large number of requests made by an app. Initially, set the value to at least 60:

http {
    server {
        ...

        location / {
            ...

            limit_req zone=one burst=60 nodelay;
        }
    }
}

Increase the value if browser developer tools or a network traffic tool indicates that requests are receiving a 503 - Service Unavailable status code.

For more information on production Nginx web server configuration, see Creating NGINX Plus and NGINX Configuration Files.

Apache

To deploy a Blazor WebAssembly app to Apache:

1. Create the Apache configuration file. The following example is a simplified configuration file (blazorapp.config):

   <VirtualHost *:80>
       ServerName www.example.com
       ServerAlias *.example.com
   
       DocumentRoot "/var/www/blazorapp"
       ErrorDocument 404 /index.html
   
       AddType application/wasm .wasm
   
       <Directory "/var/www/blazorapp">
           Options -Indexes
           AllowOverride None
       </Directory>
   
       <IfModule mod_deflate.c>
           AddOutputFilterByType DEFLATE text/css
           AddOutputFilterByType DEFLATE application/javascript
           AddOutputFilterByType DEFLATE text/html
           AddOutputFilterByType DEFLATE application/octet-stream
           AddOutputFilterByType DEFLATE application/wasm
           <IfModule mod_setenvif.c>
               BrowserMatch ^Mozilla/4 gzip-only-text/html
               BrowserMatch ^Mozilla/4.0[678] no-gzip
               BrowserMatch bMSIE !no-gzip !gzip-only-text/html
           </IfModule>
       </IfModule>
   
       ErrorLog /var/log/httpd/blazorapp-error.log
       CustomLog /var/log/httpd/blazorapp-access.log common
   </VirtualHost>
   

1. Place the Apache configuration file into the /etc/httpd/conf.d/ directory.

2. Place the app's published assets (/bin/Release/{TARGET FRAMEWORK}/publish/wwwroot, where the {TARGET FRAMEWORK} placeholder is the target framework) into the /var/www/blazorapp directory (the location specified to DocumentRoot in the configuration file).

3. Restart the Apache service.

For more information, see mod_mime and mod_deflate.

GitHub Pages

The default GitHub Action, which deploys pages, skips deployment of folders starting with underscore, for example, the _framework folder. To deploy folders starting with underscore, add an empty .nojekyll file to the Git branch.

Git treats JavaScript (JS) files, such as blazor.webassembly.js, as text and converts line endings from CRLF (carriage return-line feed) to LF (line feed) in the deployment pipeline. These changes to JS files produce different file hashes than Blazor sends to the client in the blazor.boot.json file. The mismatches result in integrity check failures on the client. One approach to solving this problem is to add a .gitattributes file with *.js binary line before adding the app's assets to the Git branch. The *.js binary line configures Git to treat JS files as binary files, which avoids processing the files in the deployment pipeline. The file hashes of the unprocessed files match the entries in the blazor.boot.json file, and client-side integrity checks pass. For more information, see ASP.NET Core Blazor WebAssembly .NET runtime and app bundle caching.

To handle URL rewrites, add a wwwroot/404.html file with a script that handles redirecting the request to the index.html page. For an example, see the SteveSandersonMS/BlazorOnGitHubPages GitHub repository:

• wwwroot/404.html
• Live site

When using a project site instead of an organization site, update the <base> tag in wwwroot/index.html. Set the href attribute value to the GitHub repository name with a trailing slash (for example, /my-repository/). In the SteveSandersonMS/BlazorOnGitHubPages GitHub repository, the base href is updated at publish by the .github/workflows/main.yml configuration file.

Note

The SteveSandersonMS/BlazorOnGitHubPages GitHub repository isn't owned, maintained, or supported by the .NET Foundation or Microsoft.

Standalone with Docker

A standalone Blazor WebAssembly app is published as a set of static files for hosting by a static file server.

To host the app in Docker:

• Choose a Docker container with web server support, such as Ngnix or Apache.
• Copy the publish folder assets to a location folder defined in the web server for serving static files.
• Apply additional configuration as needed to serve the Blazor WebAssembly app.

For configuration guidance, see the following resources:

• Nginx section or Apache section of this article
• Docker Documentation

Host configuration values

Blazor WebAssembly apps can accept the following host configuration values as command-line arguments at runtime in the development environment.

Content root

The --contentroot argument sets the absolute path to the directory that contains the app's content files (content root). In the following examples, /content-root-path is the app's content root path.

• Pass the argument when running the app locally at a command prompt. From the app's directory, execute:

  dotnet watch --contentroot=/content-root-path
  

• Add an entry to the app's launchSettings.json file in the IIS Express profile. This setting is used when the app is run with the Visual Studio Debugger and from a command prompt with dotnet watch (or dotnet run).

  "commandLineArgs": "--contentroot=/content-root-path"
  

• In Visual Studio, specify the argument in Properties > Debug > Application arguments. Setting the argument in the Visual Studio property page adds the argument to the launchSettings.json file.

  --contentroot=/content-root-path
  

Path base

The --pathbase argument sets the app base path for an app run locally with a non-root relative URL path (the <base> tag href is set to a path other than / for staging and production). In the following examples, /relative-URL-path is the app's path base. For more information, see App base path.

Important

Unlike the path provided to href of the <base> tag, don't include a trailing slash (/) when passing the --pathbase argument value. If the app base path is provided in the <base> tag as <base href="/CoolApp/"> (includes a trailing slash), pass the command-line argument value as --pathbase=/CoolApp (no trailing slash).

• Pass the argument when running the app locally at a command prompt. From the app's directory, execute:

  dotnet watch --pathbase=/relative-URL-path
  

• Add an entry to the app's launchSettings.json file in the IIS Express profile. This setting is used when running the app with the Visual Studio Debugger and from a command prompt with dotnet watch (or dotnet run).

  "commandLineArgs": "--pathbase=/relative-URL-path"
  

• In Visual Studio, specify the argument in Properties > Debug > Application arguments. Setting the argument in the Visual Studio property page adds the argument to the launchSettings.json file.

  --pathbase=/relative-URL-path
  

URLs

The --urls argument sets the IP addresses or host addresses with ports and protocols to listen on for requests.

• Pass the argument when running the app locally at a command prompt. From the app's directory, execute:

  dotnet watch --urls=http://127.0.0.1:0
  

• Add an entry to the app's launchSettings.json file in the IIS Express profile. This setting is used when running the app with the Visual Studio Debugger and from a command prompt with dotnet watch (or dotnet run).

  "commandLineArgs": "--urls=http://127.0.0.1:0"
  

• In Visual Studio, specify the argument in Properties > Debug > Application arguments. Setting the argument in the Visual Studio property page adds the argument to the launchSettings.json file.

  --urls=http://127.0.0.1:0
  

Configure the Trimmer

Blazor performs Intermediate Language (IL) trimming on each Release build to remove unnecessary IL from the output assemblies. For more information, see Configure the Trimmer for ASP.NET Core Blazor.

Change the file name extension of DLL files

This section applies to ASP.NET Core 6.x and 7.x. In ASP.NET Core in .NET 8 or later, .NET assemblies are deployed as WebAssembly files (.wasm) using the Webcil file format. In ASP.NET Core in .NET 8 or later, this section only applies if the Webcil file format has been disabled in the app's project file.

If a firewall, anti-virus program, or network security appliance is blocking the transmission of the app's dynamic-link library (DLL) files (.dll), you can follow the guidance in this section to change the file name extensions of the app's published DLL files.

Note

Changing the file name extensions of the app's DLL files might not resolve the problem because many security systems scan the content of the app's files, not merely check file extensions.

For a more robust approach in environments that block the download and execution of DLL files, use ASP.NET Core in .NET 8 or later, which packages .NET assemblies as WebAssembly files (.wasm) using the Webcil file format. For more information, see the Webcil packaging format for .NET assemblies section in an 8.0 or later version of this article.

Third-party approaches exist for dealing with this problem. For more information, see the resources at Awesome Blazor.

After publishing the app, use a shell script or DevOps build pipeline to rename .dll files to use a different file extension in the directory of the app's published output.

In the following examples:

• PowerShell (PS) is used to update the file extensions.
• .dll files are renamed to use the .bin file extension from the command line.
• Files listed in the published blazor.boot.json file with a .dll file extension are updated to the .bin file extension.
• If service worker assets are also in use, a PowerShell command updates the .dll files listed in the service-worker-assets.js file to the .bin file extension.

To use a different file extension than .bin, replace .bin in the following commands with the desired file extension.

On Windows:

dir {PATH} | rename-item -NewName { $_.name -replace ".dll\b",".bin" }
((Get-Content {PATH}\blazor.boot.json -Raw) -replace '.dll"','.bin"') | Set-Content {PATH}\blazor.boot.json

In the preceding command, the {PATH} placeholder is the path to the published _framework folder (for example, .\bin\Release\net6.0\browser-wasm\publish\wwwroot\_framework from the project's root folder).

If service worker assets are also in use:

((Get-Content {PATH}\service-worker-assets.js -Raw) -replace '.dll"','.bin"') | Set-Content {PATH}\service-worker-assets.js

In the preceding command, the {PATH} placeholder is the path to the published service-worker-assets.js file.

On Linux or macOS:

for f in {PATH}/*; do mv "$f" "`echo $f | sed -e 's/\.dll/.bin/g'`"; done
sed -i 's/\.dll"/.bin"/g' {PATH}/blazor.boot.json

In the preceding command, the {PATH} placeholder is the path to the published _framework folder (for example, .\bin\Release\net6.0\browser-wasm\publish\wwwroot\_framework from the project's root folder).

If service worker assets are also in use:

sed -i 's/\.dll"/.bin"/g' {PATH}/service-worker-assets.js

In the preceding command, the {PATH} placeholder is the path to the published service-worker-assets.js file.

To address the compressed blazor.boot.json.gz and blazor.boot.json.br files, adopt either of the following approaches:

• Remove the compressed blazor.boot.json.gz and blazor.boot.json.br files. Compression is disabled with this approach.
• Recompress the updated blazor.boot.json file.

The preceding guidance for the compressed blazor.boot.json file also applies when service worker assets are in use. Remove or recompress service-worker-assets.js.br and service-worker-assets.js.gz. Otherwise, file integrity checks fail in the browser.

The following Windows example for .NET 6 uses a PowerShell script placed at the root of the project. The following script, which disables compression, is the basis for further modification if you wish to recompress the blazor.boot.json file.

ChangeDLLExtensions.ps1::

param([string]$filepath,[string]$tfm)
dir $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\_framework | rename-item -NewName { $_.name -replace ".dll\b",".bin" }
((Get-Content $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\_framework\blazor.boot.json -Raw) -replace '.dll"','.bin"') | Set-Content $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\_framework\blazor.boot.json
Remove-Item $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\_framework\blazor.boot.json.gz
Remove-Item $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\_framework\blazor.boot.json.br

If service worker assets are also in use, add the following commands:

((Get-Content $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\service-worker-assets.js -Raw) -replace '.dll"','.bin"') | Set-Content $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\_framework\wwwroot\service-worker-assets.js
Remove-Item $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\_framework\wwwroot\service-worker-assets.js.gz
Remove-Item $filepath\bin\Release\$tfm\browser-wasm\publish\wwwroot\_framework\wwwroot\service-worker-assets.js.br

In the project file, the script is executed after publishing the app for the Release configuration:

<Target Name="ChangeDLLFileExtensions" AfterTargets="AfterPublish" Condition="'$(Configuration)'=='Release'">
  <Exec Command="powershell.exe -command &quot;&amp; { .\ChangeDLLExtensions.ps1 '$(SolutionDir)' '$(TargetFramework)'}&quot;" />
</Target>

Note

When renaming and lazy loading the same assemblies, see the guidance in Lazy load assemblies in ASP.NET Core Blazor WebAssembly.

Usually, the app's server requires static asset configuration to serve the files with the updated extension. For an app hosted by IIS, add a MIME map entry (<mimeMap>) for the new file extension in the static content section (<staticContent>) in a custom web.config file. The following example assumes that the file extension is changed from .dll to .bin:

<staticContent>
  ...
  <mimeMap fileExtension=".bin" mimeType="application/octet-stream" />
  ...
</staticContent>

Include an update for compressed files if compression is in use:

<mimeMap fileExtension=".bin.br" mimeType="application/octet-stream" />
<mimeMap fileExtension=".bin.gz" mimeType="application/octet-stream" />

Remove the entry for the .dll file extension:

- <mimeMap fileExtension=".dll" mimeType="application/octet-stream" />

Remove entries for compressed .dll files if compression is in use:

- <mimeMap fileExtension=".dll.br" mimeType="application/octet-stream" />
- <mimeMap fileExtension=".dll.gz" mimeType="application/octet-stream" />

For more information on custom web.config files, see the Use a custom web.config section.

Prior deployment corruption

Typically on deployment:

• Only the files that have changed are replaced, which usually results in a faster deployment.
• Existing files that aren't part of the new deployment are left in place for use by the new deployment.

In rare cases, lingering files from a prior deployment can corrupt a new deployment. Completely deleting the existing deployment (or locally-published app prior to deployment) may resolve the issue with a corrupted deployment. Often, deleting the existing deployment once is sufficient to resolve the problem, including for a DevOps build and deploy pipeline.

If you determine that clearing a prior deployment is always required when a DevOps build and deploy pipeline is in use, you can temporarily add a step to the build pipeline to delete the prior deployment for each new deployment until you troubleshoot the exact cause of the corruption.

Resolve integrity check failures

When Blazor WebAssembly downloads an app's startup files, it instructs the browser to perform integrity checks on the responses. Blazor sends SHA-256 hash values for DLL (.dll), WebAssembly (.wasm), and other files in the blazor.boot.json file, which isn't cached on clients. The file hashes of cached files are compared to the hashes in the blazor.boot.json file. For cached files with a matching hash, Blazor uses the cached files. Otherwise, files are requested from the server. After a file is downloaded, its hash is checked again for integrity validation. An error is generated by the browser if any downloaded file's integrity check fails.

Blazor's algorithm for managing file integrity:

• Ensures that the app doesn't risk loading an inconsistent set of files, for example if a new deployment is applied to your web server while the user is in the process of downloading the application files. Inconsistent files can result in a malfunctioning app.
• Ensures the user's browser never caches inconsistent or invalid responses, which can prevent the app from starting even if the user manually refreshes the page.
• Makes it safe to cache the responses and not check for server-side changes until the expected SHA-256 hashes themselves change, so subsequent page loads involve fewer requests and complete faster.

If the web server returns responses that don't match the expected SHA-256 hashes, an error similar to the following example appears in the browser's developer console:

Failed to find a valid digest in the 'integrity' attribute for resource 'https://myapp.example.com/_framework/MyBlazorApp.dll' with computed SHA-256 integrity 'IIa70iwvmEg5WiDV17OpQ5eCztNYqL186J56852RpJY='. The resource has been blocked.

In most cases, the warning doesn't indicate a problem with integrity checking. Instead, the warning usually means that some other problem exists.

For Blazor WebAssembly's boot reference source, see the Boot.WebAssembly.ts file in the dotnet/aspnetcore GitHub repository.

Note

Documentation links to .NET reference source usually load the repository's default branch, which represents the current development for the next release of .NET. To select a tag for a specific release, use the Switch branches or tags dropdown list. For more information, see How to select a version tag of ASP.NET Core source code (dotnet/AspNetCore.Docs #26205).

Diagnosing integrity problems

When an app is built, the generated blazor.boot.json manifest describes the SHA-256 hashes of boot resources at the time that the build output is produced. The integrity check passes as long as the SHA-256 hashes in blazor.boot.json match the files delivered to the browser.

Common reasons why this fails include:

• The web server's response is an error (for example, a 404 - Not Found or a 500 - Internal Server Error) instead of the file the browser requested. This is reported by the browser as an integrity check failure and not as a response failure.
• Something has changed the contents of the files between the build and delivery of the files to the browser. This might happen:
  • If you or build tools manually modify the build output.
  • If some aspect of the deployment process modified the files. For example if you use a Git-based deployment mechanism, bear in mind that Git transparently converts Windows-style line endings to Unix-style line endings if you commit files on Windows and check them out on Linux. Changing file line endings change the SHA-256 hashes. To avoid this problem, consider using .gitattributes to treat build artifacts as binary files.
  • The web server modifies the file contents as part of serving them. For example, some content distribution networks (CDNs) automatically attempt to minify HTML, thereby modifying it. You may need to disable such features.
• The blazor.boot.json file fails to load properly or is improperly cached on the client. Common causes include either of the following:
  • Misconfigured or malfunctioning custom developer code.
  • One or more misconfigured intermediate caching layers.

To diagnose which of these applies in your case:

1. Note which file is triggering the error by reading the error message.
2. Open your browser's developer tools and look in the Network tab. If necessary, reload the page to see the list of requests and responses. Find the file that is triggering the error in that list.
3. Check the HTTP status code in the response. If the server returns anything other than 200 - OK (or another 2xx status code), then you have a server-side problem to diagnose. For example, status code 403 means there's an authorization problem, whereas status code 500 means the server is failing in an unspecified manner. Consult server-side logs to diagnose and fix the app.
4. If the status code is 200 - OK for the resource, look at the response content in browser's developer tools and check that the content matches up with the data expected. For example, a common problem is to misconfigure routing so that requests return your index.html data even for other files. Make sure that responses to .wasm requests are WebAssembly binaries and that responses to .dll requests are .NET assembly binaries. If not, you have a server-side routing problem to diagnose.
5. Seek to validate the app's published and deployed output with the Troubleshoot integrity PowerShell script.

If you confirm that the server is returning plausibly correct data, there must be something else modifying the contents in between build and delivery of the file. To investigate this:

• Examine the build toolchain and deployment mechanism in case they're modifying files after the files are built. An example of this is when Git transforms file line endings, as described earlier.
• Examine the web server or CDN configuration in case they're set up to modify responses dynamically (for example, trying to minify HTML). It's fine for the web server to implement HTTP compression (for example, returning content-encoding: br or content-encoding: gzip), since this doesn't affect the result after decompression. However, it's not fine for the web server to modify the uncompressed data.

Troubleshoot integrity PowerShell script

Use the integrity.ps1 PowerShell script to validate a published and deployed Blazor app. The script is provided for PowerShell Core 7 or later as a starting point when the app has integrity issues that the Blazor framework can't identify. Customization of the script might be required for your apps, including if running on version of PowerShell later than version 7.2.0.

The script checks the files in the publish folder and downloaded from the deployed app to detect issues in the different manifests that contain integrity hashes. These checks should detect the most common problems:

• You modified a file in the published output without realizing it.
• The app wasn't correctly deployed to the deployment target, or something changed within the deployment target's environment.
• There are differences between the deployed app and the output from publishing the app.

Invoke the script with the following command in a PowerShell command shell:

.\integrity.ps1 {BASE URL} {PUBLISH OUTPUT FOLDER}

In the following example, the script is executed on a locally-running app at https://localhost:5001/:

.\integrity.ps1 https://localhost:5001/ C:\TestApps\BlazorSample\bin\Release\net6.0\publish\

Placeholders:

• {BASE URL}: The URL of the deployed app. A trailing slash (/) is required.
• {PUBLISH OUTPUT FOLDER}: The path to the app's publish folder or location where the app is published for deployment.

Note

When cloning the dotnet/AspNetCore.Docs GitHub repository, the integrity.ps1 script might be quarantined by Bitdefender or another virus scanner present on the system. Usually, the file is trapped by a virus scanner's heuristic scanning technology, which merely looks for patterns in files that might indicate the presence of malware. To prevent the virus scanner from quarantining the file, add an exception to the virus scanner prior to cloning the repo. The following example is a typical path to the script on a Windows system. Adjust the path as needed for other systems. The {USER} placeholder is the user's path segment.

C:\Users\{USER}\Documents\GitHub\AspNetCore.Docs\aspnetcore\blazor\host-and-deploy\webassembly\_samples\integrity.ps1

Warning: Creating virus scanner exceptions is dangerous and should only be performed when you're certain that the file is safe.

Comparing the checksum of a file to a valid checksum value doesn't guarantee file safety, but modifying a file in a way that maintains a checksum value isn't trivial for malicious users. Therefore, checksums are useful as a general security approach. Compare the checksum of the local integrity.ps1 file to one of the following values:

• SHA256: 32c24cb667d79a701135cb72f6bae490d81703323f61b8af2c7e5e5dc0f0c2bb
• MD5: 9cee7d7ec86ee809a329b5406fbf21a8

Obtain the file's checksum on Windows OS with the following command. Provide the path and file name for the {PATH AND FILE NAME} placeholder and indicate the type of checksum to produce for the {SHA512|MD5} placeholder, either SHA256 or MD5:

CertUtil -hashfile {PATH AND FILE NAME} {SHA256|MD5}

If you have any cause for concern that checksum validation isn't secure enough in your environment, consult your organization's security leadership for guidance.

For more information, see Overview of threat protection by Microsoft Defender Antivirus.

Disable integrity checking for non-PWA apps

In most cases, don't disable integrity checking. Disabling integrity checking doesn't solve the underlying problem that has caused the unexpected responses and results in losing the benefits listed earlier.

There may be cases where the web server can't be relied upon to return consistent responses, and you have no choice but to temporarily disable integrity checks until the underlying problem is resolved.

To disable integrity checks, add the following to a property group in the Blazor WebAssembly app's project file (.csproj):

<BlazorCacheBootResources>false</BlazorCacheBootResources>

BlazorCacheBootResources also disables Blazor's default behavior of caching the .dll, .wasm, and other files based on their SHA-256 hashes because the property indicates that the SHA-256 hashes can't be relied upon for correctness. Even with this setting, the browser's normal HTTP cache may still cache those files, but whether or not this happens depends on your web server configuration and the cache-control headers that it serves.

Note

The BlazorCacheBootResources property doesn't disable integrity checks for Progressive Web Applications (PWAs). For guidance pertaining to PWAs, see the Disable integrity checking for PWAs section.

We can't provide an exhaustive list of scenarios where disabling integrity checking is required. Servers can answer a request in arbitrary ways outside of the scope of the Blazor framework. The framework provides the BlazorCacheBootResources setting to make the app runnable at the cost of losing a guarantee of integrity that the app can provide. Again, we don't recommend disabling integrity checking, especially for production deployments. Developers should seek to solve the underlying integrity problem that's causing integrity checking to fail.

A few general cases that can cause integrity issues are:

• Running on HTTP where integrity can't be checked.
• If your deployment process modifies the files after publish in any way.
• If your host modifies the files in any way.

Disable integrity checking for PWAs

Blazor's Progressive Web Application (PWA) template contains a suggested service-worker.published.js file that's responsible for fetching and storing application files for offline use. This is a separate process from the normal app startup mechanism and has its own separate integrity checking logic.

Inside the service-worker.published.js file, following line is present:

.map(asset => new Request(asset.url, { integrity: asset.hash }));

To disable integrity checking, remove the integrity parameter by changing the line to the following:

.map(asset => new Request(asset.url));

Again, disabling integrity checking means that you lose the safety guarantees offered by integrity checking. For example, there is a risk that if the user's browser is caching the app at the exact moment that you deploy a new version, it could cache some files from the old deployment and some from the new deployment. If that happens, the app becomes stuck in a broken state until you deploy a further update.