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Content Security Policy

Content Security Policy (CSP) is a supplementary security approach which helps you detect and handle specific security attacks such as Cross-Site Scripting (XSS) and data-injection ones.

If the strict CSP mode is enabled, some browser features are disabled by default:

  • Inline JavaScript, such as <script></script>, or DOM event attributes, such as onclick, are blocked. All script code must reside in separate files that are served from a white-listed domain.

  • Dynamic code evaluation through eval() and string arguments for both setTimeout and setInterval are blocked.

(For R1 2023 SP1 and Later) Working with Telerik UI for ASP.NET MVC Components

As of R1 2023 release, the Kendo UI scripts address the unsafe-eval directive for all components except for the Spreadsheet. For the bigger part of its core engine, the Kendo UI for jQuery Spreadsheet uses the Function evaluation, and rewriting the logic of the component will lead to a great number of breaking changes.

The rest of the Kendo UI components and internal mechanisms have been completely rewritten to discard the usage of the eval() and new Function() calls.

When Kendo UI components are initialized from HTML helpers there are inline scripts that are generated automatically after each component HTML markup. When the CSP is enabled, you will get the following error: Refused to execute inline script because it violates the following Content Security Policy directive: "script-src 'unsafe-eval' 'self'

To resolve the issue and prevent the components from being dependent on the unsafe-eval directive, the R1 2023 SP1 release introduces the following features:

Deferring Scripts to File

The DeferredScriptFile method simulates loading the initialization scripts as a JS file through a HttpModule to ensure strict CSP compliance. You can either use the global deferred initialization functionality to configure all Telerik UI for ASP.NET MVC components as deferred globally or defer each component separately by using the Deferred method.

Call the method after all components declarations to serialize the deferred initialization scripts as a JS file.


Creating Content Security Policy Templates

Most of the components support templating options, which use the Kendo UI Templates syntax, for example, Grid templates, DropDownList templates, and more. To avoid using the inline and external Kendo UI templates and remove the unsafe-eval keyword from the meta tag of your Telerik UI for ASP.NET MVC application, you can define functional templates. For more information on the CSP-compatible templates, refer to the CSP-compatible templates section.

The example below demonstrates how to define a CSP-compatible client detail template of a Grid.

        .Columns(columns =>
            columns.Bound(product => product.CategoryID);
            columns.Bound(product => product.CategoryName);
        .DataSource(dataSource => dataSource
            .Read(read => read.Action("Categories_Read", "Home"))

    function clientDetailTemplateHandler(data) {
        return `<p>${data.CategoryDetails}</p>`

The engine for the inline and external templates will remain available. However, if you are using the previous template syntax, you must include the usafe-eval directive in the meta tag.

(Prior to R1 2023 SP1) Working with Telerik UI for ASP.NET MVC Components

The Telerik UI for ASP.NET MVC releases prior to the R1 2023 SP1 one does not support the strict CSP mode. Thus, in these previous versions, if the Content Security Policy (CSP) is enabled, you could set the script-src policy as follows:

  1. Defer the individual components:

            .Items(items =>
  2. Render the initialization logic in a script using nonce.

        <script type="text/javascript" nonce="kendoInlineScript">
  3. Extend the meta CSP tag to include the unsafe-eval keyword and the nonce signature for enabling the CSP mode:

        <meta http-equiv="Content-Security-Policy" content="script-src 'unsafe-eval' 'self' 'nonce-kendoInlineScript';">

See Also

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