Content Security Policy for ASP.NET Web Forms Applications

Dec 01, 2023
5 minutes to read

A Content Security Policy (CSP) is an additional layer of security built into most modern browsers. It allows the browser to recognize and mitigate certain types of risks, including Cross Site Scripting (XSS) and data injection attacks. These attacks include, but are not limited to, data theft, page spoofing, and malware distribution.

The CSP defines a list of policies/directives and initial values that specify which resources your site will allow/disallow.

To enable CSP, specify a Content-Security-Policy header or use the <meta> tag to explicitly define authorized functionality with CSP directives.

The following meta tag specifies minimum required directives for DevExpress Reporting components:

<head>
<!--...-->
    <meta http-equiv="Content-Security-Policy" content="default-src 'self';
    img-src data: https: http:;
    script-src 'self' 'unsafe-inline';
    style-src 'self' 'unsafe-inline';
    connect-src 'self';
    worker-src 'self' blob:;
    frame-src 'self' blob:;" />
<!--...-->
</head>

default-src 'self';

Fallback for other fetch directives.

img-src data: https: http:;

Allows components to load specific images and document pages.

script-src 'self' 'unsafe-inline';

Allows the use of inline scripts.

You can remove the unsafe-inline keyword. Refer to the following section for more information: Disallow Inline Styles and Inline Scripts.

style-src 'self' 'unsafe-inline';

Allows the use of inline style elements.

You can remove the unsafe-inline keyword. Refer to the following section for more information: Disallow Inline Styles and Inline Scripts.

connect-src 'self' (optional) http:my_backend_url;

The optional my_backend_url value specifies the server endpoint. This is necessary for JavaScript-Based reporting apps when client and server applications have different URLs.

worker-src 'self' blob:;

Required for printing.

frame-src 'self' blob: (optional) http:my_backend_url;

Required for printing. The optional my_backend_url value specifies the server endpoint. This is necessary if client and server applications have different URLs.

Disallow Inline Styles and Inline Scripts (Nonce-Based CSP)

In ASP.NET Web Forms applications, you can implement a nonce-based CSP to remove the unsafe-inline keyword from script-src and style-src directives. The main idea behind this approach is to generate a cryptographic nonce (“number used once”), include it in policy directives, and specify a matching nonce attribute in every script/style tag. The browser only executes inline code/styles that include the correct nonce value. The nonce value should regenerate each time you reload the page.

To disable inline styles and execution of inline scripts, do the following:

Generate the nonce value and pass it to ASPxReportDesigner.Nonce / ASPxWebDocumentViewer.Nonce properties.

Note
We are using the placeholder test-random-value to denote the nonce. You need to generate a random number, unique for each HTTP request.

Define a CSP on your page and add the same nonce value to both script-src and style-src directives.

default-src 'self';
img-src data: https: http:;
script-src 'self' 'nonce-test-random-value' 'sha256-UITiqbXyaWS7NpwiFrMIbdXAZy5EXLRUHkpylF4504k='
'sha256-Coceh0j8H9OB5c9s1/Cs2tJRvke+y3A8t4m+/OBFuBs=';
style-src 'self' 'nonce-test-random-value';
connect-src 'self';
worker-src 'self' blob:;
frame-src 'self' blob:;

Create a CSS class that modifies the component. The class may look as follows:
```
.my-reporting-component {
        height: 850px;
        width: 100%;
    }
```

The following code configures the Report Designer in an ASP.NET Web Forms application:

<dx:ASPxWebDocumentViewer ID="ASPxWebDocumentViewer1" runat="server" Nonce="test-random-value" CssClass="my-reporting-component">
</dx:ASPxWebDocumentViewer>

If you are using DevExpress Components in trial mode, the web page may display a Trial Message that violates the CSP specified for Reporting Components. Add additional style-src hashes as required to avoid CSP violation errors.

Troubleshooting

Error with ‘unsafe-eval’ Directive

The use strict directive in scripts may generate a “Refused to evaluate a string as JavaScript because ‘unsafe-eval’ is not an allowed source of script…” error.

To address this error, include the Knockout library in the global scope and use an alias to reference the library.

Custom Templates Do Not Work

DevExpress Reporting custom templates are based on the Knockout JavaScript library. The Knockout library uses the data-bind attribute to render a value in the following manner: it generates a function as a JavaScript string and passes the string to the new Function constructor.

To function properly, Knockout templates require the script-src 'unsafe-eval' CSP directive.

Important

We do not recommend the inclusion of the script-src 'unsafe-eval' directive in your content security policy. This directive may introduce a vulnerability as it enables script execution from a string on your page.

DevExpress Reporting stores JavaScript functions related to data-bind attributes in the cache, thus eliminating the need to run the script on the page. Our components do not need the ‘unsafe-eval’ directive.

Follow the steps below to use custom templates.

Call the addToBindingsCache Function

To add a custom template to the function cache, call the addToBindingsCache function before the component is rendered. You can handle the BeforeRender event to call the function.

Example: DevExtreme Template

DevExtreme Template
Function

<div data-options="dxTemplate: { name: 'content' }"></div>

DevExpress.Analytics.Utils.addToBindingsCache('dxTemplate: { name: \'content\' }', function () { return { dxTemplate: { name: 'content' } } });

Example: Knockout Binding

DevExtreme Template
Function

<div data-bind="text: text, attr: { title: text }"></div>

DevExpress.Analytics.Utils.addToBindingsCache('text: text, attr: { title: text }', function($context,$element) {
return {
    'text':function(){ 
        return $context.$data.text 
    },
    'attr':function(){ 
        return {
            'title':$context.$data.text
            } 
        }
    } 
});

Use the CLI Utility

v22.2 and later ships with our @devexpress/analytics-core-cli CLI utility package. It includes the processBindings command. You can use this command to automatically generate a file with the code that calls the addToBindingsCache function to add your templates to the cache.

Run the following command to install the package:

npm i @devexpress/analytics-core-cli

To process custom templates, execute the following command:

node node_modules/@devexpress/analytics-core-cli/utils/processBindings <templates folder path> <result file path>

Command parameters are as follows:

templates folder path: A folder that contains template files (.HTML)
result file path: Path to the file being created

When prompted, select application type (Modules or Namespaces):

CLI Template Utility

The generated file contains JavaScript code that must be run in the DevExpress Reporting component’s BeforeRender event handler.

Controls

Tools

Controls and Extensions

Tools

Maintenance Mode

Controls

Tools

Controls and Extensions

Tools

Maintenance Mode

Disallow Inline Styles and Inline Scripts (Nonce-Based CSP)

Troubleshooting

Error with ‘unsafe-eval’ Directive

Custom Templates Do Not Work

Call the addToBindingsCache Function

Use the CLI Utility