Content Security Policy for ASP.NET Core Applications
- 6 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-inlinekeyword. 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-inlinekeyword. 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_urlvalue 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_urlvalue 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 Core Applications, you can implement a nonce-based CSP to remove the unsafe-inline keyword for script-src and style-src directives. The main idea behind this approach is to generate a cryptographic nonce (“number used once”), include it in the specific policy, and add the nonce value in the 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 implement the nonce-based CSP in ASP.NET Core applications with DevExpress Reporting components, create a cryptographic nonce and pass it to the ReportDesignerBuilder.Nonce / WebDocumentViewerBuilder.Nonce methods.
Example
This example demonstrates how to implement a nonce-based Content Security Policy (CSP) for an ASP.NET Core Application through an HTTP response header:
In the HomeController.cs file, generate the nonce value. The RandomNumberGenerator class is used to generate cryptographically strong random values. Add an HTTP header with the Content Security Policy with nonce for the script-src directive.
The following code snippet shows how to add a nonce-based CSP for the Report Designer component:
using DevExpress.XtraReports.Web.ReportDesigner;
using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Security.Cryptography;
using System.Threading.Tasks;
//...
public async Task<IActionResult> Designer(
[FromServices] IReportDesignerClientSideModelGenerator clientSideModelGenerator,
[FromQuery] string reportName) {
var nonceBytes = new byte[32];
using var generator = RandomNumberGenerator.Create();
generator.GetBytes(nonceBytes);
var nonce = Convert.ToBase64String(nonceBytes);
HttpContext.Response.Headers.Add("Content-Security-Policy",
string.Format("script-src 'self' 'nonce-{0}';", nonce) +
"img-src data: https: http:;" +
"style-src 'self';" +
"connect-src 'self';" +
"worker-src 'self' blob:;" +
"frame-src 'self' blob:;"
);
Models.ReportDesignerCustomModel model = new Models.ReportDesignerCustomModel();
model.ReportDesignerModel = await CreateDefaultReportDesignerModel(clientSideModelGenerator, reportName, null);
model.Nonce = nonce;
return View(model);
}
//...
The new nonce value is generated each time the page loads.
On the page, pass the nonce value to the Nonce method: On the page, pass the nonce value to the Nonce method:
@{
var designerRender = Html.DevExpress().ReportDesigner("reportDesigner")
.Height(null)
.Width(null)
.Nonce(Model.Nonce)
.CssClassName("my-reporting-component")
.Bind(Model.ReportDesignerModel);
@designerRender.RenderHtml()
}
Note
Set the Height and Width properties to null for the application to work properly. You can specify the required height and width in a CSS class.
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
<div data-options="dxTemplate: { name: 'content' }"></div>Example: Knockout Binding
<div data-bind="text: text, attr: { title: text }"></div>
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):
The generated file contains JavaScript code that must be run in the DevExpress Reporting component’s BeforeRender event handler.