XHTML Validation in Angular Rich Text Editor Component

22 Aug 20259 minutes to read

The Syncfusion Angular Rich Text Editor provides features to ensure content complies with XHTML standards and is secure against cross-site scripting (XSS) attacks. The enableXhtml property enforces continuous XHTML validation, while enableHtmlSanitizer and the beforeSanitizeHtml event protect against XSS vulnerabilities. These features are essential for maintaining standards-compliant and secure content, particularly when editorMode is set to HTML.

XHTML validation

The editor includes an enableXhtml property that allows for continuous validation of the Rich Text Editor’s source content against the XHTML standard. When content is entered or modified in the editor, this feature ensures ongoing compliance by automatically removing invalid elements and attributes. This feature requires editorMode: 'HTML' to function.

Validating attributes

When enableXhtml is set to true, the editor enforces the following attribute rules:

• Case Sensitivity: Attributes must be lowercase (e.g., class, not CLASS).
• Quotation Marks: Attribute values must be enclosed in quotation marks.
• Validity: Only valid attributes for corresponding HTML elements are allowed.
• Required Attributes: Required attributes for HTML elements must be included (e.g., alt for <img>).

Validating HTML elements

The editor also enforces these element rules:

• Case Sensitivity: HTML tags must be lowercase (e.g., <p>, not <P>).
• Proper Closing: All opening tags must have corresponding closing tags.
• Element Validity: Only valid HTML elements are permitted.
• Nesting: Elements must be properly nested to maintain structure.
• Root Element: The content must have a single root element.
• Element Hierarchy: Inline elements cannot contain block elements (e.g., <span> cannot contain <div>).

The following example demonstrates enabling XHTML validation:

app.component.ts

import { Component } from '@angular/core';
import { RichTextEditorModule, ToolbarService, LinkService, ImageService, HtmlEditorService, QuickToolbarService, TableService, PasteCleanupService } from '@syncfusion/ej2-angular-richtexteditor';

@Component({
    imports: [
        RichTextEditorModule
    ],
    standalone: true,
    selector: 'app-root',
    template: `<ejs-richtexteditor id='editor' [enableXhtml] = 'xhtml'></ejs-richtexteditor>`,
    providers: [ToolbarService, LinkService, ImageService, HtmlEditorService, QuickToolbarService, TableService, PasteCleanupService],
})

export class AppComponent {
    public xhtml = true;
}

main.ts

import { bootstrapApplication } from '@angular/platform-browser';
import { AppComponent } from './app.component';
import 'zone.js';
bootstrapApplication(AppComponent).catch((err) => console.error(err));

Cross-Site scripting (XSS) prevention

The Rich Text Editor allows users to edit the content with security by preventing cross-site scripting (XSS). By default, it provides built-in support to remove elements from editor content that cause XSS attacks. The editor removes the elements based on the attributes if it is possible to execute a script.

Enabling XSS prevention

The enableHtmlSanitize, enabled by default, activates XSS prevention. When active, the editor automatically removes elements like <script> and attributes like onmouseover from the content.
The following example shows XSS prevention removing a <script> tag and onmouseover attribute:

app.component.ts

import { Component } from '@angular/core';
import { RichTextEditorModule, ToolbarService, LinkService, ImageService, HtmlEditorService, QuickToolbarService, TableService, PasteCleanupService } from '@syncfusion/ej2-angular-richtexteditor';

@Component({
    imports: [RichTextEditorModule],
    standalone: true,
    selector: 'app-root',
    template: `<ejs-richtexteditor id='editor' [value]='editorValue'></ejs-richtexteditor>`,
    providers: [ToolbarService, LinkService, ImageService, HtmlEditorService, QuickToolbarService, TableService, PasteCleanupService]
})
export class AppComponent {
    public editorValue: string = `<div onmouseover='javascript:alert(1)'>Prevention of Cross Sit Scripting (XSS) </div><script>alert('hi')</script>`;
}

main.ts

import { bootstrapApplication } from '@angular/platform-browser';
import { AppComponent } from './app.component';
import 'zone.js';
bootstrapApplication(AppComponent).catch((err) => console.error(err));

The XSS prevention feature is only applicable when the editorMode is set to HTML.

Custom XSS prevention

For more precise control over XSS prevention, you can implement custom filtering logic using the beforeSanitizeHtml event.

Implementing custom cross-site scripting and fililtering in Rich Text Editor

1. Use the beforeSanitizeHtml event to define custom filtering rules.
2. Utilize the helper function from the event argument to apply your custom filters.
3. Set the cancel argument to true if you want to override the built-in XSS prevention entirely.

The following sample demonstrates how to filter the script tag by value.

app.component.ts

import { Component } from '@angular/core';
import { RichTextEditorModule, ToolbarService, LinkService, ImageService, HtmlEditorService, BeforeSanitizeHtmlArgs, QuickToolbarService, TableService, PasteCleanupService } from '@syncfusion/ej2-angular-richtexteditor';
import { detach } from '@syncfusion/ej2-base';

@Component({
    imports: [RichTextEditorModule],
    standalone: true,
    selector: 'app-root',
    template: `<ejs-richtexteditor id='editor' [value]='editorValue' (beforeSanitizeHtml)='onBeforeSanitizeHtml($event)'></ejs-richtexteditor>`,
    providers: [ToolbarService, LinkService, ImageService, HtmlEditorService, QuickToolbarService, TableService, PasteCleanupService]
})
export class AppComponent {
    public editorValue: string = `<div>Prevention of Cross Sit Scripting (XSS)</div><script>alert('hi')</script>`;
    public onBeforeSanitizeHtml(e: BeforeSanitizeHtmlArgs): void {
        e.helper = (value: string) => {
            e.cancel = true;
            let temp: HTMLElement = document.createElement('div');
            temp.innerHTML = value;
            let scriptTag: HTMLElement = temp.querySelector('script') as HTMLElement;
            if (scriptTag) {
                detach(scriptTag);
            }
            return temp.innerHTML;
        }
    }
}

main.ts

import { bootstrapApplication } from '@angular/platform-browser';
import { AppComponent } from './app.component';
import 'zone.js';
bootstrapApplication(AppComponent).catch((err) => console.error(err));

You can also filter out the e.selectors.tags and e.selector.attributs in the beforeSanitizeHtml event to control which HTML tags and attributes are allowed to appear.

For instance, if you want to display <iframe>, By manipulating the e.selectors.tags property in this event, you can selectively remove tags like <iframe>. This approach ensures that your application can safely display iframes while preventing potential security risks associated with XSS vulnerabilities.

The following sample demonstrates how to filter the iframe tag.

app.component.ts

import { Component } from '@angular/core';
import { RichTextEditorModule, ToolbarService, LinkService, ImageService, HtmlEditorService, BeforeSanitizeHtmlArgs, QuickToolbarService, TableService, PasteCleanupService } from '@syncfusion/ej2-angular-richtexteditor';

@Component({
    imports: [RichTextEditorModule],
    standalone: true,
    selector: 'app-root',
    template: `<ejs-richtexteditor id='editor' [value]='editorValue' (beforeSanitizeHtml)='onBeforeSanitizeHtml($event)'></ejs-richtexteditor>`,
    providers: [ToolbarService, LinkService, ImageService, HtmlEditorService, QuickToolbarService, TableService, PasteCleanupService]
})
export class AppComponent {
    public editorValue: string = `<div>Prevention of Cross-Site Scripting (XSS)</div><script>alert('hi')</script><iframe srcdoc="<p>The Rich Text Editor component is WYSIWYG ('what you see is what you get') editor that provides the best user experience to create and update the content. Users can format their content using standard toolbar commands.</p>"></iframe>`;
    public onBeforeSanitizeHtml(e: BeforeSanitizeHtmlArgs): void {
          if (e.selectors && e.selectors.tags) {
            e.selectors.tags = e.selectors.tags.filter((tag: string) => tag !== 'iframe:not(.e-rte-embed-url)');
            e.selectors.tags = [('iframe[src^="https://"]')];
        }
    }
}

main.ts

import { bootstrapApplication } from '@angular/platform-browser';
import { AppComponent } from './app.component';
import 'zone.js';
bootstrapApplication(AppComponent).catch((err) => console.error(err));