$sanitize stripping valid HTML content

$sanitize Stripping Valid HTML Content in AngularJS

In AngularJS, the $sanitize service is designed to ensure that the HTML content being rendered on the page is safe and does not contain any potentially malicious or harmful code. It is primarily used to prevent Cross-Site Scripting (XSS) attacks by sanitizing the HTML content before it is rendered in the browser.

However, there may be situations where $sanitize inadvertently strips valid HTML content, even when the content is intended to be rendered safely. This issue can arise due to certain elements, attributes, or styles that $sanitize does not consider safe.

In this explanation, we will look at why $sanitize may strip valid HTML content, how to work around it, and ways to customize it to handle specific use cases.

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Why Does $sanitize Strip Valid HTML?

1. Security Concerns The primary role of $sanitize is to protect the application from XSS attacks. It achieves this by removing potentially dangerous elements and attributes, such as:
   • <script> tags
   • javascript: URIs in links
   • on* event handlers (e.g., onclick, onload, onerror)
   • Dangerous CSS properties and styles
   • Forms of inline JavaScript
   This is done to ensure that any malicious content (such as user input) does not result in the execution of harmful scripts when rendered on the page.
2. Stripping Specific Tags and Attributes $sanitize comes with a default whitelist of safe HTML elements and attributes. This whitelist includes most common elements like <div>, <p>, <span>, etc., but excludes some elements that might be considered risky, like:
   • <iframe>
   • <object>
   • <form>
   If you try to include one of these elements or other non-whitelisted tags or attributes, $sanitize will remove them from the HTML content.
3. Default Behavior and Whitelisting The $sanitize service by default will strip tags that are not in the allowed list. For example, <b> or <i> tags are allowed, but less commonly used tags like <marquee> or <blink> are not. Additionally, it may strip attributes that it does not consider safe. For instance, attributes like onclick, href="javascript:alert('XSS')" are often removed by default for security reasons.

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How to Prevent $sanitize from Stripping Valid HTML Content

To prevent $sanitize from stripping content that you need to retain (but that AngularJS deems unsafe), there are a few strategies you can employ.

1. Customize the $sanitize Whitelist

One solution to avoid $sanitize stripping valid HTML is to extend or modify its whitelist to allow specific tags or attributes that you need.

Unfortunately, $sanitize doesn’t expose a built-in API to modify the whitelist directly in AngularJS. However, you can implement a custom sanitization strategy by using third-party libraries or by extending AngularJS’s sanitization service.

For example, you can use the ngSanitize module and implement a custom filter to allow additional tags or attributes:

angular.module('myApp').filter('customSanitize', ['$sanitize', function($sanitize) {
  return function(input) {
    // Custom logic to extend the whitelist or modify sanitization
    var sanitized = $sanitize(input);

    // Add any additional custom HTML logic here if necessary
    // For instance, keep some tags or attributes in the sanitized HTML

    return sanitized;
  };
}]);

In this example, we are applying the $sanitize filter and then processing it further to allow additional tags or attributes manually.

2. Use ng-bind-html with Trusted HTML

If you want to trust certain HTML content from a trusted source (e.g., from a backend server or specific user input), you can use AngularJS’s $sce (Strict Contextual Escaping) service to mark content as trusted.

The $sce service allows you to bypass $sanitize for specific pieces of HTML that you know are safe. Here’s an example:

angular.module('myApp').controller('myCtrl', ['$sce', function($sce) {
  var safeHtml = $sce.trustAsHtml('<div style="color:red;">This is trusted HTML</div>');
}]);

And in your HTML, use ng-bind-html to bind the content:

<div ng-bind-html="safeHtml"></div>

By using $sce.trustAsHtml(), you inform AngularJS that this content is safe to render without sanitization, effectively bypassing $sanitize.

3. Use Custom HTML Sanitization

If $sanitize is stripping too much valid HTML content and you need more control over which tags and attributes are allowed, you can implement a custom HTML sanitization function. You can use libraries like DOMPurify or sanitize-html to create a more fine-grained approach to sanitization.

Example with DOMPurify:

1. First, install DOMPurify via a CDN or NPM: <script src="https://cdn.jsdelivr.net/npm/dompurify@2.0.17/dist/purify.min.js"></script>
2. In your AngularJS controller, use DOMPurify to clean and allow specific tags: angular.module('myApp').controller('myCtrl', function() { var rawHtml = '<div style="color:red;">This is a div</div><script>alert("XSS")</script>'; var sanitizedHtml = DOMPurify.sanitize(rawHtml, { ALLOWED_TAGS: ['div', 'span'], // Allow div and span tags ALLOWED_ATTR: ['style'] // Allow style attribute }); console.log(sanitizedHtml); // Output the sanitized HTML });

This allows for more flexibility in controlling what gets sanitized and ensures only valid and safe content is rendered.

4. Validate and Sanitize HTML Server-Side

While client-side sanitization is important, it’s equally crucial to sanitize any incoming HTML content on the server side before rendering it. This adds an additional layer of security, as it ensures that malicious scripts or dangerous content are prevented from being processed on the client side.

You can use libraries such as OWASP Java HTML Sanitizer (for Java backends) or sanitize-html (for Node.js) to sanitize content server-side.