Cross-site scripting (XSS) vulnerability in ng-bind-html

Cross-site Scripting (XSS) Vulnerability in ng-bind-html

Cross-site scripting (XSS) is a significant security vulnerability that can allow an attacker to inject malicious scripts into web pages viewed by other users. This can lead to a variety of serious issues such as data theft, session hijacking, or unauthorized actions being performed on behalf of the user. In AngularJS, the ng-bind-html directive is a common source of XSS vulnerabilities when not used with caution. Here’s a detailed explanation of why this happens and how to mitigate such risks.

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What is ng-bind-html?

ng-bind-html is an AngularJS directive used to bind HTML content to an element. It allows you to dynamically insert HTML content into the DOM without Angular’s automatic escaping mechanism for HTML. This is particularly useful when you need to display rich content like formatted text, images, or links fetched from a database or user input.

Example:

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

In this example, someHtmlContent could be a variable in the AngularJS scope that holds HTML content. However, this directive can present security risks if the data being inserted is not properly sanitized.

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XSS Vulnerability with ng-bind-html

The XSS vulnerability arises because ng-bind-html bypasses Angular’s default behavior of escaping HTML and JavaScript, which means that it allows potentially dangerous user inputs, including malicious scripts, to be executed within the browser. If user-generated content (such as from a form or an external source) is inserted using ng-bind-html without proper sanitization, an attacker can inject malicious JavaScript into the page.

For example, if an attacker submits the following input:

<script>alert('XSS attack!');</script>

And this input is inserted into the DOM via ng-bind-html, the malicious JavaScript will be executed when the page is loaded, resulting in an XSS attack.

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How ng-bind-html Works

When you use ng-bind-html, AngularJS does not escape HTML by default. This means it directly inserts the raw HTML content into the DOM. While this behavior is desirable in many cases where you need to display user-generated HTML (like blog posts with embedded HTML), it also exposes your application to the possibility of injecting malicious scripts.

Potential Exploit Scenario

1. User Input Handling: Suppose you have a content management system (CMS) where users can submit HTML-formatted posts. Without proper sanitation, a user could inject a script tag into their post, as shown below: <p>This is a normal post</p> <script>alert('Hacked');</script>
2. Content Binding: If this post is then bound to an element using ng-bind-html, AngularJS will insert the post into the DOM as is, and the browser will execute the malicious script, causing an XSS attack.
3. Impact: This can have severe consequences:
   • Data Theft: The attacker can steal sensitive information like session cookies or credentials.
   • Session Hijacking: An attacker could take over a user’s session and perform actions on their behalf.
   • Malicious Redirects: The attacker could redirect the user to a malicious website or launch further attacks.
   • Defacing the Application: An attacker can alter the UI by injecting scripts that modify the DOM.

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How to Mitigate XSS in ng-bind-html

To mitigate the risk of XSS attacks when using ng-bind-html, it is crucial to sanitize the input before binding it to the DOM. Fortunately, AngularJS provides a service called $sanitize to handle this.

Step-by-Step Solution:

1. Use $sanitize Service AngularJS includes a $sanitize service, which helps sanitize HTML content by stripping out any potentially dangerous code, such as <script> tags, event handlers, and other forms of malicious code. You should always use this service before binding HTML to the DOM with ng-bind-html. Here’s how to safely use ng-bind-html: angular.module('app', ['ngSanitize']) .controller('MainCtrl', function($scope, $sanitize) { // Assume content comes from user input $scope.rawHtmlContent = '<p>This is some <b>bold</b> text</p><script>alert("XSS attack!")</script>'; // Sanitize the content $scope.safeHtmlContent = $sanitize($scope.rawHtmlContent); }); And in your HTML: <div ng-bind-html="safeHtmlContent"></div> The $sanitize service will remove any script tags, event handlers, or other malicious content from the HTML, ensuring that only safe HTML is injected into the DOM.
2. Enable ngSanitize Module The $sanitize service is part of the ngSanitize module, which is not included by default in AngularJS. You need to explicitly include it in your application. To use it, you need to include the angular-sanitize.js script in your project: <script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.8.2/angular-sanitize.min.js"></script> Then, include it in your AngularJS module: angular.module('app', ['ngSanitize']);
3. Sanitize User Input on the Server Side While sanitizing input on the client side is important, it is also essential to sanitize any user-generated content on the server side before storing it in a database or rendering it in the application. Client-side sanitization should not be relied upon as the sole line of defense. Use appropriate libraries or functions in your server-side language to sanitize user inputs. For example:
   • In Node.js, you can use libraries like DOMPurify or sanitize-html.
   • In PHP, you can use the htmlspecialchars function.
   • In Java, libraries like OWASP Java HTML Sanitizer can be used.
4. Avoid Binding Unsafe HTML Directly Avoid using ng-bind-html for untrusted or user-generated content. For content that is purely text and doesn’t require HTML formatting (like a user’s name or a comment), prefer using ng-bind or regular interpolation, which automatically escapes the input. <span ng-bind="userComment"></span> This will escape any potentially dangerous HTML or JavaScript, preventing XSS attacks.
5. Use Content Security Policy (CSP) A Content Security Policy (CSP) is a browser feature that helps detect and mitigate certain types of attacks, including XSS and data injection attacks. By defining a CSP header in your web server, you can limit the sources from which scripts can be loaded, helping prevent the execution of injected scripts. Example CSP header: Content-Security-Policy: default-src 'self'; script-src 'self'; object-src 'none'; This policy restricts the loading of scripts to only those hosted on the same domain as your application, blocking the execution of any externally injected scripts.