Browser Extension Security Testing: Part 1
A browser extension, also known as a browser add-on or plugin, is a small software program that extends the functionality of a web browser. These extensions are designed to enhance the browsing experience by adding new features, modifying existing ones, or customizing the browser’s behavior according to the user’s preferences.
Browser extensions are typically written in languages such as HTML, CSS, and JavaScript, and they integrate seamlessly with the browser’s user interface. They can interact with web pages, manipulate content, and access browser features through APIs provided by the browser vendor.
Before delving into the intricacies of static and dynamic analysis of extensions, it’s imperative to understand the core components that constitute an extension and their interplay. The vulnerability often stems from the utilization and flow of these components.
Manifest File: The manifest file serves as a central configuration file for the extension, defining its metadata, permissions, and dependencies. It specifies how different components of the extension should behave and interact.
Extension Core: The extension core serves as the central logic and infrastructure of the extension, coordinating the activities of other components and managing its overall functionality. The extension core encapsulates the main functionality of the extension and facilitates communication between different parts of the extension architecture.
Background Script: This script runs in the background and can communicate with both the manifest file and content scripts. It often serves as a bridge between different components, handling events and coordinating tasks. It’s often used for tasks that don’t require direct user interaction, like managing data or listening for events.
Content Script: Content scripts run within the context of specific web pages and can communicate with the background script through messaging. They interact with the DOM of web pages to modify content and respond to events. Content scripts are loaded whenever the user navigates to a matching page, in our case any page matching the “*://web.telegram.org/*”
document_idle: Whenever possible
document_start: After any files from css, but before any other DOM is constructed or any other script is run.
document_end: Immediately after the DOM is complete, but before sub resources like images and frames have loaded.
UI Elements: UI elements include components like pop-ups, browser actions (toolbar buttons), and options pages. They provide user interaction points and allow users to access the extension’s features and settings.
APIs: Browser extensions interact with the browser through APIs (Application Programming Interfaces) provided by the browser vendor. These APIs allow extensions to perform various tasks, such as manipulating tabs, storing data locally, sending HTTP requests, and more.
Let’s try to understand the same with an example:
Suppose you’re developing a browser extension called “Highlighter” that allows users to highlight specific keywords on web pages for easy identification. The extension provides a UI element (such as a browser action button) that users can click to activate the highlighting functionality.
- The manifest file specifies that the extension should inject the content script (
contentScript.js) into all web pages ("matches": ["<all_urls>"]).
- The content script listens for messages from the background script. When it receives a ‘highlight’ message with a keyword, it searches for occurrences of the keyword in the page’s HTML content and highlights them by wrapping them in
<span>elements with a yellow background color.
- The background script listens for clicks on the browser action button. When the button is clicked, it prompts the user to enter a keyword and sends a message to the content script to highlight that keyword on the current web page.
I hope you’ve gained a fundamental understanding of browser extensions and the interplay between their components. In the next blog post, we’ll explore how to conduct static analysis on browser extensions using various tools and techniques.
STAY CURIOUS STAY PROTECTED!!
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