How a Keylogger Chrome Extension Works

Legal and ethical note

Creating, distributing, or using keyloggers to capture other people’s inputs without explicit consent is illegal and unethical in most jurisdictions. Keylogging research should be confined to controlled, consented testing environments.

How keystroke capture typically works

1. Content scripts inject listeners onto page elements and the document:
   • document.addEventListener('keydown', handler) or 'input' events on specific fields.
2. Handlers read event properties (event.key, event.code, value of target element) to reconstruct typed text, including special keys.
3. Data is sanitized, chunked, and sent to the background script via chrome.runtime.sendMessage or chrome.runtime.connect.
4. Background script queues and persists captured data and triggers exfiltration (POST to an endpoint) or syncs with cloud storage.
5. Optionally, the extension monitors navigation (tabs.onUpdated) to keep tracking across pages, or injects scripts on matching domains.

The Trojan Horse Strategy

Malicious developers often create extensions that appear legitimate. They might clone the code of a popular open-source extension but add a few lines of malicious keylogging code in the minified JavaScript.

• Utility Disguise: Extensions promising to improve browser functionality (e.g., "Dark Mode for All Sites," "Auto-Fill Forms," "Screenshot Tools") are common vectors.
• Fake Reviews: Attackers often use botnets to generate thousands of fake 5-star reviews to push the rating up, tricking the Chrome Web Store algorithm and human users into trusting the extension.