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CRX is the package format for Google Chrome extensions — ZIP archives with a special header containing a signature and public key for verification. When you install an extension from the Chrome Web Store, Chrome downloads a .crx file, verifies its signature, and unpacks it into your browser's extension directory.
Inside a CRX file, you'll find the extension's manifest.json (metadata, permissions, entry points), JavaScript files, HTML popups, CSS, icons, and any other assets. The manifest declares what the extension can access — tabs, browsing history, cookies, page content, network requests — and Chrome enforces these permission boundaries. Extensions can modify web pages (content scripts), intercept network traffic (declarativeNetRequest), and run background processes (service workers).
Chrome extensions are powerful and that power comes with risk. Malicious extensions have been caught stealing passwords, injecting ads, tracking browsing activity, and mining cryptocurrency. Google reviews extensions submitted to the Chrome Web Store, but the review process isn't foolproof. Side-loading .crx files from outside the Web Store bypasses even this limited review. Chrome has progressively restricted side-loading, and modern versions require developer mode to install unpacked extensions.