Browser extensions are one of the most underestimated attack surfaces in everyday computing. The reason comes down to what permissions the browser grants them by default. When you install an extension and click Add to Chrome or Add to Firefox you are presented with a permissions dialogue that most people dismiss without reading. Those permissions are not cosmetic. They are binding access grants that determine what the extension can touch inside your browser. The most dangerous permission is tabs and activeTab combined with access to all URLs. An extension with this permission can read the full content of every webpage you visit. Every form field. Every input box. Everything rendered on screen. A password manager extension needs this to function legitimately. A malicious extension uses it to silently capture credentials, banking details, and session cookies as you type them. webRequest and webRequestBlocking permissions allow an extension to intercept, inspect, and modify network traffic before it leaves your browser and before responses reach your page. A malicious extension sitting in this position can strip HTTPS from redirects, inject content into pages you visit, redirect specific requests to attacker controlled servers, and read unencrypted traffic in transit. Cookie access is a separate and critical vector. Session cookies are the tokens that prove to a website you are already logged in. An extension with cookie permissions can read every session cookie in your browser for every domain you are authenticated to. The attacker does not need your password. They take the cookie and they are you. This is called session hijacking and extensions make it trivial. Clipboard access allows extensions to read everything you copy. Copy a crypto wallet address to paste it somewhere and a malicious extension can swap it silently for an attacker controlled address before it reaches the destination field. This specific attack has drained significant amounts of cryptocurrency from users who watched the paste happen and never noticed the substitution. The supply chain angle is the most dangerous evolution of this threat. Legitimate extensions with established user bases get acquired by malicious actors who push an update containing new functionality. The extension you installed two years ago from a trusted developer now belongs to someone else. The browser auto updates it silently. Your 200,000 user extension just became a data collection tool overnight with no indication to any of those users that anything changed. The Chrome Web Store has removed thousands of malicious extensions but the review process relies heavily on automated scanning that sophisticated attackers have learned to evade by activating malicious functionality only after a time delay or only on specific domains. The practical defence is treating extensions like you treat app permissions on your phone. Install as few as possible. Review what permissions each one requests before installing. Periodically audit installed extensions and remove anything you no longer actively use. Check the developer behind an extension before trusting it with access to your browsing session. An extension you forgot you installed three years ago sitting with access to all site data is not a passive piece of software. It is a privileged process running inside your browser with more access to your digital life than most applications on your computer.