What Is a Browser Extension? How They Work, Risks & Security Guide

Yes—and it happens. Extensions with access to page content can read what you type into form fields, including login forms. If a malicious extension is running while you enter credentials, it can capture and transmit them without any visible sign that something went wrong.

This isn't theoretical. There have been documented cases of extensions that appeared legitimate—some with hundreds of thousands of users—that were quietly harvesting credentials and session tokens in the background.

The risk is highest when extensions have broad permissions (access to all sites, ability to read page content) and when they've been granted access to sensitive domains like your email, banking, or company tools. That's exactly why permission hygiene matters: an extension that can only run on one specific site has a much smaller blast radius than one running everywhere.

If you use a password manager, keep it as a separate, verified extension from the official provider—not something that came bundled with another install.

A good rule of thumb: if you can't explain what it does or why it's there, remove it.

More specifically, consider removing extensions that:

• You haven't actively used in the last 30 days
• Were installed alongside another piece of software and you didn't choose them intentionally
• Request permissions that don't match their stated purpose
• Come from publishers you can't verify
• Have been flagged or removed from the official extension store
• Duplicate functionality you already get from another trusted tool

Toolbars, "speed boosters," free VPNs from unknown providers, and shopping coupon injectors are among the most common culprits. They're often not worth the risk they introduce.

When in doubt, remove it, see if anything breaks, and reinstall intentionally if you actually needed it. The friction of reinstalling a legitimate extension is almost always lower than the cost of a compromised session.

Three primary methods: (1) Build a malicious extension from scratch and publish it to an official store before it's detected and removed; (2) Purchase an established extension with an existing user base and push a malicious update — users who already approved the permissions receive the malicious version automatically; (3) Compromise an extension developer's account and push a malicious update without the developer's knowledge. The third method is especially dangerous because the extension's reputation is genuine.