The invisible threat: How browser extensions can spy on you

Have you ever installed a Chrome extension to make your browser more productive, stylish, or fun? What if I told you that one of those extensions could silently siphon off your most sensitive data—without you ever knowing?

Browser extensions have been a staple of our online experience for years, enhancing productivity and personalizing our browsing. But they also pose a long-standing security risk—one that’s just as relevant today as it was a decade ago. Despite increased awareness, many users remain oblivious to how extensions can be weaponized to steal sensitive data and monitor online activity.

In this post, we’ll revisit this persistent issue and explore the dangers of blindly installing browser extensions, including a chilling demo of how an innocent-looking extension can become a hacker’s gateway to your digital life. Along the way, I’ll also share tools to help you assess the risk level of your installed extensions.

Why are browser extensions a security risk?

Browser extensions often request permissions that go beyond their advertised functionality. For example, an extension designed to improve your productivity might request access to:

• Read and change all your data on the websites you visit
• Access your browser tabs
• Intercept and modify network requests

While these permissions might be necessary for the extension’s functionality, they also open the door to abuse. An extension with malicious intent—or one that becomes compromised in a supply chain attack—can:

• Steal your browsing history
• Extract credentials entered on websites
• Monitor your online activity in real-time

These risks aren’t hypothetical. There have been numerous instances where seemingly harmless extensions were caught collecting and selling user data.

The demo: How a malicious extension can exploit you

To make this issue tangible, I created a Chrome extension and a server to demonstrate how easy it is to exploit unsuspecting users. The setup includes:

1. The attacker’s server: A Node.js server that listens for incoming data.
2. The victim’s browser: A Chrome instance where the malicious extension is installed.

The scenario

On one side of the screen, you’ll see the attacker’s server capturing data in real-time. On the other side, the victim is using their browser, checking emails, performing searches, and logging into accounts—completely unaware that every keystroke and website is being sent to the attacker.

How victims install malicious extensions

Attackers use several strategies to get victims to install their malicious extensions. These include:

1. Phishing campaigns
   Sending emails or messages with links to a fake extension, often claiming to improve security or productivity. Victims may unknowingly download the extension, granting attackers access to sensitive data.
2. Supply chain attacks
   Attackers often compromise legitimate extensions through various means, including injecting malicious code during updates or outright purchasing extensions from developers. Another emerging tactic involves submitting malicious pull requests to open-source projects associated with popular extensions. These requests may appear legitimate but contain hidden malware, allowing attackers to infiltrate trusted extensions.
   Additionally, threat actors target developer accounts, using phishing or credential theft to gain access. Once inside, they can push compromised updates to unsuspecting users, effectively weaponizing extensions already trusted by millions.
3. Social engineering
   Leveraging fake reviews, misleading ads, or enticing features to lure users into installing malicious extensions. These strategies often exploit users' need for convenience or curiosity.
4. USB-based attacks
   Tools like Bash Bunny and Rubber Ducky can act as physical attack vectors, delivering malicious payloads that manipulate browsers. For instance, such tools could automatically install extensions or change browser settings to allow malicious behavior without user consent.

Watch the recording

Mitigating the risks

While browser extensions can significantly enhance your online experience, you need to evaluate their safety critically. Here’s how:

1. Use my risk assessment tool

I built a Chrome extension that analyzes your installed extensions and assigns a risk score based on their permissions and origin. It’s a simple way to see which extensions might pose a threat to your security. It has passed the Chrome Web Store review and can be found here: Advanced Extension Risk Checker. The extension has one permission only, and that is management.

2. Check permissions

Before installing any extension, review the permissions it requests. Ask yourself: does it need all these permissions to perform its stated function?

3. Limit installed extensions

Only keep the extensions you actively use. The more extensions you have, the higher your attack surface.

4. Research extensions

Look for extensions with a strong reputation, verified publishers, and a history of consistent updates. Avoid installing extensions with few reviews or questionable origins.

5. Regularly audit rour extensions

Review your installed extensions periodically and remove those you no longer need or trust.

Final thoughts

Browser extensions are powerful tools that can enhance your productivity and online experience - but with great power comes great responsibility. Always think twice before clicking “Add to Chrome,” and remember that even trusted extensions can become compromised.

Stay vigilant, and don’t let your browser become a hacker’s playground.

What are your thoughts?

Have you ever encountered a malicious browser extension? What steps do you take to secure your online activity? Share your experiences in the comments below.