Why Browser Extensions Are the Most Dangerous Spyware You’ll Ever Install Yourself

Browser extensions are these small tools that blend seamlessly into your browser, but these tiny tools can turn into giants in the world of spyware. You install them with a single click of a button. That gives you access to a great amount of functionality, but it also gives them the ability to access everything from browsing history and passwords to cookies, form data, and active sessions. This means they could potentially capture almost all of your online activity, far more effectively than any other software.

By comparison, even the most insecure website can only access the data you provide directly. It can’t capture your saved passwords, monitor every keystroke, or see your entire browsing history. Extensions operate through APIs that grant them permissions to read and modify website content, intercept network requests, and store data both remotely and locally. While these permissions are given out of necessity for functionality, they also double as the perfect tools for exploitation.

Here are a few past cases that demonstrate how quickly a helpful tool can become spyware.

1 In 2019, the DataSpii leak revealed that extensions like Hoverzoom and SpeakIT! were transmitting entire browsing histories and authentication cookies of approximately 4 million users to third-party data brokers. The leaked datapoints included everything from tax records and medical records to travel itineraries and even corporate memos. (Source: Wikipedia)

2 In 2024, a supply-chain attack compromised at least 25 Chrome extensions, including “Reader Mode” and “Email Hunter.” This attack affected roughly 2.29 million users by extracting session cookies and authentication tokens. Link

3 In 2025, security researchers discovered a network of 58 extensions installed on around 6 million devices. All of these carried hidden code to track users, access cookies, and execute remote commands.

Read more on it here.

All of these cases share a common thread. These extensions were initially marketed as legitimate tools, but once a large number of users started trusting them, they transformed from useful utilities into sophisticated spyware.

The core of the problem lies in how these tools are monetized. The real money comes from data collection and advertising networks, not from direct sales. Behavioral data harvested from millions of users is sold to advertisers, analytics firms, and sometimes third-party brokers, who then create highly detailed user profiles. These profiles are used for targeted ads, influence campaigns, and even identity theft. The more granular the data, the higher its value, which creates a huge incentive to collect as much as possible.

The sheer volume of extensions makes it impossible for browser vendors to perform thorough checks or in-depth reviews. As a result, oversight is usually limited to basic automated scans and occasional manual checks, neither of which are fully effective. Once an extension gains traction, a developer or attacker can easily introduce functionality that harvests data without raising any visible suspicion. Automatic updates then push these changes instantly to millions of users, turning a trusted tool into surveillance software almost overnight.

Many vendors have tried to restrict access to certain powerful APIs, limit background execution, and require extensions to declare more transparent data-handling practices. While these measures do reduce some high-risk capabilities, the core paradox remains: extensions need deep privileges to be useful, and those same privileges create the perfect opportunity for data harvesting.

The conclusion is simple: unless this paradox between revenue models and technical architecture is resolved, browser extensions will continue to be a fertile ground for covert surveillance.