When Andrey Meshkov, the co-founder of AdGuard ad blocker, decided to take a look at some of the more popular knockoffs of his software, he found things that were disconcerting. Extensions were created by design to look like actual well known ad blockers; and with at least one that had 10 million users, compromised privacy was a pretty sure thing.

While many of the extensions had millions of downloads, AdRemover, in particular, was way out in front with 10 million. Meshkov decided to download it and take a look inside. In the process, he saw that it contained code hidden within an image that tells a server about websites the user visits–something that goes against Google policy.

Motherboard reports, “Though Meshkov didn’t immediately see what the extension was collecting data for, he said having this link to a remote server is dangerous because it could change your browser behavior in many ways. Meshkov said it could alter the appearance of pages, scrape information from the user, or load additional extensions that a user hasn’t installed.”

Essentially, this kind of stealthy data collection could be used by hackers to compromise the cybersecurity of users and run malicious activities that could lead to bigger problems.

Get the full story at Motherboard.