Malicious browser extensions are malicious add-ons that install themselves in the user's browser to collect data, spy on the user or perform other harmful actions. These extensions often gain unauthorized access to a user's browsing history, passwords and other personal information.
They are often distributed for free or in the form of attractive offers, and users may not even suspect that the extension is harmful. It is advisable to download extensions only from trusted sources, read the reviews, and regularly check and remove unused or suspicious extensions from the browser.