A malicious extension is a browser add-on intentionally designed to harm users or their devices. Unlike legitimate extensions that enhance browsing, malicious ones may steal personal information, monitor online activities, inject unwanted ads, or install additional malware. These extensions often appear useful or harmless but can compromise privacy and security once installed. They may be distributed through official browser stores or third-party websites, sometimes masquerading as popular tools. Users can reduce risk by installing extensions only from trusted sources and regularly reviewing browser add-ons for unfamiliar or suspicious items.