Extension malware is any browser extension that was developed intentionally with coding that causes undesirable behaviors or whose code has been compromised by an attacker to do so.
Like other types of software, browser extensions can be designed to carry out attacks. In January 2018, for example, a security company called ICEBERG reported that they had detected four malicious extensions available from the Chrome Web Store. The four -- Change HTTP Request Header, Nyoogle - Custom Logo for Google, Lite Bookmarks and Stickies - Chrome's Post-it Notes -- were apparently designed to conduct click fraud and black hat SEO practices. ICEBERG warned that the same access methods that enabled those behaviors could also make it possible for the attackers to breach corporate networks and gather sensitive data.
Although Google subsequently removed the four extensions from the store, ICEBERG noted that the extensions may still exist on the machines of people who downloaded them and that they may also still be available through third-party repositories.
Security experts recommend that users be judicious when installing browser extensions. Whenever possible, you should check what permissions an extension requires. It’s wise, as well, to refrain from installing extensions from unknown companies and developers.