1Password Extension Hijack

SquareX's article, Polymorphic Extensions: The Sneaky Extension That Can Impersonate Any Browser Extension  goes into some detail and includes their video demonstration.

An important factor to consider in this case is that this is a clever and pernicious example of the consequences of a user installing malware on their computer, not an exploit of a vulnerability in the 1Password plugin, application, browser, or platform.

Browser plugins currently are a ripe area for exploitation of probably the largest and most important software platform, and protections are severely limited - usually to users exercising good judgement (and even then with imperfect ongoing results).