Pwned? per PwnedExtDev

Will monitor sites and check your submitted passwords against Have I Been Pwned's database of passwords. When you enter a pwned password, the icon and popup change, so this extension works best if you have it pinned. You can also actively check your passwords using the popup of the locked icon.

Since security is the main concern for this extension, passwords are never saved or sent over the internet. Each password is hashed and then a snippet of that is sent to the api (to ensure k-anonymity) which will return a list of possible matches. These matches are then compared to the original hash.