Seems really weird to advocate people reveal their passwords to a random untrusted 3rd party.
They do have an API that allows you to search for your password based on a truncated checksum, so you can find out if your password was leaked, without revealing the password.
A more sane approach would be to just put your passwords in a file, maybe by export from your database manager. Take a sha1 of each password, then submit those. That way you aren't trusting any random 3rd party sites to run safe code.
You're either not reading the "how this protects your password" link that's on top of the page or claiming it's wrong. [1]
It sends the first N characters of the SHA1 hash of the password you provided to the server, the server replies with all the hashes it knows with that prefix, and then the client-side JS compares it to the rest of the hash it has.
If you don't believe me, you can look at the request said site issues for some arbitrary string - it's just the first 5 characters of the SHA1 hash, and the response from the server is as I (and that link) describe.
So you are trusting the HTML/CSS and javscript downloaded from troyhunt.com with your plaintext password? Not to mention various bits from cloudflare.com, and other places.
How about if you go to another device which you have not used before (maybe a library or internet cafe), do not identify yourself to the web in any way, open a sole link to that page and enter the passwords you wish to check. They are checked, but there is nothing to link them to you?
Of course, if we don't completely trust Troy Hunt and everybody associated with the site then we could assume that now those passwords have been added to a secret list of known unknowns, to use when trying to crack the hashed files they already have stored.
Security sure is difficult! I know it says at the top of the article that it is pitched at non-technical people but most of the people I know would have glazed over in the first few paragraphs..
Please don't make false assertions about what I was suggesting without any evidence.
Pwned Passwords consists of a number of tools, which one you choose to use depends on the concerns you have and the effort you choose to put in. Both the API and the SHA download files provide secure means of checking if your password is present in this data dump.
I would certainly not put any live passwords into the webform.
You replies in general have be combative and seeked to push people into positions that you could argue against for internet points.
You could have made your points in a much more constructive and concise manner:
Pwned Passwords is a great data set, I would recommend against using the webform to check your password, instead download the hash file or utilize the extremely simple api. The webform is insecure because...
They do have an API that allows you to search for your password based on a truncated checksum, so you can find out if your password was leaked, without revealing the password.