Sergio,
The kpasswd field is only relevent for Kerberos passwords, such as you would be using when authenticating against Microsoft Active Directory, so it is not relevant for web-based authentication.
If the password is a field submitted via a HTTP GET or POST method then it probably can be found via a filter something like:-
http.request.method == POST && (data-text-lines contains yourpassword)
http.request.method == GET && (http.request.uri contains contains yourpassword)
Of course best practice would see that any passwords are protected via SSL (HTTPS) so you may not see a lot unless you have access to the server's SSL certificate private key.
Regards, Martin
MartinVisser99@xxxxxxxxx