Undescore password problem

[PlayJames]

Hi guy
I have a little problem with a password compose.
In my list of passw, I have a sequence with a "prefix_number". The problem is that if I want to login whit: "my Login" and "someone else passw" my login pass true in every case.
Why I can login with other passw in to my box? How I can do to solve this bug?
Many Thank's

PJ