[ADMIN] user password problem

[julien piaser]

I'm using postgresql7.0.2 on Linux Redhat 6.1.

It seems that any user can log in psql with any other user account
without giving any password

i.e: if I'm logged in a shell as julien (who is a single user, not an
administrator) I just have to write: psql -U postgres mydatabase to go
into mydatabase with postgres permissions, without giving any password
(or a wrong one as well)

Not really securised... any suggestions?

regards

J.Piaser