Security problem on shared folders

[Rui Paz]

Hello to all,
I'm new to this list and I have a question about setting up a Linux server
to authenticate windows machines, I've success on this but I found one
security problem that I don't know if is possible to solve. The problem is
this:

When a user login on the server from a windows machine he can write on is
home on the linux server but is also possible to gain access for read on the
other users area making run \\server\otheruser

Tanks,

Rui Paz