[cobalt-security] about cd .. command can change home directory up to root server

["ThaiTOP .COM" <sanukk@xxxxxxxxxxx>]

I am a user at a host, i can telnet to my account ... and can use cd .. 
[enter] to change directory up and up to other home directory and can chage 
to root server
or /etc/passwd ... can use cat command to view passwd file.. or to see other 
php or perl script..
to test...

telnet yourhost.com
enter username
enter password
type cd .. and press enter ..... do this again and again and type ls and 
enter ... Wow amazing....

How can i protect myself or do not let other to see my php or perl script..
What root should do to solve this..

Thanks you.

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