Privacy Issue

When i go to my inbox (message plugin) the link is someting like
http://www.mydomain.com/messages/inbox/myuserid

when i replace "myuserid" with my "otheruserid" i.e.,
http://www.mydomain.com/messages/inbox/otheruserid

I am able to see his inbox and every other stuff....

Please tell me a way to fix it... i m using elgg 1.8.8...!!!