Environment variables are not true global variables, because changes made by a child process are not propagated back to the parent. Supposing this was not the case. Show how a hacker might use this...

Environment variables are not true global variables, because changes made by a child process are not propagated back to the parent. Supposing this was not the case. Show how a hacker might use this loophole to cause trouble on a multiuser system on which he has an account.