In general, the goal of every security measure on a system is to prevent people from doing things they shouldn't. Given the all-or-nothing structure of Unix privileges, in practical terms this means you are trying to prevent unauthorized access to the root account—it also implies that the root account is what the bad guys are trying to gain access to. When they cannot do so directly because the root password has been well chosen, they may try other, indirect routes through the filesystem to gain superuser status.
So, how can you get root access from an ordinary, unprivileged user account? One way is to get root to execute commands like these:
cp /bin/sh /tmp/.junk#
chmod 4755 /tmp/.junk
These commands create a setuid root version of the Bourne shell: any user can start a shell with this file, and every command that he runs within it will be executed as if he were root. Of course, no reputable system administrator will run these commands on demand, so a cracker will have to trick her into doing it anyway by hiding these commands—or other commands just as deadly—within something that she will execute. One large class of system attack revolves around substituting hacked, pernicious copies of normally benign system entities: Unix command executables, login or other initialization files, and so on. Making sure that the filesystem is protected will prevent many of them from succeeding.
In this section, we'll consider the types of vulnerabilities ...