Almost every Unix system comes with a special user in the /etc/passwd file with a UID of 0. This user is known as the superuser and is normally given the username root. The password for the root account is usually called simply the "root password."
The root account is the identity used by the operating system itself to accomplish its basic functions, such as logging users in and out of the system, recording accounting information, and managing input/output devices. For this reason, the superuser exerts nearly complete control over the operating system: nearly all security restrictions are bypassed for any program that is run by the root user, and most of the checks and warnings are turned off.
 On a few systems, it's possible to restrict root's capabilities as part of the kernel boot process, so that even if the superuser account is compromised, some kinds of damage are not possible unless the attacker is physically at the console and has an additional password. Systems that use MAC often do not have a superuser at all, so the discussion in this section does not apply to such systems.
Any process that has an effective UID of 0 (see Section 5.3.1 later in this chapter) runs as the superuser?that is, any process with a UID of 0 runs without security checks and is allowed to do almost anything. Normal security checks and constraints are ignored for the superuser, although most systems do audit and log some of the superuser's actions.
Some of the things that the superuser can do include:
Change the nice value of any process (see Section B.1.3.3).
Send any signal to any process (see Signals).
Alter "hard limits" for maximum CPU time as well as maximum file, data segment, stack segment, and core file sizes (see Chapter 23).
Turn accounting and auditing on and off (see Chapter 21).
Bypass login restrictions prior to shutdown. (Note that this may not be possible if you have configured your system so that the superuser cannot log into terminals.)
Change his process UID to that of any other user on the system.
Log out all users and prevent new logins.
Access any working device.
Shut down or reboot the computer.
Set the date and time.
Read or modify any memory location.
Create new devices (anywhere in the filesystem) with the mknod command.
Run network services on "trusted" ports (see Chapter 17).
Reconfigure the network.
Put the network interface into "promiscuous mode" and examine all packets on the network (possible only with certain kinds of networks and network interfaces).
Read, modify, or delete any file or program on the system (see Chapter 6).
Run any program.
 If a program has a file mode of 000, root must set the execute bit of the program with the chmod( ) system call before the program can be run, although shell scripts can be run by feeding their input directly into /bin/sh.
Change a disk's electronic label.
 Usually stored on the first 16 blocks of a hard disk or floppy disk formatted with the Unix filesystem.
Mount and unmount filesystems.
Add, remove, or change user accounts.
Enable or disable quotas and accounting.
Use the chroot( ) system call, which changes a process's view of the filesystem root directory.
Write to the disk after it is "100 percent" full. The Berkeley Fast Filesystem and the Linux ext2 File System both allow the reservation of some minfree amount of the disk. Normally, a report that a disk is 100% full implies that there is still 10% left. Although this space can be used by the superuser, it shouldn't be: filesystems run faster when their disks are not completely filled.
Despite all of the powers listed in the previous section, there are some things that the superuser can't do, including:
Make a change to a filesystem that is mounted read-only. (However, the superuser can make changes directly to the raw device, or can unmount a read-only filesystem and remount it read/write, provided that the media is not physically write-protected.)
Unmount a filesystem that contains open files, or one in which some running process has set its current directory.
 Many BSD variants (including NetBSD and FreeBSD) provide an -f option to umount, which forcibly unmounts a busy filesystem.
Write directly to a directory, or create a hard link to a directory (although these operations are allowed on some Unix systems).
Decrypt the passwords stored in the shadow password file, although the superuser can modify the /bin/login and su system programs to record passwords when they are typed. The superuser can also use the passwd command to change the password of any account.
Terminate a process that has entered a wait state inside the kernel, although the superuser can shut down the computer, effectively killing all processes.
As we noted in Section 5.1, any account that has a UID of 0 has superuser privileges. The username root is merely a convention. Thus, in the following sample /etc/passwd file, both root and beth can execute commands without any security checks:
root:x:0:1:Operator:/:/bin/ksh beth:x:0:101:Beth Cousineau:/u/beth:/bin/csh rachel:x:181:181:Rachel Cohen:/u/rachel:/bin/ksh
You should immediately be suspicious of accounts on your system that have a UID of 0 that you did not install; accounts such as these are frequently added by people who break into computers so that they will have a simple way of obtaining superuser access in the future.
The superuser is the main security weakness in the Unix operating system. Because the superuser can do anything, after a person gains superuser privileges?for example, by learning the root password and logging in as root?that person can do virtually anything to the system. This explains why most attackers who break into Unix systems try to become the superuser.
Most Unix security holes that have been discovered are of the kind that allow regular users to obtain superuser privileges. Thus, most Unix security holes result in a catastrophic bypass of the operating system's security mechanisms. After a flaw is discovered and exploited, the entire computer is compromised.
There are a number of techniques for minimizing the impact of such system compromises, including:
Storing sensitive files on removable media, and mounting the media only when you need to access the files. An attacker who gains superuser privileges while the media are unmounted will not have access to critical files.
Encrypting your files. Being the superuser grants privileges only on the Unix system; it does not magically grant the mathematical prowess necessary to decrypt a well-coded file or the necessary clairvoyance to divine encryption keys. (Encryption is discussed in Chapter 7.) Best practice is to encrypt with a passphrase other than your login password, which an attacker might capture.
Mounting disks read-only when possible.
Taking advantage of filesystem features like immutable and append-only files if your system supports them.
Keeping your backups of the system current. This practice is discussed further in Chapter 16.
There are many other defenses, too, and we'll continue to present them in this chapter and throughout this book.