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Root Account

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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 Solaris it is possible to restrict root's capabilities via RBAC. In such cases even if the superuser account is compromised, some kinds of damage are not possible. Trusted Solaris has additional features and allows  not to have a superuser at all.

The root account has virtually unlimited access to all programs, files, and resources on a system. The root account is the special user in the /etc/passwd file with the userid (UID) of 0  and is commonly given the user name, root.

It is not the user name that makes the root account so special, but the UID value of 0. This means that any user that has a UID of 0  also has the same privileges as the root user. Also, the root account is always authenticated by means of the local security files.

The root account should always have a password, which should be selected with additional care. This is one of the few cases when random generator of password might be useful. 

There are multiple method of slightly increasing root account security. See Root Security. The best solution is provided by Solaris RBAC.

Attention: Routinely operating as the root user can result in accidental operation which damage the system to the extent making it unusable. See Admin Horror Storiesd  It is much safer to work from your own account, using  sudo for any operation that requires root privileges.

What the Superuser Can Do

Any process that has an effective UID of 0  runs as the superuseróthat 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.

While it is generally understood that root can do anything, in reality those pawers are concentrted in two areas: process control and device control:

What the Superuser Can't Do

There are also several things that the superuser can't do, including:

Root is just default username for superuser, Any Username Can Be a Superuser

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 Problem with the Superuser

The key problem with root account is not that is omnipotent but that regular user accounts are so powerless. So calling the root account the main security weakness in the Unix operating system is equivalent to barking to the wrong tree. 

At the same time there are some security issues related to having "omnipotent" account like root. Most Unix security exploits are based on the obtaining root privileges by exploiting some badly written or buggy program or interaction of several processes. Thus, most Unix security exploits result in a catastrophic failures of  server security mechanisms. It is enough to have a single unpatched program or component to compromise the entire computer (the weakest link effect).

There are a number of techniques for minimizing the impact of such system compromises, including:

Overall, setting the secure level to 1 or 2 enables you to increase the overall security of a Unix system; it also makes the system dramatically harder to administer. If you need to take an action that's prohibited by the current security level, you must reboot the system to do so. Furthermore, the restrictions placed on the  root user at higher secure levels may not be sufficient; it may be possible, given enough persistence, for a determined attacker to circumvent the extra security that the secure level system provides. In this regard, setting the level higher may create a false sense of security that lulls the administrator into failing to put in the proper safeguards. Nevertheless, if you can run your system at a secure level higher than 0 without needing to constantly reboot it, it's probably worthwhile to do so.

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BSD Kernel Security Levels

FreeBSD, Mac OS X, and other operating systems have kernel security levels, which can be used to significantly reduce the power that the system allots to the root  user. Using kernel security levels, you can decrease the chances that an attacker who gains root  access to your computer will be able to hide this fact in your logfiles.

The kernel security level starts at 0; it can be raised as part of the system startup, but never lowered. The secure level is set with the ;Level 1 is used for secure mode. Level 2 is used for "very secure" mode. Level 3 is defined as the "really-really secure mode."

At security level 1, the following restrictions are in place:

At security level 2, the following restriction is added: Reads from raw disk partitions are not permitted.

At security level 3, the following restriction is added:: Changes to the IP filter are not permitted.

This list is not comprehensive.

Linux Capabilities

Another mechanism for limiting the power of the superuser is the Linux capabilities system, invented on other operating systems five decades ago and included with the Linux 2.4 kernel. Some other high-security Unix systems and security add-ons to Unix have used capabilities for years, and the POSIX committee drafted a standard (POSIX 1003.1e) but later withdrew it.

The Linux capabilities system allows certain privileged tasks to be restricted to processes that have a specific "capability." This capability can be used, transferred to other processes, or given up. Once a process gives up a capability, it cannot regain that capability unless it gets a copy of the capability from another process that was similarly endowed. At startup, the

Some of the capabilities that a program can give up in the Linux 2.4.19 kernel are shown in Table 5-2. (This table also provides a nice illustration of the power of the superuser!)

Table 5-2. Some capabilities in Linux 2.4.19
Capability Description
CAP_CHOWN Can change file owner and group
CAP_FOWNER Can override file restrictions based on file owner ID
CAP_FSETIDCAP_SETUIDCAP_SETGID Can override requirements for setting SUID and SGID bits on files
CAP_KILL Can send signals to any process
CAP_LINUX_IMMUTABLE Can change the immutable or append-only attributes on files
CAP_NET_BIND_SERVICE Can bind to TCP/UDP ports below 1024
CAP_NET_BROADCAST Can transmit broadcasts
CAP_NET_ADMIN Can configure interfaces, bind addresses, modify routing tables and packet filters, and otherwise manage networking
CAP_NET_RAW Can use raw and packet sockets
CAP_IPC_LOCK Can lock shared memory
CAP_IPC_OWNER Can override IPC ownership checks
CAP_SYS_MODULE Can load and remove kernel modules




Can enable, disable, or configure process accounting
CAP_SYS_ADMIN Can configure disk quotas, configure kernel logging, set hostnames, mount and unmount filesystems, enable and disable swap, tune disk devices, access system bios, set up serial ports, and many other things


Can change process priorities and scheduling
CAP_SYS_RESOURCE Can set or override limits on resources, quotas, reserved filesystem space, and other things
CAP_SYS_TIME Can manipulate system clock
CAP_SYS_TTY_CONFIG Can configure tty devices
CAP_SETPCAP Can transfer or remove capabilities from any other process

Unfortunately, at the time this edition is being written, few Linux systems are designed to take advantage of the kernel capabilities and few system programs have been written to shed capabilities.



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