EDX Linux Foundation Ch11:Local Security Principle section 1~3

Section 1 Understanding Linux Security


User Accounts

For each user, the following seven fields are maintained in the /etc/passwd file:

Types of Accounts

By default, Linux distinguishes between several account types in order to isolate processes and workloads. Linux has four types of accounts:

  • root
  • System
  • Normal
  • Network

The last utility, which shows the last time each user logged into the system, can be used to help identify potentially inactive accounts which are candidates for system removal.

Understanding the root Account

root is the most privileged account on a Linux/UNIX system. This account has the ability to carry out all facets of system administration, including adding accounts, changing user passwords, examining log files, installing software, etc. Utmost care must be taken when using this account. It has no security restrictions imposed upon it.

section 2: Understanding the usage of root

Operations that Require root Privileges

root privileges are required to perform operations such as:

  • Creating, removing and managing user accounts.
  • Managing software packages.
  • Removing or modifying system files.
  • Restarting system services.


Creating a New User in Linux

useradd passwd

Operations That Do Not Require root Privileges

SUID(Set owner User ID upon execution)

SUID (similar to the Windows “run as” feature)is a special kind of file permission given to a file. SUID provides temporary permissions to a user to run a program with the permissions of the file owner (which may be root) instead of the permissions held by the user.

當 s 這個標誌出現在檔案擁有者的 x 權限上時,例如/usr/bin/passwd 這個檔案的權限狀態:『-rwsr-xr-x』,此時就被稱為 Set UID,簡稱為 SUID 的特殊權限。明明 /etc/shadow 就不能讓 vbird 這個一般帳戶去存取的,為什麼還能夠修改這個檔案內的密碼呢? 這就是 SUID 的功能!

  1. vbird 對於 /usr/bin/passwd 這個程式來說是具有 x 權限的,表示 vbird 能執行 passwd;
  2. passwd 的擁有者是 root 這個帳號;
  3. vbird 執行 passwd 的過程中,會『暫時』獲得 root 的權限;
  4. /etc/shadow 就可以被 vbird 所執行的 passwd 所修改。

另外,SUID 僅可用在binary program 上, 不能夠用在 shell script 上面!這是因為 shell script 只是將很多的 binary 執行檔叫進來執行而已!

Section 3:Comparing sudo and su

Users’ authorization for using sudo is based on configuration information stored in the /etc/sudoers file and in the /etc/sudoers.d directory.

The sudoers File

Whenever sudo is invoked, a trigger will look at /etc/sudoers and the files in /etc/sudoers.d to determine if the user has the right to use sudo and what the scope of their privilege is. Unknown user requests and requests to do operations not allowed to the user.

Edit sudoers

You can edit the sudoers file by using visudo, which ensures that only one person is editing the file at a time.

The basic structure of an entry is:

who where = (as_whom) what


Most Linux distributions now prefer you add a file in the directory /etc/sudoers.d with a name the same as the user. This file contains the individual user’s sudo configuration, and one should leave the master configuration file untouched except for changes that affect all users.

Command Logging

By default, sudo commands and any failures are logged in /var/log/auth.log under the Debian distribution family, and in /var/log/messages or /var/log/secure on other systems.This is an important safeguard to allow for tracking and accountability of sudo use. A typical entry of the message contains:

Running a command such as sudo whoami results in a log file entry such as: Dec 8 14:20:47 server1 sudo: op : TTY=pts/6 PWD=/var/log USER=root COMMAND=/usr/bin/whoami

Process Isolation

Linux is considered to be more secure than many other operating systems because processes are naturally isolated from each other. One process normally cannot access the resources of another process, even when that process is running with the same user privileges.

Hardware Device Access

device special file (often called a device node) under the /dev directory that corresponds to the device being accessed.


Hard disks, for example, are represented as /dev/sd*. While a root user can read and write to the disk in a raw fashion (for example, by doing something like:

$ echo hello world > /dev/sda1

寫入divice node很容易會毀掉整個檔案系統,所以絕對不要直接存取device node。

Keeping Current

The best practice is to take advantage of your Linux distribution’s mechanism for automatic updates and never postpone them. It is extremely rare that such an update will cause new problems.