System Administration Guide: Security Services

Using UNIX Permissions to Protect Files

Files can be secured through UNIX file permissions and through ACLs. Files with sticky bits, and files that are executable, require special security measures.

Commands for Viewing and Securing Files

This table describes the commands for monitoring and securing files and directories.

Table 7–1 Commands for Securing Files and Directories


Command	Description	Man Page
`ls`	Lists the files in a directory and information about the files.	ls(1)
`chown`	Changes the ownership of a file.	chown(1)
`chgrp`	Changes the group ownership of a file.	chgrp(1)
`chmod`	Changes permissions on a file. You can use either symbolic mode, which uses letters and symbols, or absolute mode, which uses octal numbers, to change permissions on a file.	chmod(1)

File and Directory Ownership

Traditional UNIX file permissions can assign ownership to three classes of users:

user – The file or directory owner, which is usually the user who created the file. The owner of a file can decide who has the right to read the file, to write to the file (make changes to it), or, if the file is a command, to execute the file.
group – Members of a group of users.
others – All other users who are not the file owner and are not members of the group.

The owner of the file can usually assign or modify file permissions. Additionally, users or roles with administrative capabilities, such as superuser or the Primary Administrator role, can change a file's ownership. To override system policy, see Example 7–2.

A file can be one of seven types. Each type is displayed by a symbol:

- (Minus symbol): Text or program
b: Block special file
c: Character special file
d: Directory
l: Symbolic link
s: Socket
D: Door
P: Named pipe (FIFO)

UNIX File Permissions

The following table lists and describes the permissions that you can give to each class of user for a file or directory.

Table 7–2 File and Directory Permissions


Symbol	Permission	Object	Description
`r`	Read	File	Designated users can open and read the contents of a file.
		Directory	Designated users can list files in the directory.
`w`	Write	File	Designated users can modify the contents of the file or delete the file.
		Directory	Designated users can add files or add links in the directory. They can also remove files or remove links in the directory.
`x`	Execute	File	Designated users can execute the file, if it is a program or shell script. They also can run the program with one of the `exec(2)` system calls.
		Directory	Designated users can open files or execute files in the directory. They also can make the directory and the directories beneath it current.
`-`	Denied	File and Directory	Designated users cannot read, write, or execute the file.

These file permissions apply to regular files, and to special files such as devices, sockets, and named pipes (FIFOs).

For a symbolic link, the permissions that apply are the permissions of the file that the link points to.

You can protect the files in a directory and its subdirectories by setting restrictive file permissions on that directory. Note, however, that superuser has access to all files and directories on the system.

Special File Permissions (`setuid`, `setgid` and Sticky Bit)

Three special types of permissions are available for executable files and public directories: setuid, setgid, and sticky bit. When these permissions are set, any user who runs that executable file assumes the ID of the owner (or group) of the executable file.

You must be extremely careful when you set special permissions, because special permissions constitute a security risk. For example, a user can gain superuser capabilities by executing a program that sets the user ID (UID) to 0, which is the UID of root. Also, all users can set special permissions for files that they own, which constitutes another security concern.

You should monitor your system for any unauthorized use of the setuid permission and the setgid permission to gain superuser capabilities. A suspicious permission grants ownership of an administrative program to a user rather than to root or bin. To search for and list all files that use this special permission, see How to Find Files With Special File Permissions.

`setuid` Permission

When setuid permission is set on an executable file, a process that runs this file is granted access on the basis of the owner of the file. The access is not based on the user who is running the executable file. This special permission allows a user to access files and directories that are normally available only to the owner.

For example, the setuid permission on the passwd command makes it possible for users to change passwords. A passwd command with setuid permission would resemble the following:

-r-sr-sr-x   3 root     sys       28144 Jun 17 12:02 /usr/bin/passwd

This special permission presents a security risk. Some determined users can find a way to maintain the permissions that are granted to them by the setuid process even after the process has finished executing.

Note –

The use of setuid permissions with the reserved UIDs (0–100) from a program might not set the effective UID correctly. Use a shell script, or avoid using the reserved UIDs with setuid permissions.

`setgid` Permission

The setgid permission is similar to the setuid permission. The process's effective group ID (GID) is changed to the group that owns the file, and a user is granted access based on the permissions that are granted to that group. The /usr/bin/mail command has setgid permissions:

-r-x--s--x   1 root   mail     67504 Jun 17 12:01 /usr/bin/mail

When the setgid permission is applied to a directory, files that were created in this directory belong to the group to which the directory belongs. The files do not belong to the group to which the creating process belongs. Any user who has write and execute permissions in the directory can create a file there. However, the file belongs to the group that owns the directory, not to the group that the user belongs to.

You should monitor your system for any unauthorized use of the setgid permission to gain superuser capabilities. A suspicious permission grants group access to such a program to an unusual group rather than to root or bin. To search for and list all files that use this permission, see How to Find Files With Special File Permissions.

Sticky Bit

The sticky bit is a permission bit that protects the files within a directory. If the directory has the sticky bit set, a file can be deleted only by the file owner, the directory owner, or by a privileged user. The root user and the Primary Administrator role are examples of privileged users. The sticky bit prevents a user from deleting other users' files from public directories such as /tmp:

drwxrwxrwt 7  root  sys   400 Sep  3 13:37 tmp

Be sure to set the sticky bit manually when you set up a public directory on a TMPFS file system. For instructions, see Example 7–5.

Default `umask` Value

When you create a file or directory, you create it with a default set of permissions. The system defaults are open. A text file has 666 permissions, which grants read and write permission to everyone. A directory and an executable file have 777 permissions, which grants read, write, and execute permission to everyone. Typically, users override the system defaults in their /etc/profile file, .cshrc file, or .login file.

The value assigned by the umask command is subtracted from the default. This process has the effect of denying permissions in the same way that the chmod command grants them. For example, the chmod 022 command grants write permission to group and others. The umask 022 command denies write permission to group and others.

The following table shows some typical umask settings and their effect on an executable file.

Table 7–3 umask Settings for Different Security Levels


Level of Security	`umask` Setting	Permissions Disallowed
Permissive (`744`)	`022`	`w` for group and others
Moderate (`740`)	`027`	`w` for group, `rwx` for others
Moderate (`741`)	`026`	`w` for group, `rw` for others
Severe (`700`)	`077`	`rwx` for group and others

For more information on setting the umask value, see the umask(1) man page.

File Permission Modes

The chmod command enables you to change the permissions on a file. You must be superuser or the owner of a file or directory to change its permissions.

You can use the chmod command to set permissions in either of two modes:

Absolute Mode – Use numbers to represent file permissions. When you change permissions by using the absolute mode, you represent permissions for each triplet by an octal mode number. Absolute mode is the method most commonly used to set permissions.
Symbolic Mode – Use combinations of letters and symbols to add permissions or remove permissions.

The following table lists the octal values for setting file permissions in absolute mode. You use these numbers in sets of three to set permissions for owner, group, and other, in that order. For example, the value 644 sets read and write permissions for owner, and read-only permissions for group and other.

Table 7–4 Setting File Permissions in Absolute Mode


Octal Value	File Permissions Set	Permissions Description
`0`	`---`	No permissions
`1`	`--x`	Execute permission only
`2`	`-w-`	Write permission only
`3`	`-wx`	Write and execute permissions
`4`	`r--`	Read permission only
`5`	`r-x`	Read and execute permissions
`6`	`rw-`	Read and write permissions
`7`	`rwx`	Read, write, and execute permissions

The following table lists the symbols for setting file permissions in symbolic mode. Symbols can specify whose permissions are to be set or changed, the operation to be performed, and the permissions that are being assigned or changed.

Table 7–5 Setting File Permissions in Symbolic Mode


Symbol	Function	Description
`u`	`who`	User (owner)
`g`	`who`	Group
`o`	`who`	Others
`a`	`who`	All
`=`	`operator`	Assign
`+`	`operator`	Add
`-`	`operator`	Remove
`r`	`permissions`	Read
`w`	`permissions`	Write
`x`	`permissions`	Execute
`l`	`permissions`	Mandatory locking, `setgid` bit is on, group execution bit is off
`s`	`permissions`	`setuid` or `setgid` bit is on
`t`	`permissions`	Sticky bit is on, execution bit for others is on

The who operator permissions designations in the function column specify the symbols that change the permissions on the file or directory.

who: Specifies whose permissions are to be changed.
operator: Specifies the operation to be performed.
permissions: Specifies what permissions are to be changed.

You can set special permissions on a file in absolute mode or symbolic mode. However, you must use symbolic mode to set or remove setuid permissions on a directory. In absolute mode, you set special permissions by adding a new octal value to the left of the permission triplet. The following table lists the octal values for setting special permissions on a file.

Table 7–6 Setting Special File Permissions in Absolute Mode


Octal Value	Special File Permissions
`1`	Sticky bit
`2`	`setgid`
`4`	`setuid`