Comparison of privilege authorization features

A number of computer operating systems employ security features to help prevent malicious software from gaining sufficient privileges to compromise the computer system. Operating systems lacking such features, such as DOS, Windows implementations prior to Windows NT (and its descendants), CP/M-80, and all Mac operating systems prior to Mac OS X, had only one category of user who was allowed to do anything. The problem was that in addition to granting legitimate applications administrative privileges, this gave malicious software such as viruses and worms high privileges as well. With these privileges, malicious software could wreak havoc.

To avoid these problems, modern operating systems use one of two methods. The simpler method is to give users limited-privilege accounts, switching to a higher-privileged account only when necessary. Another method is to have users run with administrative privileges as before, but require explicit permission to grant a specific program or process administrator rights.

Introduction to implementations

• su is a command line tool for Unix. It allows users to switch the terminal to a different account by entering the username and password of that account. If no user name is given, the operating system's superuser account (known as "root") is used, thus providing a fast method to obtain a login shell with full privileges to the system. Issuing an exit command returns the user to their own account.
• sudo, created around 1980, is a Unix command line tool similar to su, but it allows certain users to run programs with root privileges instead of having to switch to the root account.
• gksudo is a graphical frontend to sudo included with Ubuntu. It comes up automatically when a supported application needs to perform an action requiring root privileges.
• kdesu is a graphical front-end to the su command.
• kdesudo is a graphical front-end to sudo that has replaced kdesu in Kubuntu, starting with the Gutsy Gibbon Tribe 5 pre-release version.
• Mac OS X includes the Authenticate dialog, which prompts the user to input their password in order to perform administrator tasks.
• User Account Control (UAC), included with Windows Vista and later Microsoft Windows operating systems, prompts the user for authorization when an application tries to perform an administrator task.
• runas is a command-line tool introduced with Windows 2000 that allows running a program, control panel applet, or a MMC snap-in as a different user. Runas makes use of the "Secondary Login" Windows service, also introduced with Windows 2000. This service provides the capability to allow applications running as a separate user to interact with the logged-in user's desktop. This is necessary to support drag-and-drop, clipboard sharing, and other interactive login features.

• 
  sudo
• 
  gksudo
• 
  System Policy Authentication
• 
  kdesu
• 
  Authenticate
• UAC confirmation dialog
  UAC confirmation dialog
• 
  UAC credentials dialog

Security considerations

Spoofed input

A major security consideration is the ability of malicious applications to simulate keystrokes or mouse clicks, thus tricking or spoofing the security feature into granting malicious applications higher privileges.

• su and sudo run in the terminal, where they are vulnerable to spoofed input. Of course, if the user was not running a multitasking environment (i.e. a single user in the shell only), this would not be a problem.
• By default, gksudo "locks" the keyboard, mouse, and window focus, preventing anything but the actual user from inputting the password or otherwise interfering with the confirmation dialog.
• By default, UAC runs in the Secure Desktop, preventing malicious applications from simulating clicking the "Allow" button or otherwise interfering with the confirmation dialog. In this mode, the user's desktop appears dimmed and cannot be interacted with.

If either gksudo's "lock" feature or UAC's Secure Desktop were compromised or disabled, malicious applications could gain administrator privileges by using keystroke logging to record the administrator's password; or, in the case of UAC if running as an administrator, spoofing a mouse click on the "Allow" button. For this reason, voice recognition is also prohibited from interacting with the dialog. Note that since gksu password prompt runs without special privileges malicious applications can still do keystroke logging using e.g. the strace tool.

Spoofed dialogs

Another security consideration is the ability of malicious software to spoof dialogs that look like legitimate security confirmation requests. If the user were to input credentials into a fake dialog, thinking the dialog was legitimate, the malicious software would then know the user's password. If the Secure Desktop or similar feature were disabled, the malicious software could use that password to gain higher privileges.

• Though it is not the default behavior for usability reasons, UAC may be configured to require the user to press Ctrl+Alt+Del (known as the secure attention sequence) as part of the authentication process. Because only Windows can detect this key combination, requiring this additional security measure would prevent spoofed dialogs from behaving the same way as a legitimate dialog. For example, a spoofed dialog might not ask the user to press Ctrl+Alt+Del, and the user could realize that the dialog was fake. Or, when the user did press Ctrl+Alt+Del, the user would be brought to the screen Ctrl+Alt+Del normally brings them to instead of a UAC confirmation dialog. Thus the user could tell whether the dialog was an attempt to trick them into providing their password to a piece of malicious software.

Usability considerations

Another consideration that has gone into these implementations is usability, or in other words, ease of use.

Separate administrator account

• su and kdesu require the user to know the password to at least two accounts: the regular-use account, and an account with higher privileges such as root.
• sudo and gksudo use a simpler approach. With these programs, the user is pre-configured to be granted access to specific administrative tasks, but must explicitly authorize applications to run with those privileges. The user enters their own password instead of the superuser or some another account.
• UAC and Authenticate combine these two ideas into one. With these programs, administrators explicitly authorize programs to run with higher privileges. Non-administrators are prompted for an administrator username and password.

Simplicity of dialog

• In order to grant an application administrative privileges, sudo, gksudo, and Authenticate prompt administrators to re-enter their password.
• With UAC, when logged in as a standard user, the user must enter an administrator's name and password each time they need to grant an application elevated privileges; but when logged in as a member of the Administrators group, they by default simply confirm or deny, instead of re-entering their password each time, though that is an option. While the default approach is simpler, it is also less secure, since if the user physically walks away from the computer without locking it another person could walk up and have administrator privileges over the system. However, because the user does not type in his credentials, this dialog cannot be spoofed to the effect of stealing his password.

Saving credentials

• UAC prompts for authorization each time it is called to elevate a program.

• sudo, gksudo, and kdesu do not ask the user to re-enter their password every time it is called to elevate a program. Rather, the user is asked for their password once at the start. If the user has not used their administrative privileges for a certain period of time (sudo's default is 5 minutes), the user is once again restricted to standard user privileges until they enter their password again.

sudo's approach is a trade-off between security and usability. On one hand, a user only has to enter their password once to perform a series of administrator tasks, rather than having to enter their password for each task. But at the same time, the surface area for attack is larger because all programs that run in that tty (for sudo) or all programs not running in a terminal (for gksudo and kdesu) receive administrator privileges for a period of time. Security-conscious users may remove the temporary administrator privileges upon completing the tasks requiring them by using the sudo -k command when from each tty or pts in which sudo was used (in the case of pts's, closing the terminal emulator is not sufficient). The equivalent command for kdesu is kdesu -s. There is no gksudo option to do the same; however, running sudo -k not within a terminal instance (e.g. through the Alt + F2 "Run Application" dialogue box, unticking "Run in terminal") will have the desired effect.

Identifying when administrative rights are needed

In order for an operating system to know when to prompt the user for authorization, an application or action needs to identify itself as requiring elevated privileges. While it is technically possible for the user to be prompted at the exact moment that an operation requiring such privileges is executed, it is often not ideal to ask for privileges partway through completing a task. If the user were unable to provide proper credentials, the work done before requiring administrator privileges would have to be undone because the task could not be seen though to the end.

In the case of user interfaces such as the Control Panel in Microsoft Windows, and the Preferences panels in Mac OS X, the exact privilege requirements are hard-coded into the system so that the user is presented with an authorization dialog at an appropriate time (for example, before displaying information that only administrators should see). Different operating systems offer distinct methods for applications to identify their security requirements:

• sudo centralises all privilege authorization information in a single configuration file, /etc/sudoers, which contains a list of users and the privileged applications and actions that those users are permitted to use. The grammar of the sudoers file is intended to be flexible enough to cover many different scenarios, such as placing restrictions on command-line parameters. For example, a user can be granted access to change anybody's password except for the root account, as follows:

 pete    ALL = /usr/bin/passwd [A-z]*, !/usr/bin/passwd root

• User Account Control uses a combination of heuristic scanning and "application manifests" to determine if an application requires administrator privileges. Manifest files, first introduced with Windows XP, are XML files with the same name as the application and a suffix of ".manifest", e.g. Notepad.exe.manifest. When an application is started, the manifest is looked at for information about what security requirements the application has. For example, this XML fragment will indicate that the application will require administrator access, but will not require unfettered access to other parts of the user desktop outside the application:

<security>
    <requestedPrivileges>
        <requestedExecutionLevel level="requireAdministrator" uiAccess="false" />
    </requestedPrivileges>
</security>

Manifest files can also be compiled into the application executable itself as an embedded resource. Heuristic scanning is also used, primarily for backwards compatibility. One example of this is looking at the executable's file name; if it contains the word "Setup", it is assumed that the executable is an installer, and a UAC prompt is displayed before the application starts.

UAC also makes a distinction between elevation requests from a signed executable and an unsigned executable; and if the former, whether or not the publisher is 'Windows Vista'. The color, icon, and wording of the prompts are different in each case: for example, attempting to convey a greater sense of warning if the executable is unsigned than if not.

See also

• Privilege escalation, a term used to describe a type of security exploit
• Principle of least privilege, a security design pattern