Comparison of Privilege Authorization Features - Identifying When Administrative Rights Are Needed

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:

• User Account Control uses a combination of heuristic scanning and "application manifests" to determine if an application requires administrator privileges. Manifest (.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:

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.

• Applications using PolicyKit ask for specific privileges when prompting for authentication, and PolicyKit performs those actions on behalf of the application. Before authenticating, users are able to see which application requested the action and which action was requested.