User Account Control
User Account Control (UAC) is a technology and security infrastructure introduced with
Microsoft's Windows Vista operating system. It aims to improve the security of Microsoft Windowsby limiting application softwareto standard user privileges until an administrator authorizes an increase in privilege level. In this way, only applications that the user trusts receive higher privileges, and malwareshould be kept from receiving the privileges necessary to compromise the operating system. In other words, a user account may have administrator privileges assigned to it, but applications that the user runs do not also have those privileges unless they are approved beforehand or the user explicitly authorizes it to have higher privileges.
To reduce the possibility of lower-privilege applications communicating with higher-privilege ones, another new technology,
User Interface Privilege Isolationis used in conjunction with User Account Control to isolate these processes from each other. [cite web
title=The Windows Vista and Windows Server 2008 Developer Story: Windows Vista Application Development Requirements for User Account Control (UAC)
month=April | year=2007
work=The Windows Vista and Windows Server 2008 Developer Story Series
Microsoft] One prominent use of this is Internet Explorer 7's "Protected Mode". [cite web
title=Understanding and Working in Protected Mode Internet Explorer
month=January | year=2006
coauthors=Marc Silbey, Peter Brundrett
Differentiation of a superuser and userland has been common in mainframes and servers for decades. This had an obvious security component, but also an administrative component, in that it prevented users from accidentally changing system settings.
Microsoft home operating systems (such as
Windows 95, Windows 98and Windows Me) did not have a concept of different user accounts on the same machine, and all actions were performed as super user. Windows NTintroduced multiple user accounts, but in practice most users continued to operate as super user administrator for their normal operations. Further, many applications tend to assume that the user is super user, and will simply not work if they are not.cite web
title=UAC - What. How. Why.
March 5 2007
Subsequent versions of Windows and Microsoft applications have encouraged the use of non-administrator user logins, but the uptake has been slow. User Account Control is a stronger approach to do this introduced in Vista. But it is difficult to introduce new security features without breaking existing applications.
When logging into Vista as a standard user, a logon session is created and a token containing only the most basic privileges is assigned. In this way, the new logon session is incapable of making changes that would affect the entire system. When logging in as a user in the Administrators group, two separate tokens are assigned. The first token contains all privileges typically awarded to an administrator, and the second is a restricted token similar to what a standard user would receive. User applications, including the
Windows Shell, are then started with the restricted token, resulting in a reduced privilege environment even under an Administrator account. When an application requests higher privileges or "Run as administrator" is clicked, UAC will prompt for confirmation and, if consent is given, start the process using the unrestricted token.cite web
title=Windows Vista for Developers – Part 4 – User Account Control
September 29 2006
Tasks that trigger a UAC prompt
Tasks that require administrator privileges will trigger a UAC prompt (if UAC is enabled) are typically marked by a 4-color security shield symbol. In the case of executable files, the icon will have a security shield overlay. Ed Bott's "Windows Vista Inside Out" lists the following tasks which require administrator privileges: [cite web
title=What triggers User Account Control prompts?
*Changes to system-wide settings or to files in %SystemRoot% or %ProgramFiles%
*Installing and uninstalling applications
*Installing device drivers
*Installing ActiveX controls
*Changing settings for Windows Firewall
*Changing UAC settings
*Configuring Windows Update
*Adding or removing user accounts
*Changing a user’s account type
*Configuring Parental Controls
*Running Task Scheduler
*Restoring backed-up system files
*Viewing or changing another user’s folders and files
Common tasks, such as changing the time zone, do not require administrator privileges [cite web
title=Security Features vs. Convenience |work=Windows Vista Team Blog
accessdate=2007-03-04] (although changing the system time itself does, since the system time is commonly used in security protocols). A number of tasks that required administrator privileges in earlier versions of Windows, such as installing critical Windows updates, no longer do so in Vista. [cite web
title=User Account Control Overview
Microsoft] Any program can be run as administrator by right-clicking its icon and clicking "Run as administrator".
User Account Control asks for credentials in a "Secure Desktop" mode, where the entire screen is temporarily darkened and
Windows Aerodisabled and only the authorization window is enlightened, to present only the elevation user interface (UI). This is to prevent spoofing of the UI or the mouse by the application requesting elevation. [cite web
title=User Account Control Prompts on the Secure Desktop
publisher=MSDN Blogs] If an administrative activity comes from a minimized application, the secure desktop request will also be minimized so as to prevent the focus from being lost. It is possible to disable "Secure Desktop", though this is inadvisable from a security perspective. [cite web
title=Why you need to be discriminating with those Vista tips
February 2 2007
work=Ed Bott's Windows Expertise]
Applications written with the assumption that the user will be running with administrator privileges experienced problems in earlier versions of Windows when run from limited user accounts, often because they attempted to write to machine-wide or system directories (such as "Program Files") or registry keys (notably HKLM)cite web
title=UAC - What. How. Why.
March 5 2007
Microsoft] . UAC attempts to alleviate this using "File and Registry Virtualization", which redirects writes (and subsequent reads) to a per-user location within the user’s profile. For example, if an application attempts to write to “C:program filesappnamesettings.ini” and the user doesn’t have permissions to write to that directory, the write will get redirected to “C:UsersusernameAppDataLocalVirtualStoreProgram Filesappnamesettings.ini”.
There are a number of configurable UAC settings. It is possible to: [cite web
title=Chapter 2: Defend Against Malware
work=Windows Vista Security Guide
November 8 2006
*Require administrators to re-enter their password for heightened security;
*Require the user to press Ctrl+Alt+Del as part of the authentication process for heightened security;
*Disable "Admin Approval Mode" (UAC prompts for administrators) entirely;
Command Prompt windows that are running elevated will prefix the title of the window with the word "Administrator", so that a user can discern which instances are running with elevated privileges. [cite web
title=Administrator Marking for Command Prompt
August 1 2006
A distinction is made between elevation requests from a signed executable and an unsigned executable; and if the former, whether 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. [cite web
title=Accessible UAC Prompts
work=Windows Vista Blog
Internet Explorer 7's "Protected Mode" feature uses UAC to run with a 'low' integrity level (a Standard user token has an integrity level of 'medium'; an elevated (Administrator) token has an integrity level of 'high'). As such, it effectively runs in a sandbox, unable to write to most of the system (apart from the Temporary Internet Files folder) without elevating via UAC.cite web
title=Inside Windows Vista User Account Control
month=June | year=2007
accessdate=2007-12-08] Since toolbars and ActiveX controls run within the Internet Explorer process, they will run with low privileges as well, and will be severely limited in what damage they can do to the system. [cite web |url=http://blogs.msdn.com/ie/archive/2006/02/09/528963.aspx |title=Protected Mode in Vista IE7 |first=Mike |last=Friedman |publisher=IEBlog]
UAC can be configured via security settings (secpol.msc -> Local Policies -> Security Options), though this is only available for the Business and Ultimate editions. All configuration items are prefixed with “User Account Control”.
*" 'Behaviour of the elevation prompt for administrators in admin approval mode'. " Can be set to:
**'Turn off UAC' (no prompt).
**'Prompt for consent' (default).
**'Prompt for credentials'.
*" 'Behaviour of the elevation prompt for standard users'. " This setting determines what happens if you run as a standard user and start a program that needs administrator rights (for the cases UAC can determine admin rights are required e.g. does not work for MMC snapins). Can be set to:
**'No prompt: fail and do not start the program if it required admin rights'.
**'Prompt for credentials' (default).
*" 'Admin approval mode for the built-in administrator account'." This setting can be used to disable UAC for the built-in Administrator account. Can be set to" 'Enable' " or" 'Disable'. " The built-in administrator account is disabled by default in Windows Vista. The default value of this setting depends on the configuration and on how the system was installed - see http://msdn.microsoft.com/en-us/library/bb756883.aspx for discussion.
*" 'Detect application installations and prompt for elevation'." Windows by default uses some heuristics to determine if an EXE is an installer (which most likely requires elevation). Can be set to" 'Enable' "(the default) or" 'Disable'. "
*" 'Switch to the secure desktop when prompting for elevation'." Can be set to" 'Enable' "(the default) or" 'Disable'. "
*" 'Only execute executables that are signed and validated'." If enabled an additional check is done after the elevation prompt. If the EXE is not signed the EXE will not be started. Can be set to 'Enable' or 'Disable' (the default).
*" 'Virtualize file and registry write failures to per-user locations'." Can be set to" 'Enable' "(the default) or" 'Disable'. "
*" 'Run all administrators in Admin Approval Mode'." To switch off UAC set this setting to disabled and reboot. All UAC behavior will be disabled, including file and registry virtualization. Can be set to" 'Enable' "(the default) or" 'Disable'. "
The other way to configure UAC is using tool called LEACfg. This is commandline tool and running this takes effect immediately and without a system restart.
A program can request elevation in a number of different ways. One way for program developers is to add a requestedPrivileges section to an XML document, known as the manifest, that is then embedded into the application. A manifest can specify dependencies, visual styles, and now the appropriate security context:
Setting the level attribute for requestedExecutionLevel to "asInvoker" will make the application run with the token that started it, "highestAvailable" will present a UAC prompt for administrators and run with the usual reduced privileges for standard users, and "requireAdministrator" will require elevation. [cite web
title=Making Your Application UAC Aware
publisher=The Code Project] In both highestAvailable and requireAdministrator modes, failure to provide confirmation results in the program not being launched.
An executable that is marked as "
requireAdministrator" in its manifest cannot be started from a non-elevated process using
ERROR_ELEVATION_REQUIREDwill be returned.
ShellExecuteEx()must be used instead. If an
HWNDis not supplied, then the dialog will show up as a blinking item in the taskbar.
Inspecting an executable's manifest to determine if it requires elevation is not recommended, as elevation may be required for other reasons (setup executables, application compatibility). However, it is possible to programmatically detect if an executable will require elevation by using
CreateProcess()and setting the
CREATE_SUSPENDED. If elevation is required, then
ERROR_ELEVATION_REQUIREDwill be returned. [cite web
title=Programmatically determine if an application requires elevation in Windows Vista
publisher=MSDN Blogs] If elevation is not required, a success return code will be returned at which point you can use
TerminateProcess()on the newly created, suspended process. This will not allow you detect that an executable requires elevation if you are already executing in an elevated process, however.
A new process with elevated privileges can be spawned from within a .NET application using the "
runas" verb. An example using
C++/CLI:In a native Win32application the same "
runas" verb can be added to a
ShellExecuteEx()call.In the absence of a specific directive stating what privileges the application requests, UAC will apply heuristics, to determine whether or not the application needs administrator privileges. For example, if UAC detects that the application is a setup program, from clues such as the filename, versioning fields, or the presence of certain sequences of bytes within the executable, in the absence of a manifest it will assume that the application needs administrator privileges. [cite web
title=Understanding and Configuring User Account Control in Windows Vista
There have been complaints that UAC notifications slow down various tasks on the computer such as the initial installation of software onto Windows Vista. [cite web
title=Disabling the UAC feature
date=2007-03-10] It is possible to turn off UAC while installing software, and reenable it at a later time. [cite web
title=Windows Vista upgrade power tips] However, this is not recommended since, as File & Registry Virtualization is only active when UAC is turned on, user settings and configuration files may be installed to a different place (a system directory rather than a user-specific directory) if UAC is switched off than they would be otherwise. [cite web
title=Why you need to be discriminating with those Vista tips
work=Ed Bott's Windows Expertise
accessdate=2007-07-05] Also note that
Internet Explorer 7's "Protected Mode", whereby the browser runs in a sandbox with lower privileges than the standard user, relies on UAC; and will not function if UAC is disabled. Yankee Groupanalyst Andrew Jaquith stated that "while the new security system shows promise, it is far too chatty and annoying."cite web
title=Report: Vista to hit anti-spyware, firewall markets
publisher=CNet] However, this statement was made over six months before Vista was actually released (even before Beta 2 was released). By the time Windows Vista was released in November 2006, Microsoft had drastically reduced the number of operating system tasks that triggered UAC prompts, and added file and registry virtualization to reduce the number of legacy applications that trigger UAC prompts. However, David Cross, a product unit manager at Microsoft, stated during the RSA Conference 2008 that UAC was in fact designed to "annoy users", and force independent software vendors to make their programs more secure so that UAC prompts would not be triggered. [ [http://news.zdnet.com/2100-9590_22-6237191.html Microsoft: Vista feature designed to 'annoy users' | Tech News on ZDNet ] ]
Comparison of privilege authorization features
Features new to Windows Vista
Least user access(LUA)
Security and safety features of Windows Vista
* [http://www.microsoft.com/technet/windowsvista/security/uacppr.mspx UAC (User Account Control)] — Information at Microsoft TechNet
* [http://technet2.microsoft.com/WindowsVista/en/library/00d04415-2b2f-422c-b70e-b18ff918c2811033.mspx?mfr=true UAC Understanding and Configuring] More Information at Microsoft Technet
* [http://blogs.msdn.com/uac/ UAC Team Blog]
* [http://theinvisiblethings.blogspot.com/2007/02/running-vista-every-day.html UAC – The Good and The Bad] (Joanna Rutkowska - invisiblethings.org).
* [http://www.softwaretipspalace.com/MS_Windows_Vista/Tips-and-Tricks/deactivate-UAC-in-windows-vista.php Deactivate User Account Control] in Windows Vista
* [http://windowsvistablog.com/blogs/windowsvista/archive/2007/01/23/security-features-vs-convenience.aspx Security Features vs. Convenience]
* [http://www.edbott.com/weblog/?p=1602 What triggers User Account Control prompts?]
* [http://youtech.tv/node/19 How to disable User Account Control]
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