Control-Alt-Delete: Difference between revisions

Revision as of 21:07, 13 January 2009

Revision as of 04:03, 25 January 2009

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Windows NT is designed so that, unless security is already compromised in some other way, only the WinLogon process, a trusted system process, can receive notification of this keystroke combination. This is because the kernel remembers the [[Process ID]] of the WinLogon process, and allows only that process to receive the notification. This keystroke combination is thus called the [[secure attention key|Secure Attention Sequence]]. A user pressing Control-Alt-Delete can be sure that it is the operating system (specifically the WinLogon process), rather than a third party program, that is responding to the key combination, and that it is therefore safe to enter a password. It was chosen as the secure attention key in Windows (instead of, for example, the [[SysRq|System Request]] key), because on the PC platform no program could reasonably expect to redefine this keystroke combination for its own purposes.<ref>{{cite web |url=http://blogs.msdn.com/larryosterman/archive/2005/01/24/359850.aspx |title=Why is Control-Alt-Delete the secure attention sequence (SAS)? |last=Osterman |first=Larry |work=Larry Osterman's WebLog |publisher=[[MSDN Blogs]] |date={{Date|2005-01-24}} |accessdate=2007-03-15}}</ref>

It is also a reliable method for bringing up the Task Manager (in Windows Server 20032000 and older). All other keystroke combinations could potentially be exclusively tied up by a process that is stuck, but a user process is not able to intercept the Control-Alt-Delete sequence. It can be however disabled by Windows Group Policies. Ctrl+Shift+Esc also brings up the task manager in all Windows NT versions starting with NT 4.0, even if pressing Ctrl+Alt+Del is set to bring up the Windows Security dialog. The task manager can be however disabled by Windows Group Policies.

As a side effect, users who do not have physical access to the [[Computer power supply|computer's power supply]] and power/reset switches can be denied the ability to shut down or restart the computer, where previously (on MS-DOS and other variants of Windows) they could always use Control-Alt-Delete. However, as both the Task Manager and Windows Security have options for shutting down the computer, this operation can still be executed unless the entire system (including the WinLogon process) is unresponsive. Also, it is sometimes impossible to access and/or see the Task Manager after a full-screen application has frozen, although the Windows Security dialog, which is on a separate "secure desktop" almost always appears.

====Windows Vista and later====

In Windows Vista and other NT operating systems based on it (such as [[Windows Server 2008]] and [[Windows 7]], most Ctrl+Alt+Del actions remain the same, with a few differences. The classic logon screen has been completely removed, replaced with an updated welcome screen, similar to that from Windows XP. Unlike in Windows XP, the welcome screen can now be configured to require the user to press Ctrl+Alt+Del before logon. While the default behavior is to have icons for each user, the welcome screen can be configured to prompt for a username and password, though it retains the new UI. Additionally, pressing Ctrl+Alt+Del when logged in always brings up the Windows Security screen, regardless of the logon method. Users can also lock the computer even if fast user switching is enabled.

== OS/2 ==<!-- This section is linked from [[Three-finger salute (computing)]] -->