Windows implements CTRL+ALT+DEL as a secure attention sequence (SAS) that creates a trusted path to the operating system's authentication subsystem. When you press this key combination:

// Simplified pseudocode of Windows SAS handling
void HandleSecureAttention() {
    if (KeyboardInterrupt == CTRL_ALT_DEL) {
        DisplaySecureDesktop();  // Switches to Winlogon desktop
        PurgeKeyboardBuffer();   // Prevents keyloggers from capturing input
        EnableSecureUIInput();   // Bypasses hook-based interception
    }
}

Unlike Unix terminals which directly communicate with /bin/login, Windows must defend against:

• Fake login screens (credential phishing)
• User-mode keyloggers
• Session hijacking attempts

The Windows kernel (ntoskrnl.exe) processes this interrupt at IRQL 2 (DISPATCH_LEVEL):

// Kernel-mode handling (simplified)
NTSTATUS NtUserInitialize(/*...*/) {
    RegisterSasRoutine(WinlogonSasHandler);
    SetConsoleCtrlHandler(HandlerRoutine, TRUE);
}

VOID WinlogonSasHandler() {
    CsrSwitchToWinlogonDesktop();  // Secure desktop
    WlxSasNotify();                // Notifies GINA/credential providers
}

For headless servers where SAS isn't practical:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]
"DisableCAD"=dword:00000001

Windows implements CTRL+ALT+DEL as a Secure Attention Sequence (SAS) - a trusted path mechanism that cannot be intercepted by malicious software. Unlike regular key combinations that applications can capture, this sequence triggers an interrupt (INT 0x2E) handled directly by the Windows kernel.

The security stems from these architectural decisions:

// Simplified kernel-level handling (conceptual)
VOID KiSystemServiceHandler(IN ULONG ServiceNumber, ...) {
    if (ServiceNumber == SAS_SERVICE_ID) {
        if (UserModeRequest) return STATUS_ACCESS_DENIED;
        // Switch to secure desktop
        WinlogonSwitchDesktop();
    }
}

The SAS combats several attack vectors:

• Credential phishing: Prevents fake login screens by ensuring only the genuine Winlogon process receives credentials
• Keyloggers: The sequence isn't logged as regular keystrokes
• Session hijacking: Forces a clean context switch to the secure desktop

UNIX systems achieve similar security through different means:

// Linux getty implementation example
void secure_tty_io() {
    ioctl(STDIN_FILENO, TIOCSCTTY, 1);
    setsid();
    // Drop all unnecessary capabilities
    cap_drop_all(); 
}

Modern Windows versions may skip SAS when:

• Using Windows Hello biometric authentication
• Running in Safe Mode
• Certain domain-joined configurations with Group Policy overrides

For applications requiring similar security:

// C# example of secure desktop usage
using System.Security;

public void ShowSecureDialog() {
    SecureString password = new SecureString();
    // This uses protected UI path similar to SAS
    Microsoft.Win32.PasswordBox.ShowDialog(password); 
}

The SAS originates from IBM's original PC design (INT 09h keyboard handler) and was repurposed by Windows NT's architects as a security boundary. Interestingly, early UNIX systems actually did use attention sequences (BREAK or ESC) on serial terminals to ensure clean session starts.