How Interactive Logon Works

Article
03/19/2017

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

In this section

The interactive logon process is the first step in user authentication and authorization. Interactive logon is mandatory in the Windows Server 2003, Windows XP, Microsoft Windows 2000, and Windows NT 4.0 operating systems. Interactive logon provides a way to identify authorized users and determine whether they are allowed to log on and access the system.

This section describes the interactive logon architecture, the process of an interactive logon, and the way in which the Windows Server 2003 interactive logon process locks the workstation and helps to keep it secure.

Interactive Logon Architecture

The Windows Server 2003 interactive logon architecture includes the following components:

Winlogon
Graphical Identification and Authentication (GINA) dynamic-link library (DLL)
Local Security Authority (LSA)
Authentication packages (NTLM and Kerberos)

For more information about these components, see Interactive Logon Components later in this section.

Windows Server 2003 interactive logons begin with the user pressing CTRL+ALT+DEL to initiate the logon process. The CTRL+ALT+DEL keystroke is called a secure attention sequence (SAS); Winlogon registers this sequence during the boot process to keep other programs and processes from using it. The GINA DLL generates the logon dialog box. The following figure shows the Windows Server 2003 logon dialog box.

Windows Server 2003 Logon Dialog Box

Windows Server 2003 Logon Dialog Box

A user who logs on to a computer using either a local or domain account must enter a user name and password, which form the user's credentials and are used to verify the user's identity. For smart card logons, a user's credentials are contained on the card's security chip, which is read by an external device, a smart card reader. During a smart card logon, a user enters a personal identification number (PIN) instead of a user name, domain, and password.

Local Logon

Local logons give users access to local computer applications and resources but not to domain applications and resources. When users log on locally, their identities are validated by authentication packages to local account information stored in the Security Accounts Manager (SAM) database. The SAM operates in the security context of the LSA; it protects and manages user and group information in the form of security accounts stored in the local computer registry. Because user accounts are stored on the local computer, network access is not required for local logons. However, if a computer has a network connection and a user logs on to a local account, there is no interaction with the network.

Local logons can be performed on Windows client operating systems, such as Windows XP and Windows 2000 Professional. Windows server operating systems, such as Windows NT Server, Windows 2000 Server, and Windows Server 2003, also permit local logons.

The following figure shows the local logon architecture.

Interactive Local Logon

Interactive Local Logon

A successful local logon begins when a user presses CTRL+ALT+DEL. Winlogon and the GINA DLL collect the user's credentials and then send the credentials to the LSA. The LSA verifies the user's identity and then returns a logon success and the user's access token to Winlogon and the GINA DLL. Winlogon and the GINA DLL then activate the user's shell by creating a new process, such as Explorer.exe.

Domain Logon

Domain logons give users access to resources throughout the domain. Domain user accounts are stored in an Active Directory domain. Active Directory is deployed on each domain controller, and domain user accounts are replicated throughout a domain.

Before a user can log on to a computer by using a domain account, the computer must be joined to a domain. If the computer has access to a network connection, the user can log on to a domain if the user has an account in the domain's Active Directory.

The computer must transparently authenticate to the domain's Active Directory. This form of logon is called a computer logon. Both users and computers are considered equal security principals in Active Directory; to be granted access to network resources, both must be able to verify their identities.

Users can use a domain account to log on to Windows client operating systems, such as Windows 8. Windows server operating systems, such as Windows Server 2012 R2, also permit domain logons. Only server operating systems can function as domain controllers and deploy Active Directory.

On a domain-joined computer, Windows is hard-coded to show only the last logged on user or Other user. Additional tiles for other users to log on are available only for computers joined to a workgroup.

The following figure shows the domain logon architecture.

Interactive Domain Logon

Interactive Domain Logon

Unlike a local logon in which the local LSA validates the user, during a domain logon, the LSA on a domain controller validates the user. The LSA evaluates the user's credentials to determine if the logon should be processed as a logon to a local account or a logon to a domain account. After determining the logon type, either the NTLM or Kerberos authentication package validates the user. If the authenticating domain controller is a computer running Windows 2000 or Windows Server 2003, the LSA will use Kerberos, the default authentication package for domain and network logons. The LSA uses NTLM to process domain logons in Windows NT 4.0 mixed environments.

Single Sign On

Single sign-on (SSO) uses credentials collected during an interactive domain logon to allow the user to authenticate to a network one time and, thereafter, to have access to all authorized network resources without additional authentication. The network resources can range from hardware devices, such as printers, to applications, files, and other types of data, all of which may be located throughout an enterprise on servers of various types, possibly in different domains and running different operating systems.

Cached Credentials

After a successful domain logon, information is cached; this means that later a user can log on to the computer with the domain account even if the domain controller that authenticated the user is not available. Because the user has already been authenticated, Windows uses the cached credentials to log the user on locally. For example, if a mobile user logs on to a portable computer that is a domain member with a domain account and then takes the portable computer to a location where the domain is unavailable, Windows will attempt to use the cached credentials from the last successful logon with a domain account to locally log on the user and allocate access to local computer resources.

Interactive Logon Components

The components of the interactive logon process are responsible for helping to establish secure user authentication. This section describes the roles and functions of the user logon components.

Winlogon

Winlogon (%windir%\System32\Winlogon.exe) is the executable file responsible for managing secure user interactions. Winlogon initiates the logon process for Windows Server 2003, Windows 2000, Windows XP, and Windows NT 4.0.

Winlogon is responsible for the following tasks.

Desktop lockdown
Standard SAS recognition
SAS routine dispatching
User profile loading
Screen saver control
Multiple network provider support

Desktop Lockdown

Winlogon helps prevent unauthorized users from gaining access to system resources by locking the computer desktop. At any time, Winlogon is in one of three possible states: logged on, logged off, or workstation locked, as shown in the following figure.

Winlogon States

Winlogon States

Winlogon switches between three desktops—Winlogon, screen saver, and default—depending on its state and user activity. The following table lists and describes each of these desktops.

Winlogon Desktops

Desktop	Description
Winlogon	Also called the secure desktop. There are five dialog boxes in the Winlogon desktop, which are described in the following table.
Screen saver	Used when a screen saver is running.
Default	Displayed when a user has successfully logged on to Windows Server 2003 and the user's shell has been activated. The user's applications run in this desktop.

The following table lists and describes the five dialog boxes in the Winlogon desktop.

Winlogon Desktop Dialog Boxes

Desktop	Description
Welcome to Windows	Displayed when a computer has initially booted and no user has entered a SAS or when a user has logged off.
Log On to Windows	Displayed after a user presses CTRL+ALT+DEL when no user is currently logged on.
Windows Security and Change Password	Displayed when: - A logged-on user presses CTRL+ALT+DEL when the workstation is not locked. OR - When the workstation is not locked, after a user presses CTRL+ALT+DEL and clicks Change Password.
Computer Locked	Displayed when the workstation is locked either explicitly by the user or when a screen saver is set as secure and is dismissed.
Unlock Computer	Displayed when a user presses CTRL+ALT+DEL when the workstation is locked.

By switching between the desktops for different logon tasks, Winlogon ensures that its processes will be invisible to applications not involved in the logon process, preventing logon data from being compromised.

Standard SAS Recognition

Winlogon registers the default SAS during the boot process, which ensures that no other program can access the same key sequence and compromise the logon process by impersonating Winlogon and its logon dialog boxes.

SAS Routine Dispatching

When Winlogon recognizes a SAS event or the GINA delivers a SAS, Winlogon calls one of the SAS processing functions of the GINA.

User Profile Loading

After a successful logon, Winlogon loads user profiles into the HKEY_CURRENT_USER registry key.

Screen Saver Control

Winlogon monitors keyboard and mouse activity to determine when to activate screen savers. When a screen saver that is set as secure is dismissed, Winlogon treats the workstation as locked and the GINA displays the Computer Locked dialog box. When a screen saver not set as secure is dismissed, the user is able to access the application desktop without being prompted to reenter credentials.

Multiple Network Provider Support

If there are multiple network providers installed on a Windows-based system, they can be included in the authentication process and in password-updating operations. This inclusion lets additional network providers gather identification and authentication information during a logon, using the secure desktop.

GINA

The GINA is a DLL module that operates in the security context of Winlogon. Winlogon loads the GINA early in the boot process. The GINA is responsible for processing SAS events and activating the user's shell. The GINA can generate SAS events in some instances.

In Windows Server 2003, Windows XP, and Windows 2000, Msgina.dll (%windir%\System32\Msgina.dll) is the default GINA; it can be replaced to support specific and unique authentication methods. GINA customization is enabled to accommodate the use of authentication hardware tokens, such as retinal scanners and proprietary smart card solutions.

LSA

The LSA (%windir%\System32\lsass.exe) is a protected security subsystem that helps create secure user interactions in Windows Server 2003. Winlogon and the GINA call the LSA to process logon credentials.

The following components involved in user logon run in the security context of the LSA:

Authentication packages
SAM
Active Directory

LSA Responsibilities

Role	Description
Application programming interfaces (APIs)	The LSA provides APIs for: - Packages to create a user's logon token after authentication. - Applications to call directly for user logon. - Microsoft Security Support Provider Interface (SSPI) requests and the routing of requests to the appropriate authentication package. - Applications to enumerate available authentication packages and query package capabilities. - Applications to resolve Security Identifier-to-Name and Name-to-Security Identifier. - Applications to store secret information in the system.
Logon session state	Manages the state of the logon session for any user authenticated by the authentication packages.
Notification to installed authentication packages	Notifies installed authentication packages of user logon, logoff, and credential updates.
Authentication packages	Loads and unloads authentication packages.
Encryption assistance	Includes subcomponents that provide encryption services to users of the local computer.

The following figure shows the LSA architecture. Winlogon, which runs as a separate process from lsass.exe, is shown here to illustrate the way in which the logon process interacts with the LSA.

LSA Architecture

LSA Architecture

Note

Some components shown in this figure run only on domain controllers or non-domain controllers. The SAM runs only on non-domain controllers and Windows NT computers; Directory Services runs only on domain controllers.

As the figure shows, all calls from Winlogon are sent by using the Secur32.dll LsaLogonUser call. LsaLogonUser returns the call through local procedure call (LPC) to the LSA Server service. The LSA Server service then determines which authentication package to use to process the logon.

You can set user rights for local computers by using the Local Security Policy snap-in of the Microsoft Management Console (MMC). On Windows Server 2003 non-domain controllers, the Group Policy User Rights Assignment contains information about which users are authorized to perform different tasks, including logging on to the system locally. Domain controllers use the Domain Security Policy snap-in of the MMC to set user rights for domain computers. The User Rights Assignment on domain controllers contains information that applies to the entire domain.

Authentication Packages

Authentication packages on the user's local computer communicate with server authentication packages to authenticate users. Windows Server 2003 applies default authentication packages for user authentication and also supports custom authentication packages.

The following table lists the Windows Server 2003 authentication packages used for interactive logons.

Interactive Logon Authentication Packages

Name	Associated Protocol	Environment
Kerberos version 5 (V5)	Kerberos.dll	Windows 2000, Windows XP, and Windows Server 2003
NTLM	MSV1_0.dll	Windows NT 4.0 and mixed environments

SAM

The SAM stores information about local user accounts in the Windows registry. Passwords are encrypted by the NTLM authentication package. The outcome of the encryption is a hashed password transformed into ciphertext, a string of numbers and letters that appears meaningless. The hashing process occurs by means of a hashing algorithm. NTLM uses the same algorithm to encrypt and decrypt a user's password.

Active Directory

Domain account information is stored in Active Directory on the domain controllers. Within a particular domain, each domain controller has an identical copy of the domain's Active Directory. Users can therefore authenticate to any domain controller and their logon information will be recognized throughout the domain.

Interactive Logon Protocols

In network environments, authentication protocols are used to authenticate users, computers, and groups. For local logons, Windows Server 2003 supports the NTLM authentication protocol. For domain logons, Windows Server 2003 supports the Kerberos V5 and NTLM authentication protocols. Windows Server 2003 also supports custom authentication packages.

Logon Protocols

Component	Description
Kerberos.dll	- Kerberos V5 authentication package authentication protocol. - Standard Internet protocol and default network authentication protocol in Windows Server 2003. - Performs authentication of users and computers in Windows 2000, Windows XP, and Windows Server 2003.
MSV1_0.dll	- NTLM authentication package authentication protocol. - Default network authentication protocol in Windows NT 4.0. - Performs authentication of users and computers in Windows NT 4.0 mixed environments. - Included in Windows Server 2003 for compatibility with Windows NT 4.0 mixed environments.

For more information about how the Kerberos V5 protocol implements user authentication, see Kerberos Authentication Technical Reference.