Windows® Internals, Sixth Edition, Part 1

How SAS Is Implemented

The SAS is secure because no application can intercept the Ctrl+Alt+Delete keystroke combination or prevent Winlogon from receiving it. Win32k.sys reserves the Ctrl+Alt+Delete key combination so that whenever the Windows input system (implemented in the raw input thread in Win32k) sees the combination, it sends an RPC message to Winlogon’s message server, which listens for such notifications. The keystrokes that map to a registered hot key are otherwise not sent to any process other than the one that registered it, and only the thread that registered a hot key can unregister it, so a Trojan horse application cannot deregister Winlogon’s ownership of the SAS.

A Windows function, SetWindowsHook, enables an application to install a hook procedure that’s invoked every time a keystroke is pressed, even before hot keys are processed, and it allows the hook to squash keystrokes. However, the Windows hot key processing code contains a special case for Ctrl+Alt+Delete that disables hooks so that the keystroke sequence can’t be intercepted. In addition, if the interactive desktop is locked, only hot keys owned by Winlogon are processed.

Once the Winlogon desktop is created during initialization, it becomes the active desktop. When the Winlogon desktop is active, it is always locked. Winlogon unlocks its desktop only to switch to the application desktop or the screen-saver desktop. (Only the Winlogon process can lock or unlock a desktop.)

User Logon Steps

Logon begins when a user presses the SAS (Ctrl+Alt+Delete). After the SAS is pressed, Winlogon starts LogonUI, which calls the credential providers to obtain a user name and password. Winlogon also creates a unique local logon SID for this user that it assigns to this instance of the desktop (keyboard, screen, and mouse). Winlogon passes this SID to LSASS as part of the LsaLogonUser call. If the user is successfully logged on, this SID will be included in the logon process token—a step that protects access to the desktop. For example, another logon to the same account but on a different system will be unable to write to the first machine’s desktop because this second logon won’t be in the first logon’s desktop token.

When the user name and password have been entered, Winlogon retrieves a handle to a package by calling the LSASS function LsaLookupAuthenticationPackage. Authentication packages are listed in the registry under HKLM\SYSTEM\CurrentControlSet\Control\Lsa. Winlogon passes logon information to the authentication package via LsaLogonUser. Once a package authenticates a user, Winlogon continues the logon process for that user. If none of the authentication packages indicates a successful logon, the logon process is aborted.

Windows uses two standard authentication packages for interactive logons: Kerberos and MSV1_0. The default authentication package on a stand-alone Windows system is MSV1_0 (%SystemRoot%\System32\Msv1_0.dll), an authentication package that implements LAN Manager 2 protocol. LSASS also uses MSV1_0 on domain-member computers to authenticate to pre–Windows 2000 domains and computers that can’t locate a domain controller for authentication. (Computers that are disconnected from the network fall into this latter category.) The Kerberos authentication package, %SystemRoot%\System32\Kerberos.dll, is used on computers that are members of Windows domains. The Windows Kerberos package, with the cooperation of Kerberos services running on a domain controller, supports the Kerberos protocol. This protocol is based on Internet RFC 1510. (Visit the Internet Engineering Task Force [IETF] website, www.ietf.org, for detailed information on the Kerberos standard.)

The MSV1_0 authentication package takes the user name and a hashed version of the password and sends a request to the local SAM to retrieve the account information, which includes the hashed password, the groups to which the user belongs, and any account restrictions. MSV1_0 first checks the account restrictions, such as hours or type of accesses allowed. If the user can’t log on because of the restrictions in the SAM database, the logon call fails and MSV1_0 returns a failure status to the LSA.

MSV1_0 then compares the hashed password and user name to that obtained from the SAM. In the case of a cached domain logon, MSV1_0 accesses the cached information by using LSASS functions that store and retrieve “secrets” from the LSA database (the SECURITY hive of the registry). If the information matches, MSV1_0 generates a LUID for the logon session and creates the logon session by calling LSASS, associating this unique identifier with the session and passing the information needed to ultimately create an access token for the user. (Recall that an access token includes the user’s SID, group SIDs, and assigned privileges.)

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