Microsoft Windows Security

Security System Components

These are the core components and databases that implement Windows security:

• Security reference monitor (SRM) A component in the Windows executive (%SystemRoot%\System32\Ntoskrnl.exe) that is responsible for defining the access token data structure to represent a security context, performing security access checks on objects, manipulating privileges (user rights), and generating any resulting security audit messages.

• Local Security Authority subsystem (LSASS) A user-mode process running the image %SystemRoot%\System32\Lsass.exe that is responsible for the local system security policy (such as which users are allowed to log on to the machine, password policies, privileges granted to users and groups, and the system security auditing settings), user authentication, and sending security audit messages to the Event Log. The Local Security Authority service (Lsasrv—%SystemRoot%\System32\Lsasrv.dll), a library that LSASS loads, implements most of this functionality.

• LSASS policy database A database that contains the local system security policy settings. This database is stored in the registry in an ACL-protected area under HKLM\SECURITY. It includes such information as what domains are entrusted to authenticate logon attempts, who has permission to access the system and how (interactive, network, and service logons), who is assigned which privileges, and what kind of security auditing is to be performed. The LSASS policy database also stores “secrets” that include logon information used for cached domain logons and Windows service user-account logons. (See Chapter 4, “Management Mechanisms,” for more information on Windows services.)

• Security Accounts Manager (SAM) A service responsible for managing the database that contains the user names and groups defined on the local machine. The SAM service, which is implemented as %SystemRoot%\System32\Samsrv.dll, is loaded into the LSASS process.

• SAM database A database that contains the defined local users and groups, along with their passwords and other attributes. On domain controllers, the SAM does not store the domain-defined users, but stores the system’s administrator recovery account definition and password. This database is stored in the registry under HKLM\SAM.

• Active Directory A directory service that contains a database that stores information about objects in a domain. A domain is a collection of computers and their associated security groups that are managed as a single entity. Active Directory stores information about the objects in the domain, including users, groups, and computers. Password information and privileges for domain users and groups are stored in Active Directory, which is replicated across the computers that are designated as domain controllers of the domain. The Active Directory server, implemented as %SystemRoot%\System32\Ntdsa.dll, runs in the LSASS process. For more information on Active Directory, see Chapter 7, “Networking.”

• Authentication packages These include dynamic-link libraries (DLLs) that run both in the context of the LSASS process and client processes, and implement Windows authentication policy. An authentication DLL is responsible for authenticating a user, by checking whether a given user name and password match, and if so, returning to the LSASS information detailing the user’s security identity, which LSASS uses to generate a token.

• Interactive logon manager (Winlogon) A user-mode process running %SystemRoot%\System32\Winlogon.exe that is responsible for responding to the SAS and for managing interactive logon sessions. Winlogon creates a user’s first process when the user logs on, for example.

• Logon user interface (LogonUI) A user-mode process running %SystemRoot%\System32\LogonUI.exe that presents users with the user interface they can use to authenticate themselves on the system. LogonUI uses credential providers to query user credentials through various methods.

• Credential providers (CPs) In-process COM objects that run in the LogonUI process (started on demand by Winlogon when the SAS is performed) and used to obtain a user’s name and password, smartcard PIN, or biometric data (such as a fingerprint). The standard CPs are %SystemRoot%\System32\authui.dll and %SystemRoot%\System32\SmartcardCredentialProvider.dll.

• Network logon service (Netlogon) A Windows service (%SystemRoot%\System32\Netlogon.dll) that sets up the secure channel to a domain controller, over which security requests—such as an interactive logon (if the domain controller is running Windows NT 4) or LAN Manager and NT LAN Manager (v1 and v2) authentication validation—are sent. Netlogon is also used for Active Directory logons.

• Kernel Security Device Driver (KSecDD) A kernel-mode library of functions that implement the advanced local procedure call (ALPC) interfaces that other kernel mode security components, including the Encrypting File System (EFS), use to communicate with LSASS in user mode. KSecDD is located in %SystemRoot%\System32\Drivers\Ksecdd.sys.

• AppLocker A mechanism that allows administrators to specify which executable files, DLLs, and scripts can be used by specified users and groups. AppLocker consists of a driver (%SystemRoot%\System32\Drivers\AppId.sys) and a service (%SystemRoot%\System32\AppIdSvc.dll) running in a SvcHost process.

Figure 6-1 shows the relationships among some of these components and the databases they manage.

Figure 6-1 Windows security components

EXPERIMENT: Looking Inside HKLM\SAM and HKLM\Security

The security descriptors associated with the SAM and Security keys in the registry prevent access by any account other than the local system account. One way to gain access to these keys for exploration is to reset their security, but that can weaken the system’s security. Another way is to execute Regedit.exe while running as the local system account. This can be done using the PsExec tool from Windows Sysinternals with the –s option, as shown here:

C:\>psexec –s –i –d c:\windows\regedit.exe

The SRM, which runs in kernel mode, and LSASS, which runs in user mode, communicate using the ALPC facility described in Chapter 3, “System Mechanisms.” During system initialization, the SRM creates a port, named SeRmCommandPort, to which LSASS connects. When the LSASS process starts, it creates an ALPC port named SeLsaCommandPort. The SRM connects to this port, resulting in the creation of private communication ports. The SRM creates a shared memory section for messages longer than 256 bytes, passing a handle in the connect call. Once the SRM and LSASS connect to each other during system initialization, they no longer listen on their respective connect ports. Therefore, a later user process has no way to connect successfully to either of these ports for malicious purposes—the connect request will never complete.

Figure 6-2 shows the communication paths as they exist after system initialization.

Figure 6-2 Communication between the SRM and LSASS