Microsoft Windows Security

Account Rights and Privileges

Many operations performed by processes as they execute cannot be authorized through object access protection because they do not involve interaction with a particular object. For example, the ability to bypass security checks when opening files for backup is an attribute of an account, not of a particular object. Windows uses both privileges and account rights to allow a system administrator to control what accounts can perform security-related operations.

A privilege is the right of an account to perform a particular system-related operation, such as shutting down the computer or changing the system time. An account right grants or denies the account to which it’s assigned the ability to perform a particular type of logon, such as a local logon or interactive logon, to a computer.

A system administrator assigns privileges to groups and accounts using tools such as the Active Directory Users and Groups MMC snap-in for domain accounts or the Local Security Policy Editor (%SystemRoot%\System32\secpol.msc). You access the Local Security Policy Editor in the Administrative Tools folder of the Control Panel or the Start menu (if you’ve configured your Start menu to contain an Administrative Tools link). Figure 6-8 shows the User Rights Assignment configuration in the Local Security Policy Editor, which displays the complete list of privileges and account rights available on Windows. Note that the tool makes no distinction between privileges and account rights. However, you can differentiate between them because any user right that does not contain the words log on is an account privilege.

Figure 6-8 Local Security Policy Editor user rights assignment

Account Rights

Account rights are not enforced by the security reference monitor, nor are they stored in tokens. The function responsible for logon is LsaLogonUser. Winlogon, for example, calls the LogonUser API when a user logs on interactively to a computer, and LogonUser calls LsaLogonUser. LogonUser takes a parameter that indicates the type of logon being performed, which includes interactive, network, batch, service, and Terminal Server client.

In response to logon requests, the Local Security Authority (LSA) retrieves account rights assigned to a user from the LSA policy database at the time that a user attempts to log on to the system. LSA checks the logon type against the account rights assigned to the user account logging on and denies the logon if the account does not have the right that permits the logon type or it has the right that denies the logon type. Table 6-8 lists the user rights defined by Windows.

Windows applications can add and remove user rights from an account by using the LsaAddAccountRights and LsaRemoveAccountRights functions, and they can determine what rights are assigned to an account with LsaEnumerateAccountRights.

Table 6-8 Account Rights

Privileges

The number of privileges defined by the operating system has grown over time. Unlike user rights, which are enforced in one place by the LSA, different privileges are defined by different components and enforced by those components. For example, the debug privilege, which allows a process to bypass security checks when opening a handle to another process with the OpenProcess Windows API, is checked for by the process manager. Table 6-9 is a full list of privileges, and it describes how and when system components check for them.

When a component wants to check a token to see whether a privilege is present, it uses the PrivilegeCheck or LsaEnumerateAccountRights APIs if running in user mode and SeSinglePrivilegeCheck or SePrivilegeCheck if running in kernel mode. The privilege-related APIs are not account-right aware, but the account-right APIs are privilege-aware.

Unlike account rights, privileges can be enabled and disabled. For a privilege check to succeed, the privilege must be in the specified token and it must be enabled. The idea behind this scheme is that privileges should be enabled only when their use is required so that a process cannot inadvertently perform a privileged security operation.

Table 6-9 Privileges

Super Privileges

Several privileges are so powerful that a user to which they are assigned is effectively a “super user” who has full control over a computer. These privileges can be used in an infinite number of ways to gain unauthorized access to otherwise off-limit resources and to perform unauthorized operations. However, we’ll focus on using the privilege to execute code that grants the user privileges not assigned to the user, with the knowledge that this capability can be leveraged to perform any operation on the local machine that the user desires.

This section lists the privileges and discusses the ways that they can be exploited. Other privileges, such as Lock Pages In Physical Memory, can be exploited for denial-of-service attacks on a system, but these are not discussed. Note that on systems with UAC enabled, these privileges will be granted only to applications running at high integrity level or higher, even if the account possesses them:

• Debug programs A user with this privilege can open any process on the system (except for a Protected Process) without regard to the security descriptor present on the process. The user could implement a program that opens the LSASS process, for example, copy executable code into its address space, and then inject a thread with the CreateRemoteThread Windows API to execute the injected code in a more-privileged security context. The code could grant the user additional privileges and group memberships.

• Take ownership This privilege allows a holder to take ownership of any securable object (even protected processes and threads) by writing his own SID into the owner field of the object’s security descriptor. Recall that an owner is always granted permission to read and modify the DACL of the security descriptor, so a process with this privilege could modify the DACL to grant itself full access to the object and then close and reopen the object with full access. This would allow the owner to see sensitive data and to even replace system files that execute as part of normal system operation, such as LSASS, with his own programs that grant a user elevated privileges.

• Restore files and directories A user assigned this privilege can replace any file on the system with her own. She could exploit this power by replacing system files as described in the preceding paragraph.

• Load and unload device drivers A malicious user could use this privilege to load a device driver into the system. Device drivers are considered trusted parts of the operating system that can execute within it with System account credentials, so a driver could launch privileged programs that assign the user other rights.

• Create a token object This privilege can be used in the obvious way to generate tokens that represent arbitrary user accounts with arbitrary group membership and privilege assignment.

• Act as part of operating system LsaRegisterLogonProcess, the function a process calls to establish a trusted connection to LSASS, checks for this privilege. A malicious user with this privilege can establish a trusted-LSASS connection and then execute LsaLogonUser, a function used to create new logon sessions. LsaLogonUser requires a valid user name and password and accepts an optional list of SIDs that it adds to the initial token created for a new logon session. The user could therefore use her own user name and password to create a new logon session that includes the SIDs of more privileged groups or users in the resulting token.

Note that the use of an elevated privilege does not extend past the machine boundary to the network, because any interaction with another computer requires authentication with a domain controller and validation of domain passwords. Domain passwords are not stored on a computer either in plain text or encrypted form, so they are not accessible to malicious code.