Windows Server 2003 Service Pack 1 modifies NTLM network authentication behavior

Microsoft Windows Server 2003 Service Pack 1 (SP1) modifies NTLM network authentication behavior. After you install Windows Server 2003 SP1, domain users can use their old password to access the network for one hour after the password is changed. Existing components that are designed to use Kerberos for authentication are not affected by this change.

To reliably support network access for NTLM network authentication in distributed environments, Windows Server 2003 SP1 modifies the NTLM network authentication behavior as follows:

• After a domain user successfully changes a password by using NTLM, the old password can still be used for network access for a user-definable time period. This behavior allows accounts, such as service accounts, that are logged on to multiple computers to access the network while the password change propagates.
• The extension of the password lifetime period applies only to network access by using NTLM. Interactive logon behavior is unchanged. This behavior does not apply to accounts that are hosted on stand-alone servers or on member servers. Only domain users are affected by this behavior.
• The lifetime period of the old password can be configured by editing the registry on a domain controller. No restart is required for this registry change to take effect.

How to change the lifetime period of an old password

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

To change the lifetime period of an old password, add a DWORD entry that is named OldPasswordAllowedPeriod to the following registry subkey on a domain controller: To do this, follow these steps:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type OldPasswordAllowedPeriod as the name of the DWORD, and then press ENTER.
5. Right-click OldPasswordAllowedPeriod, and then click Modify.
6. In the Value data box, type the value in minutes that you want to use, and then click OK.
   
   Note The lifetime period is set in minutes. If this registry value is not set, the default lifetime period for an old password is 60 minutes.
7. Quit Registry Editor.
   
   Note This registry setting does not require a restart to take effect.

Note This behavior does not cause a security weakness. As long as only one user knows both passwords, the user is still securely authenticated by using either password.

If a user's password is known to be compromised, the administrator should reset the password for that user. The administrator should ask the user to change the password at the next logon to invalidate the old password as soon as possible.

To reset a user's password, follow these steps:

1. Start Active Directory Users and Computers.
2. Locate the user account whose password must be reset.
3. Right-click the user object, and then click Reset Password.
4. Type the new password in the New password box and in the Confirm password box.
5. Click to select the User must change password at next logon check box, and then click OK.

Note The behavior that is described in this article occurs only if the effective password policy on the domain controllers has Enforce Password History set to a value that specifies that two or more passwords will be remembered. The password policy should be set at the domain level. You can determine whether the policy has taken effect on the domain controllers by using the Secpol.msc snap-in.