Machine Account Password Change Process in AD

By | June 7, 2020

Many of us have questions about how often does the machine account password change in AD on various Windows operating systems

So, by default, the machine account password change is initiated by the computer every 30 days. Since Windows Server 2000, all Windows versions have the same value. Admins are allowed to modify his behaviour using the following GPO setting in AD.

Domain member: Maximum machine account password age
Setting Path:

Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options

What happens if a workstation does not change its password, will it not be allowed to log onto the network?        

Machine account password does not expire in Active Directory. They are exempted from the domain’s password policy. It is important to remember that machine account password changes are driven by the client machines, and not the AD.

As long as no one has disabled or deleted the computer account, nor tried to add a computer with the same name to the domain, the computer will continue to work no matter how long it has been since its machine account password was initiated and changed.

So, if a computer is turned off for three months nothing expires. When the computer starts up, it will notice that its password is older than 30 days and will initiate action to change it.

The Netlogon service on the client computer is responsible for doing this. This is only applicable if the machine is turned off for such a long time.

Before we set the new password locally, we ensure we have a valid secure channel to the DC. If the client was never able to connect to the DC (where never is anything prior the time of the attempt – time to refresh the secure channel), then we will not change the password locally.

The relevant Netlogon parameters that come into play and we can think about changing here are:

ScavengeInterval (default 15 minutes),
MaximumPasswordAge (default 30 days)
DisablePasswordChange (default off).

DisablePasswordChange would prevent the client computer from changing its computer account password.

Key = HKLM\SYSTEM\CurrentControlSet\Services\NetLogon\Parameters
Value = DisablePasswordChange REG_DWORD
Default = 0

Group policy setting:
Computer Configuration\windows Settings\Security settings\Local Policies\Security Options
Domain member: Disable machine account Password changes    

Warning: If you disable machine account password changes, there are security risks because the security channel is used for pass-through authentication. If someone discovers a password, he or she can potentially perform pass-through authentication to the domain controller.

ScavengeInterval controls how often the workstation scavenger thread runs – the workstation scavenger is responsible for changing the machine password if necessary:

Value: ScavengeInterval REG_DWORD 60 to 172800 Seconds (48 hours)
Default : 900 (15 minutes)

MaximumPasswordAge determines when the computer password needs to be changed.

Key = HKLM\SYSTEM\CurrentControlSet\Services\NetLogon\Parameters
Value = MaximumPasswordAge REG_DWORD
Default = 30
Range = 1 to 1,000,000 (in days)

Group policy setting:
Computer Configuration\windows Settings\Security settings\Local Policies\Security Options

Domain member: Maximum machine account Password age

To clear things up, it is 7 days on Windows NT by default, and 30 days on Windows 2000 and up. The trust password follows the same setting. So, Trust between two NT 4 domains is 7 days. Trusts between Windows 2000 and up and anything else is 30 days. So, what this means is if:

  • 2000 and NT4 trust password is 30 days
  • 2000 to 2000 is 30 days
  • 2000 to 2003 is 30 days
  • 2003 to 2003 is 30 days

After the Netlogon service starts, the Workstation service scavenger thread wakes up. If the password is not older than MaximumPasswordAge, the scavenger thread goes back to sleep and sets itself to wake up when the password will reach that age. Otherwise, the scavenger thread will attempt to change the password. If it cannot talk to a DC, it will go back to sleep and try again in ScavengeInterval minutes.    

The ScavengeInterval setting can be modified to a custom value using the group policy setting in Active Directory.

Group policy setting:
Computer Configuration\Administrative Templates\System\Netlogon\Scavenge Interval

How do computers use passwords?

Each Windows-based computer maintains a machine account password history containing the current and previous passwords used for the account. When two computers attempt to authenticate with each other and a change to the current password is not yet received, Windows then relies on the previous password. If the sequence of password changes exceeds two changes, the computers involved may be unable to communicate, and you may receive error messages.

When a client determines that the machine account password needs to be changed, it would try to contact a domain controller for the domain of which it is a member of to change the password on the domain controller. If this operation succeeds, then it would update machine account password locally.

The client first changes the password locally and then attempts to update it in Active Directory. If the domain controller is configured with security policy “Domain Controller: Refuse machine account password changes” (i.e. RefusePasswordChange), then the client rolls back locally to the previous password. If the password change fails, however, the client keeps the new password locally and keeps trying to set it on the scavenge interval until it succeeds.

The local copy of the machine password is stored under:


We store the current password and the previous password under CurrVal & OldVal Keys respectively. In Active Directory, we store the password in unicodepwd and lmpwdHistory. We also store the timestamp in the pwdlastset attribute (the method to convert it into readable format is:

  • Convert the value in the attribute from decimal to hex (using calc.exe)
  • Split the result into two equal parts (8 bits for each part)
  • Run nltest /time: rightsidehex leftsidehex

The resultant value is the date and time the password was set on this computer object in AD. If you use System Restore after the password change interval expired one time, and you restore the computer to a point before the password changes, the next password change may not occur when it is due. Instead, the operating system treats the restore as if the password was changed.

Now consider the scenario, when a machine is not connected to the network for a long period. Supposing on the client:

  • Old password = null
  • Current password = A
  • New random password = B

And on the machine account in AD:

  • unicodePWD = A

After 30 days when the Scavenger thread runs, the value would be:

  • Old Password = A
  • Current Password = B

At 60th day the same process happens again. So now the newly generated password is C and the values are:

  • Old password = B
  • Current Password = C

Now when the client connects to Active Directory, it will try the current password to authenticate. When that fails with error. Otherwise machine should be able to reset its password once it boots even after say 90 days.

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