Active Directory: Account Lockout Policy – Think Twice Before Applying

The Account Lockout Policy in Active Directory is not what it seems. Oh sure, at first glance it appears simple enough. Set a threshold, set a counter, and when that threshold is tripped in the allotted time, account locked out. What could be simpler right? Well, as with most things in IT, what should be simple isn’t. Maybe the account lockout policy was a “set it and forget it” exercise 10 years ago, but with more and more services relying on Active Directory credentials, it might be time to reconsider the trusty account lockout policy in favor of a better approach.

locks - photo by Steven Tom

The problem with the account lockout policy in many environments is that there are numerous things that can trip it. It’s not just a user manually trying to logon with the wrong password,  or a hacker attempting a brute force attack, there are also background services and automatic credentialing that can trip up your account lockout policy.

At my company, iPhones configured to receive Exchange email were the biggest problem. Once an iPhone user’s password expired, the iPhone would keep trying to log them in. With most normal account lockout policies set to 5 or 6 attempts, the iPhone WILL trip the policy. This gets even worse if a user has more than one mobile device. What happens if their iPad, also configured to receive Exchange email, was left home today? You are basically SOL. The only way to resolve that problem until the password can be changed on all mobile devices would be to put that user account in an “exceptions” policy that does not have account lockout configured.

Some additional examples of things that can trip an account lockout policy include:

  • Stale credentials for Windows Service accounts.
  • Stale credentials used to run Scheduled tasks.
  • Multiple Citrix XenApp or Remote Desktop Services sessions open when a user initiates a password change.
  • User logged into multiple computers when initiating a password change.
  • Disconnected Citrix XenApp or Remote Desktop Services sessions that are not configured to timeout.
  • Administrative Remote Desktop Connections to Windows servers left disconnected.
  • Applications with their own credential stores that authenticate against Active Directory with stale credentials.

And I’m sure this is just the tip of the iceberg.

WHY IMPLEMENT AN ACCOUNT LOCKOUT POLICY?

The biggest argument in favor of applying an Account Lockout Policy is to impede brute force attacks. For that reason, common account lockout settings like 5 or 6 failed tries until the account locks will likely lead to increased help desk calls instead of increased security. Since a brute force attack will result in many more failed logon attempts than a user will ever generate, it’s not unreasonable to set the “Account lockout threshold” to a fairly high number. Instead of the usual 5 or 6, why not go with 50, or 100? Even 500 would suffice for this purpose. (Keep in mind the highest value for this setting is 999.) Again, the purpose here is to thwart a brute force attempt, not increase your help desk call log. And the reason I think you should set the threshold to a high value is not because users will manually attempt to logon with a bad password 30 times, it’s because of the above mentioned background services that can trip up your lockout policy. You’d be surprised how high the “bad password count” can get.

ACCOUNT LOCKOUT POLICY SETTINGS IN DETAIL

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First, for those who are unfamiliar, the Account Lockout Policy can be found in any Group Policy Object in Active Directory. However, it’s usually best set in the Default Domain Policy. Open the GPO and navigate to Computer Configuration | Policies | Windows Settings | Security Settings | Account Policies | Account Lockout Policy.

Account lockout duration — This security setting determines the number of minutes a locked-out account remains locked out before automatically becoming unlocked. The available range is from 0 minutes through 99,999 minutes. If you set the account lockout duration to 0, the account will be locked out until an administrator explicitly unlocks it. If an account lockout threshold is defined, the account lockout duration must be greater than or equal to the reset time. Default: None, because this policy setting only has meaning when an Account lockout threshold is specified.

Once an account is locked out, the “Account lockout duration” is length of time the account will be locked out until resetting. If set to “0” the lockout status will not automatically reset and an administrator will need to unlock the account manually.

Account lockout threshold — This security setting determines the number of failed logon attempts that causes a user account to be locked out. A locked-out account cannot be used until it is reset by an administrator or until the lockout duration for the account has expired. You can set a value between 0 and 999 failed logon attempts. If you set the value to 0, the account will never be locked out. Failed password attempts against workstations or member servers that have been locked using either CTRL+ALT+DELETE or password-protected screen savers count as failed logon attempts. Default: 0.

The “Account lockout threshold” determines how many failed logon attempts will result in a locked account. It works in connection with the “Reset account lockout counter after” setting. If the value specified for “Account lockout threshold” is met before the counter resets, the account is locked out.

Reset account lockout counter after — This security setting determines the number of minutes that must elapse after a failed logon attempt before the failed logon attempt counter is reset to 0 bad logon attempts. The available range is 1 minute to 99,999 minutes. If an account lockout threshold is defined, this reset time must be less than or equal to the Account lockout duration. Default: None, because this policy setting only has meaning when an Account lockout threshold is specified.

This is the trickiest setting in the account lockout policy. One way to interpret this setting is that there is a window of time and in that window of time you can have (x) amount of failed logons before your account is locked out. However, that’s NOT how this setting works. This setting specifies the amount of time BETWEEN failed logon attempts. If you attempt to logon to an account with the wrong password, the “badPwdCount” attribute for your account is incremented by 1. This starts the clock. If the clock reaches the value specified in the “Reset account lockout counter after” setting, then the process starts from scratch. If another failed logon attempt is recorded before the “Reset account lockout counter after” setting is reached, then the “badPwdCount” attribute is incremented (again) by 1. If the “badPwdCount” attribute reaches the value specified for the “Account lockout threshold” the account will be locked.

Again, the important takeaway for this counter is that it’s counting BETWEEN failed logon attempts. And because of this, you should seriously consider setting this value to it’s lowest setting of 1 minute. That is assuming you ultimately decide to implement the account lockout policy. More on that in a moment.

EXAMPLE ACCOUNT LOCKOUT POLICY SCENARIO

Account lockout duration: 5 minutes
Account lockout threshold: 50 invalid logon attempts
Reset account lockout counter after: 1 minutes

Let’s walk through this. If a brute force attack against your Active Directory domain is underway, it will require 50 failed logon attempts without more than a minute between each failed logon attempt to lock an account. As you can see, that would thwart ANY effective brute force attack. Sure, the attack could be slowed down enough not to trip this example account lockout policy, but who would do that, and to what end? If you have a strong enough password policy in place, there’s no way a massively slowed down brute force attack would bear fruit for a potential hacker. There would be no reason to even try. And that assumes the hacker even knows what your account lockout policy is set to in the first place.

In this example I specified a “lockout duration” of 5 minutes. This is the length of time an account will be locked out once the lockout policy is tripped. The default is 30 minutes, and I think that’s way too high. If your lockout policy does trip for non-malicious reasons, why lockout the user for a half hour? There’s certainly no reason to lockout an account that long to impede a brute force attack. A setting of 5 minutes seems reasonable. Again, we aren’t trying to increase help desk calls with this policy.

So if your company demands you implement an account lockout policy, I would seriously consider the settings in the above example as a good starting point. With 50 invalid logon attempts required to lockout an account, the lockout policy is unlikely to be tripped unless an actual brute force attack is underway.

AN ARGUMENT AGAINST THE ACCOUNT LOCKOUT POLICY

Because it’s still possible to trip up an account lockout policy even with a threshold of 50 invalid logon attempts (trust me, I’ve experienced it), you might want to consider scrapping the idea altogether. That is assuming you can convince management. My suggestion is simple, make sure your password policy is strong. If you require strong passwords that must be changed often it reduces the need for an account lockout policy. If you still need some kind of lockout policy I would suggest looking into an edge solution. That is, something that sits between the outside world and your Active Directory domain. For the sake of this suggestion I’m assuming you aren’t worried about LAN-based brute force attacks. An example of an edge solution is the following write-up for the Citrix Netscaler product:

Preventing Brute Force logon Attacks to the Citrix NetScaler Access Gateway or AAA for TM logon Page

One of the more common requests I see is how to prevent brute force logon attacks to the Citrix Access Gateway or NetScaler AAA for Traffic Management logon pages. Like many other web applications that have a public facing HTML form used for logon, this is an assumed risk.

Part 1 of this article looks at how you can use the NetScaler HTTP Rate Limiting feature in conjunction with the Responder module to detect and respond to a potential brute force attack.

This is a good method in that it locks out the source public IP address after a specified number of failed logon attempts instead of an Active Directory account. This is a better way to implement a lockout policy on your network because it avoids the vulnerability of traditional account lockout policies to DoS attacks designed to trip the policy. And by the way, this is yet another reason to tell management that account lockout policies are old school and have worn out their welcome. But first you must sell them on the idea of strong passwords!

TIP: This PowerShell command will show you all locked out accounts.

Search-ADAccount –LockedOut

If you haven’t previously done so, you must run the below command to import the Active Directory module before the above command will work.

Import-Module activedirectory

photo by Steven Tom

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