|Platform: win2016||Date: (C)2017-08-04 (M)2018-03-15|
"Account lockout threshold"
This policy setting determines the number of failed logon attempts before a lock occurs. Authorized users can lock themselves out of an account by mistyping their password or by remembering it incorrectly, or by changing their password on one computer while logged on to another computer. The computer with the incorrect password will continuously try to authenticate the user, and because the password it uses to authenticate is incorrect, a lock occurs. To avoid accidental lockout of authorized users, set the account lockout threshold to a high number. The default value for this policy setting is 0 invalid logon attempts, which disables the account lockout feature.
Because it is possible for an attacker to use this lockout state as a denial of service (DoS) by triggering a lockout on a large number of accounts, your organization should determine whether to use this policy setting based on identified threats and the risks you want to mitigate. There are two options to consider for this policy setting.
- Configure the value for Account lockout threshold to 0 to ensure that accounts will not be locked out. This setting value will prevent a DoS attack that attempts to lock out accounts in your organization. It will also reduce help desk calls, because users will not be able to lock themselves out of their accounts accidentally. However, this setting value will not prevent a brute force attack. The following defenses should also be considered:
- A password policy that forces all users to have complex passwords made up of 8 or more characters.
- A robust auditing mechanism, which will alert administrators when a series of account lockouts occurs in the environment. For example, the auditing solution should monitor for security event 539, which is a logon failure. This event identifies that there was a lock on the account at the time of the logon attempt.
The second option is:
- Configure the value for Account lockout threshold to a value that provides users with the ability to mistype their password several times, but locks out the account if a brute force password attack occurs. This configuration will prevent accidental account lockouts and reduce help desk calls, but will not prevent a DoS attack.
Password attacks can use automated methods to try millions of password combinations for any user account. The effectiveness of such attacks can be almost eliminated if you limit the number of failed logons that can be performed.
However, a DoS attack could be performed on a domain that has an account lockout threshold configured. An attacker could programmatically attempt a series of password attacks against all users in the organization. If the number of attempts is greater than the account lockout threshold, the attacker might be able to lock out every account.
Because vulnerabilities can exist when this value is configured as well as when it is not configured, two distinct countermeasures are defined. Any organization should weigh the choice between the two based on their identified threats and the risks that they want to mitigate. The two countermeasure options are:
- Configure the Account Lockout Threshold setting to 0. This configuration ensures that accounts will not be locked out, and will prevent a DoS attack that intentionally attempts to lock out accounts. This configuration also helps reduce help desk calls because users cannot accidentally lock themselves out of their accounts. Because it will not prevent a brute force attack, this configuration should only be chosen if both of the following criteria are explicitly met:
- The password policy requires all users to have complex passwords of 8 or more characters.
- A robust audit mechanism is in place to alert administrators when a series of failed logons occur in the environment.
- Configure the Account Lockout Threshold setting to a sufficiently high value to provide users with the ability to accidentally mistype their password several times before the account is locked, but ensure that a brute force password attack will still lock the account.
If this policy setting is enabled, a locked-out account will not be usable until it is reset by an administrator or until the account lockout duration expires. This setting will likely generate a number of additional help desk calls. In fact, locked accounts cause the greatest number of calls to the help desk in many organizations.
If you enforce this setting an attacker could cause a denial of service condition by deliberately generating failed logons for multiple user, therefore you should also configure the Account Lockout Duration to a relatively low value such as 15 minutes.
If you configure the Account Lockout Threshold to 0, there is a possibility that an attacker's attempt to discover passwords with a brute force password attack might go undetected if a robust audit mechanism is not in place.
(1) GPO: Computer Configuration\Windows Settings\Security Settings\Account Policies\Account Lockout Policy
(2) REG: No Registry Info