Password complexity policies are designed to deter brute force attacks by increasing the number of possible passwords. When password complexity policy is enforced, new passwords must meet the following guidelines:

• The password does not contain all or part of the account name of the user. Part of an account name is defined as three or more consecutive alphanumeric characters delimited on both ends by white space such as space, tab, and return, or any of the following characters: comma (,), period (.), hyphen (-), underscore (_), or number sign (#).
  
• The password is at least eight characters long.
  
• The password contains characters from three of the following four categories:
  
  • Latin uppercase letters (A through Z)
    
  • Latin lowercase letters (a through z)
    
  • Base 10 digits (0 through 9)
    
  • Non-alphanumeric characters such as: exclamation point (!), dollar sign ($), number sign (#), or percent (%).
    

Passwords can be up to 128 characters long. You should use passwords that are as long and complex as possible.

Password expiration policies are used to manage the lifespan of a password. When SQL Server enforces password expiration policy, users are reminded to change old passwords, and accounts that have expired passwords are disabled.

The enforcement of password policy can be configured separately for each SQL Server login. Use ALTER LOGIN (Transact-SQL) to configure the password policy options of a SQL Server login. The following rules apply to the configuration of password policy enforcement:

• When CHECK_POLICY is changed to ON, the following behaviors occur:
  
  • CHECK_EXPIRATION is also set to ON unless it is explicitly set to OFF.
    
  • The password history is initialized with the value of the current password hash.
    
• When CHECK_POLICY is changed to OFF, the following behaviors occur:
  
  • CHECK_EXPIRATION is also set to OFF.
    
  • The password history is cleared.
    
  • The value of lockout_time is reset.
    

Some combinations of policy options are not supported.

• If MUST_CHANGE is specified, CHECK_EXPIRATION and CHECK_POLICY must be set to ON. Otherwise, the statement will fail.
  
• If CHECK_POLICY is set to OFF, CHECK_EXPIRATION cannot be set to ON. An ALTER LOGIN statement that has this combination of options will fail.
  

  Important:
  CHECK_EXPIRATION and CHECK_POLICY are only enforced on Windows Server 2003 and later versions.

  Important:
  A known issue in Windows Server 2003 might prevent the bad password count from being reset after LockoutThreshold has been reached. This might cause an immediate lockout on subsequent failed login attempts. You can manually reset the bad password count by briefly setting CHECK_POLICY = OFF, followed by CHECK_POLICY = ON.

When SQL Server is running on Windows 2000, setting CHECK_POLICY = ON will prevent the creation of passwords that are:

• Null or empty
  
• Same as name of computer or login
  
• Any of the following: "password", "admin", "administrator", "sa", "sysadmin"
  

The security policy might be set in Windows, or might be received from the domain. To view the password policy on the computer, use the Local Security Policy MMC snap-in (secpol.msc).

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