Network security: Do not store LAN Manager hash value on next password change

Updated: November 15, 2012

Applies To: Windows 7, Windows Server 2003, Windows Server 2008, Windows Server 2008 R2, Windows Vista, Windows XP

This security policy reference topic for the IT professional describes the best practices, location, values, policy management and security considerations for this policy setting. This information applies to computers running at least the Windows Server 2008 operating system.

Reference

This policy setting determines whether LAN Manager is prevented from storing hash values for the new password the next time the password is changed. Hash values are a representation of the password after the encryption algorithm is applied that corresponds to the format that is specified by the algorithm. To decrypt the hash value, the encryption algorithm must be determined and then reversed. The LAN Manager hash is relatively weak and prone to attack compared to the cryptographically stronger NTLM hash. Because the LM hash is stored on the local computer in the security database, the passwords can be compromised if the security database, Security Accounts Manager (SAM), is attacked.

By attacking the SAM file, attackers can potentially gain access to user names and password hashes. Attackers can use a password-cracking tool to determine what the password is. After they have access to this information, they can use it to gain access to resources on your network by impersonating users. Enabling this policy setting will not prevent these types of attacks, but it will make them much more difficult.

Possible values

• Enabled

• Disabled

• Not defined

Best practices

1. Set Network security: Do not store LAN Manager hash value on next password change to Enabled.

2. Require all users to set new passwords the next time they log on to the domain so that LAN Manager hashes are removed. Legacy operating systems (such as Windows 95, Windows 98, and Windows Millennium Edition) will fail, as will some non-Microsoft applications.

Location

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

Default values

The following table lists the actual and effective default values for this policy. Default values are also listed on the policy’s property page.

Operating system version differences

On Windows operating systems before Windows Server 2008 and Windows Vista, the default for this policy was Disabled.

Policy management

This section describes features and tools that are available to help you manage this policy.

Restart requirement

None. Changes to this policy become effective without a computer restart when they are saved locally or distributed through Group Policy.

Security considerations

This section describes how an attacker might exploit a feature or its configuration, how to implement the countermeasure, and the possible negative consequences of countermeasure implementation.

Vulnerability

The SAM file can be targeted by attackers who seek access to user names and password hashes. Such attacks use special tools to discover passwords, which can then be used to impersonate users and gain access to resources on your network. These types of attacks are not prevented by enabling this policy setting because LAN Manager hashes are much weaker than NTLM hashes, but it is much more difficult for these attacks to succeed.

Countermeasure

Enable the Network security: Do not store LAN Manager hash value on next password change setting. Require all users to set new passwords the next time they log on to the domain so that LAN Manager hashes are removed.

Potential impact

Earlier operating systems such as Windows 95, Windows 98, and Windows Millennium Edition, as well as some non-Microsoft applications, cannot connect to the system.