Maximum tolerance for computer clock synchronization

Applies To: Windows Server 2008, Windows Server 2008 R2

This security policy reference topic for the IT professional describes the best practices, location, values, and security considerations for this policy setting.

Reference

This security setting determines the maximum time difference (in minutes) that Kerberos V5 tolerates between the time on the client clock and the time on the domain controller that provides Kerberos authentication.

To prevent "replay attacks," the Kerberos v5 protocol uses time stamps as part of its protocol definition. For time stamps to work properly, the clocks of the client and the domain controller need to be in sync as much as possible. In other words, both computers must be set to the same time and date. Because the clocks of two computers are often out of sync, administrators can use this policy setting to establish the maximum acceptable difference to the Kerberos protocol between a client clock and domain controller clock. If the difference between a client computer clock and the domain controller clock is less than the maximum time difference that is specified in this policy, any time stamp that is used in a session between the two computers is considered to be authentic.

The possible values for this Group Policy setting are:

• A user-defined number of minutes from 1 through 99,999

• Not defined

To prevent replay attacks, the Kerberos protocol uses time stamps as part of its definition. For time stamps to work properly, the clocks of the client computer and the domain controller need to be synchronized as closely as possible. Because the clocks of two computers are often out of sync, administrators can use this policy setting to establish the maximum acceptable difference to the Kerberos protocol between a client computer clock and domain controller clock. If the difference between a client computer clock and the domain controller clock is less than Maximum tolerance for computer clock synchronization, any time stamp that is used in a session between the two computers is considered authentic.

Best practices

1. It is advisable to set Maximum tolerance for computer clock synchronization to a value of 5 minutes.

Location

GPO_name\Computer Configuration\Windows Settings\Security Settings\Account Policies\Kerberos Policy\

Default values

Operating system version differences

There are no differences in this policy in Windows operating systems beginning with Windows Server 2008.

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

To prevent "replay attacks" (which are attacks in which an authentication credential is resubmitted by a malicious user or program to gain access to a protected resource), the Kerberos protocol uses time stamps as part of its definition. For time stamps to work properly, the clocks of the client computer and the domain controller need to be closely synchronized. Because the clocks of two computers are often not synchronized, administrators can use this policy to establish the maximum acceptable difference to the Kerberos protocol between a client computer clock and a domain controller clock. If the difference between the client computer clock and the domain controller clock is less than the maximum time difference specified in this setting, any time stamp that is used in a session between the two computers is considered to be authentic.

Countermeasure

Configure the Maximum tolerance for computer clock synchronization setting to 5 minutes.

Potential impact

None. This is the default configuration.

Additional references

The following links provide additional information about topics that relate to securing domain controllers:

• Modify Default Security Policies on Windows Server 2008-Based Domain Controllers

• Best Practice Guide for Securing Windows Server Active Directory Installations