New resolution for problems with Kerberos authentication when users belong to many groups

When a user belongs to many groups, that user may have problems with authentication or with Group Policy settings. The following Microsoft Knowledge Base articles describe these symptoms in more detail:

The existing resolution that is described in these articles instructs you to modify the MaxTokenSize registry value. An improvement has been made to this resolution. If you use the hotfix that is described in this article, you may not have to edit the default MaxTokenSize value.

The hotfix that is described in this article supersedes the hotfixes that are described in Microsoft Knowledge Base articles that are listed in this section.

The user is not able to authenticate because the Kerberos token that is generated during authentication attempts has a fixed maximum size. Transports such as remote procedure call (RPC) and HTTP rely on the MaxTokenSize value when they allocate buffers for authentication. In Windows 2000 (the original released version), the MaxTokenSize value is 8,000 bytes. In Windows 2000 Service Pack 2 (SP2) and Microsoft Windows Server 2003, the MaxTokenSize value is 12,000 bytes.

If a user is a member of more than 120 groups, the buffer that is determined by the MaxTokenSize value is not large enough. As a result, users cannot authenticate, and they may receive an "out of memory" error message. Before you apply the hotfix that is described in this article, every group that is added to a user account increases this buffer by 40 bytes.

NOTE: In many scenarios, Windows NTLM authentication works as expected; you may not see the Kerberos authentication problem without analysis. However, scenarios in which Group Policy settings are applied may not work as expected.

Note Windows 7, Windows Server 2008 R2, Windows Server 2003, Windows Vista, Windows Server 2008, and Windows XP Professional include a fix for this problem.

Service pack information

To resolve this problem, obtain the latest service pack for Microsoft Windows 2000. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

Hotfix information

A supported hotfix is now available from Microsoft. However, it is intended to correct only the problem that is described in this article. Apply it only to systems that are experiencing this specific problem. This hotfix may receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next Windows 2000 service pack that contains this hotfix.

To resolve this problem immediately, contact Microsoft Customer Support Services to obtain the hotfix. For a complete list of Microsoft Customer Support Services telephone numbers and information about support costs, visit the following Microsoft Web site:Note In special cases, charges that are ordinarily incurred for support calls may be canceled if a Microsoft Support Professional determines that a specific update will resolve your problem. The usual support costs will apply to additional support questions and issues that do not qualify for the specific update in question. The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section. This problem was first corrected in Microsoft Windows 2000 Service Pack 4.

Previously, if users experienced this problem, you had to adjust the Kerberos MaxTokenSize value to resume operations. To resolve this problem, you had to update this value on all domain workstations.

If you use the hotfix that is described in this article, you do not have to modify the MaxTokenSize registry value in most cases. However, there are some scenarios in which you have to modify the MaxTokenSize registry value after you apply this hotfix. After you apply this hotfix to all the domain controllers, use the following formula to determine whether you have to modify the MaxTokenSize value: This formula uses the following values:

• d: The number of domain local groups a user is a member of plus the number of universal groups outside the user's account domain plus the number of groups represented in security ID (SID) history.
• s: The number of security global groups that a user is a member of plus the number of universal groups in a user's account domain.
• 1200: The estimated value for ticket overhead. This value can vary depending on factors such as DNS domain name length, client name, and other factors.

In scenarios in which delegation is used (for example, when users authenticate to a domain controller), Microsoft recommends that you double the token size.

If the token size that you calculate by using this formula is less than 12,000 bytes (the default size), you do not have to modify the MaxTokenSize registry value on domain clients. If the value is more than 12,000 bytes, see the following Microsoft Knowledge Base article for a description of how to adjust the MaxTokenSize registry value:

For more information about how to obtain a hotfix for Windows 2000 Datacenter Server, click the following article number to view the article in the Microsoft Knowledge Base: Notes

• When you change MaxTokenSize, you must restart the computer so that the change is effective.
• The recommended maximum value is 65535 decimal or FFFF hexadecimal. The MaxTokenSize value specifies a fixed Kerberos ticket receive buffer that contains the SIDs that represent the groups in which the account is a member. Later, the Local Security Authority (LSA) service generates the token from this SID buffer. The hard-coded limit of customer definable SIDs for this token is 1,015. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
  328889  (http://support.microsoft.com/kb/328889/ ) Users who are members of more than 1,015 groups may fail logon authentication

Therefore, a MaxTokenSize value for more than 1015 effective SIDs is not useful. In the following formula: 40d means that you must have 40 bytes for a Domain Local Group SID. 8s means 8 bytes for a Domain Global/Universal Group SID. Therefore, if you have a MaxTokenSize value of 0x0000FFFF (64K), you may be able to buffer approximately 1600 Domain Local Group SIDs or approximately 8000 Domain Global/Universal Group SIDs. If you use "trusted for delegation" accounts, the buffer requirement for each SID may be doubled. In these scenarios, you can only store approximately 800 Domain Local Group SIDs when a MaxTokenSize value of 64K is used. However, having only Domain Local Group SIDs is not a realistic scenario. A value of 64K should be sufficient even for delegation scenarios. Additionally, applications may have trouble if a token size buffer contains more than 64K.

For more information, click the following article numbers to view the articles in the Microsoft Knowledge Base:
Because you may have cross-domain logon scenarios in your forest, the value should be set forest-wide on all Windows-based systems. Therefore, we recommend that the maximum value for the MaxTokenSize value be 64K.

On SQL Server clients, you may receive the following error message when this problem occurs: To resolve this problem, you must set the MaxTokenSize registry value for all of the computers that are involved in the Kerberos authentication process, including the SQL Server clients.