Microsoft Knowledge Base Article 904766 contains the following introduction:

This article contains information about how to modify the RequireAsChecksum registry entry. This registry entry helps provide additional protection after you install Microsoft security update 899587. This security update is documented in Security Bulletin MS05-042.

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

899587 MS05-042: Vulnerabilities in Kerberos could allow denial of service, information disclosure, and spoofing