3. Service Accounts with Lower Privileges
User accounts are the stepping stones for external intruders. If intruders can log on to a system through an account that has more power than it should have, they can steal or damage potentially sensitive data and they might even be able to use the account to progress to more sensitive areas that require greater authority to access. Some of the most important aspects of Microsoft's secure-by-default goal arrest common account vulnerabilities, including vulnerabilities associated with system services and service applications that need to run with the privileges of a specific user account.
Many system services and service applications need to run under a highly privileged account, so they run under the all-powerful Local System account or an account created with Administrator or Domain Admin authority. If a service contains a security vulnerability (e.g., a buffer overflow) that lets intruders manipulate the server, running the service under such a powerful account gives intruders almost unlimited power to cause damage. . . .
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Ben Meijer June 17, 2004