Act as part of the operating system
Computer Configuration\Windows Settings\Security Settings\Local Policies\User Rights Assignment
This policy allows a process to authenticate as any user, and therefore gain access to the same resources as any user. Only low-level authentication services should require this privilege.
The potential access is not limited to what is associated with the user by default, because the calling process may request that arbitrary additional accesses be put in the access token. Of even more concern is that the calling process can build an anonymous token that can provide any and all accesses. Additionally, the anonymous token does not provide a primary identity for tracking events in the audit log.
Processes that require this privilege should use the LocalSystem account, which already includes this privilege, rather than using a separate user account with this privilege specially assigned.
By default, only the LocalSystem account has the privilege to act as part of the operating system.