The product does not drop privileges before passing control of a resource to an actor that does not have those privileges.
In some contexts, a system executing with elevated permissions will hand off a process/file/etc. to another process or user. If the privileges of an entity are not reduced, then elevated privileges are spread throughout a system and possibly to an attacker.
Compartmentalize the system to have "safe" areas where trust boundaries can be unambiguously drawn. Do not allow sensitive data to go outside of the trust boundary and always be careful when interfacing with a compartment outside of the safe area. Ensure that appropriate compartmentalization is built into the system design, and the compartmentalization allows for and reinforces privilege separation functionality. Architects and designers should rely on the principle of least privilege to decide the appropriate time to use privileges and the time to drop privileges.
Very carefully manage the setting, management, and handling of privileges. Explicitly manage trust zones in the software.
Consider following the principle of separation of privilege. Require multiple conditions to be met before permitting access to a system resource.
If privileges are not dropped, neither are access rights of the user. Often these rights can be prevented from being dropped.
If privileges are not dropped, in some cases the system may record actions as the user which is being impersonated rather than the impersonator.