The following text was copied from Security Overview: Permissions on 2006 May 8. Italics added.
A port has a set of associated port rights, which are owned by tasks. A port right specifies which task can use that port. Each port has one receive right, the owner of which can receive messages on that port. Each port also has one or more send rights; the owners of the send rights can send messages to the port. Rights can be transferred between tasks by being attached to a message.
A single task (or other Mach object, such as a thread or the computer itself) may have multiple ports that provide access to resources it supports. For example, a task might have a name port and a control port. Access to the control port allows the task to be manipulated. In contrast, access to the name port merely allows the client to obtain information about the task or perform other nonprivileged operations on it.
Each process has a port right namespace, which maps small integers known as port right names to their corresponding port rights. A port right name is meaningful only within that task’s port right namespace. A task can transfer a port right to another task by sending it the corresponding port right name. However, unless it sends the name correctly, the receiving task won’t be able use the right. The only way to transmit a port right between two tasks is by sending a Mach message and attaching the right name to that message using the correct syntax and message structure.
When you use Mach to create a task, Mach returns a port right name that references a send right for the port (the receive right for a task port is always owned by the kernel). You can send messages to this port to start and stop the task, kill the task, manipulate the task’s address space, and so forth. Therefore, whoever owns a send right for a task’s port effectively owns the task and can manipulate the task’s state without regard to BSD security policies or any higher-level security policies. In other words, an expert in Mach programming with local administrator access to a Mac OS X machine can bypass BSD and higher-level security features. Therefore, it is very important to use strong administrator passwords, keep them secure, and control physical security for any computer containing sensitive information.