A general-purpose operating system for parallel systems
must satisfy two capabilities that contradict each other:
realizing protected and time-shared execution environment,
and providing efficient parallel-execution environment.
In parallel executions with a general-purpose operating system,
performance of the protected and virtualized 
user-level 
communications/synchronizations is the most crucial issue.
We proposed a novel high-speed user-level communications/synchronizations
scheme ``Memory-Based Communication Facilities (MBCF)'' 
suitable for the general-purpose parallel operating system
with off-the-shelf communication-hardware. 
For achieving high-performance, MBCF adopts 
the direct remote-accesses to destination user-level memory-space
without address checks. 
In this paper, we discuss aspects of
protection and security on MBCF.  We conclude MBCF is qualified
for not only parallel processing but also server-client
distributed-computations which require strict protection and security.