diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 3ceb5e717c4825..187143d02cd7bf 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -2917,6 +2917,16 @@ between themselves.
 
 Suitable authentication keys can also be generated by using :func:`os.urandom`.
 
+This authentication protects :class:`Listener` and :func:`Client` connections,
+which are reachable by address.  It is not applied to the anonymous pipes
+created by :func:`~multiprocessing.Pipe` or used internally by
+:class:`~multiprocessing.Queue`.
+:mod:`multiprocessing` treats all local processes running as the same user as
+trusted; on most operating systems such processes can access each other's pipe
+file descriptors regardless.  Applications that require isolation between
+processes of the same user must arrange it at the operating-system level --
+for example, by running workers under a different user account or in a sandbox.
+
 
 Logging
 ^^^^^^^