Details for this vulnerability have not been published by NIST at this point. Descriptions from software vendor advisories for this issue are provided below.
Due to a race condition in the jail_remove(2) implementation, it
may fail to kill some of the processes.
A process running inside a jail can avoid being killed during jail
termination. If a jail is subsequently started with the same root
path, a lingering jailed process may be able to exploit the window
during which a devfs filesystem is mounted but the jail's devfs
ruleset has not been applied, to access device nodes which are
ordinarily inaccessible. If the process is privileged, it may be able
to escape the jail and gain full access to the system.