Privilege separation, or privsep, is method in OpenSSH by which operations that require root privilege are performed by a separate privileged monitor process. Its purpose is to prevent privilege escalation by containing corruption to an unprivileged process. More information is available at: http://www.citi.umich.edu/u/provos/ssh/privsep.html Privilege separation is now enabled by default; see the UsePrivilegeSeparation option in sshd_config(5). On systems which lack mmap or anonymous (MAP_ANON) memory mapping, compression must be disabled in order for privilege separation to function. When privsep is enabled, during the pre-authentication phase sshd will chroot(2) to "/var/empty" and change its privileges to the "sshd" user and its primary group. You should do something like the following to prepare the privsep preauth environment: # mkdir /var/empty # chown root:sys /var/empty # chmod 755 /var/empty # groupadd sshd # useradd -g sshd -c 'sshd privsep' -d /var/empty sshd If you are on UnixWare 7 or OpenUNIX 8 do this additional step. # ln /usr/lib/.ns.so /usr/lib/ns.so.1 /var/empty should not contain any files. configure supports the following options to change the default privsep user and chroot directory: --with-privsep-path=xxx Path for privilege separation chroot --with-privsep-user=user Specify non-privileged user for privilege separation Privsep requires operating system support for file descriptor passing and mmap(MAP_ANON). PAM-enabled OpenSSH is known to function with privsep on Linux. It does not function on HP-UX with a trusted system configuration. PAMAuthenticationViaKbdInt does not function with privsep. Note that for a normal interactive login with a shell, enabling privsep will require 1 additional process per login session. Given the following process listing (from HP-UX): UID PID PPID C STIME TTY TIME COMMAND root 1005 1 0 10:45:17 ? 0:08 /opt/openssh/sbin/sshd -u0 root 6917 1005 0 15:19:16 ? 0:00 sshd: stevesk [priv] stevesk 6919 6917 0 15:19:17 ? 0:03 sshd: stevesk@2 stevesk 6921 6919 0 15:19:17 pts/2 0:00 -bash process 1005 is the sshd process listening for new connections. process 6917 is the privileged monitor process, 6919 is the user owned sshd process and 6921 is the shell process. $Id: README.privsep,v 1.8 2002/06/24 16:49:22 stevesk Exp $