Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1 | Privilege separation, or privsep, is method in OpenSSH by which |
| 2 | operations that require root privilege are performed by a separate |
| 3 | privileged monitor process. Its purpose is to prevent privilege |
| 4 | escalation by containing corruption to an unprivileged process. |
| 5 | More information is available at: |
| 6 | http://www.citi.umich.edu/u/provos/ssh/privsep.html |
| 7 | |
| 8 | Privilege separation is now enabled by default; see the |
| 9 | UsePrivilegeSeparation option in sshd_config(5). |
| 10 | |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 11 | When privsep is enabled, during the pre-authentication phase sshd will |
| 12 | chroot(2) to "/var/empty" and change its privileges to the "sshd" user |
| 13 | and its primary group. sshd is a pseudo-account that should not be |
| 14 | used by other daemons, and must be locked and should contain a |
| 15 | "nologin" or invalid shell. |
| 16 | |
| 17 | You should do something like the following to prepare the privsep |
| 18 | preauth environment: |
| 19 | |
| 20 | # mkdir /var/empty |
| 21 | # chown root:sys /var/empty |
| 22 | # chmod 755 /var/empty |
| 23 | # groupadd sshd |
| 24 | # useradd -g sshd -c 'sshd privsep' -d /var/empty -s /bin/false sshd |
| 25 | |
| 26 | /var/empty should not contain any files. |
| 27 | |
| 28 | configure supports the following options to change the default |
| 29 | privsep user and chroot directory: |
| 30 | |
| 31 | --with-privsep-path=xxx Path for privilege separation chroot |
| 32 | --with-privsep-user=user Specify non-privileged user for privilege separation |
| 33 | |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 34 | PAM-enabled OpenSSH is known to function with privsep on AIX, FreeBSD, |
| 35 | HP-UX (including Trusted Mode), Linux, NetBSD and Solaris. |
| 36 | |
| 37 | On Cygwin, Tru64 Unix, OpenServer, and Unicos only the pre-authentication |
| 38 | part of privsep is supported. Post-authentication privsep is disabled |
| 39 | automatically (so you won't see the additional process mentioned below). |
| 40 | |
| 41 | Note that for a normal interactive login with a shell, enabling privsep |
| 42 | will require 1 additional process per login session. |
| 43 | |
| 44 | Given the following process listing (from HP-UX): |
| 45 | |
| 46 | UID PID PPID C STIME TTY TIME COMMAND |
| 47 | root 1005 1 0 10:45:17 ? 0:08 /opt/openssh/sbin/sshd -u0 |
| 48 | root 6917 1005 0 15:19:16 ? 0:00 sshd: stevesk [priv] |
| 49 | stevesk 6919 6917 0 15:19:17 ? 0:03 sshd: stevesk@2 |
| 50 | stevesk 6921 6919 0 15:19:17 pts/2 0:00 -bash |
| 51 | |
| 52 | process 1005 is the sshd process listening for new connections. |
| 53 | process 6917 is the privileged monitor process, 6919 is the user owned |
| 54 | sshd process and 6921 is the shell process. |