2002-06-27 22:31:32 +00:00
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Privilege separation, or privsep, is method in OpenSSH by which
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operations that require root privilege are performed by a separate
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privileged monitor process. Its purpose is to prevent privilege
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2004-02-26 10:38:49 +00:00
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escalation by containing corruption to an unprivileged process.
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2002-06-27 22:31:32 +00:00
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More information is available at:
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http://www.citi.umich.edu/u/provos/ssh/privsep.html
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Privilege separation is now enabled by default; see the
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UsePrivilegeSeparation option in sshd_config(5).
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2002-06-29 11:34:13 +00:00
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When privsep is enabled, during the pre-authentication phase sshd will
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2002-06-27 22:31:32 +00:00
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chroot(2) to "/var/empty" and change its privileges to the "sshd" user
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2002-06-29 11:34:13 +00:00
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and its primary group. sshd is a pseudo-account that should not be
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used by other daemons, and must be locked and should contain a
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"nologin" or invalid shell.
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You should do something like the following to prepare the privsep
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preauth environment:
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2002-06-27 22:31:32 +00:00
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# mkdir /var/empty
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# chown root:sys /var/empty
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# chmod 755 /var/empty
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# groupadd sshd
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2002-06-29 11:34:13 +00:00
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# useradd -g sshd -c 'sshd privsep' -d /var/empty -s /bin/false sshd
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2002-06-27 22:31:32 +00:00
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/var/empty should not contain any files.
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configure supports the following options to change the default
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privsep user and chroot directory:
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--with-privsep-path=xxx Path for privilege separation chroot
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--with-privsep-user=user Specify non-privileged user for privilege separation
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2005-09-03 06:59:33 +00:00
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PAM-enabled OpenSSH is known to function with privsep on AIX, FreeBSD,
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HP-UX (including Trusted Mode), Linux, NetBSD and Solaris.
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2002-06-27 22:31:32 +00:00
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2018-05-06 12:27:04 +00:00
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On Cygwin, Tru64 Unix and OpenServer only the pre-authentication part
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of privsep is supported. Post-authentication privsep is disabled
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2004-10-28 16:03:53 +00:00
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automatically (so you won't see the additional process mentioned below).
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2003-04-23 16:53:02 +00:00
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2002-06-27 22:31:32 +00:00
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Note that for a normal interactive login with a shell, enabling privsep
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will require 1 additional process per login session.
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Given the following process listing (from HP-UX):
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UID PID PPID C STIME TTY TIME COMMAND
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root 1005 1 0 10:45:17 ? 0:08 /opt/openssh/sbin/sshd -u0
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root 6917 1005 0 15:19:16 ? 0:00 sshd: stevesk [priv]
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stevesk 6919 6917 0 15:19:17 ? 0:03 sshd: stevesk@2
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stevesk 6921 6919 0 15:19:17 pts/2 0:00 -bash
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process 1005 is the sshd process listening for new connections.
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process 6917 is the privileged monitor process, 6919 is the user owned
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sshd process and 6921 is the shell process.
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