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65 lines
2.6 KiB
Plaintext
65 lines
2.6 KiB
Plaintext
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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escalation by containing corruption to an unprivileged process.
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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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On systems which lack mmap or anonymous (MAP_ANON) memory mapping,
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compression must be disabled in order for privilege separation to
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function.
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When privsep is enabled, during the pre-authentication phase sshd will
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chroot(2) to "/var/empty" and change its privileges to the "sshd" user
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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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# 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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# useradd -g sshd -c 'sshd privsep' -d /var/empty -s /bin/false sshd
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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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Privsep requires operating system support for file descriptor passing.
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Compression will be disabled on systems without a working mmap MAP_ANON.
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PAM-enabled OpenSSH is known to function with privsep on Linux.
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It does not function on HP-UX with a trusted system
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configuration.
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On Compaq Tru64 Unix, only the pre-authentication part of privsep is
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supported. Post-authentication privsep is disabled automatically (so
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you won't see the additional process mentioned below).
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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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$Id: README.privsep,v 1.12 2003/08/26 00:48:15 djm Exp $
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