145 lines
5.0 KiB
Plaintext
145 lines
5.0 KiB
Plaintext
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README smrsh - sendmail restricted shell.
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@(#)README 8.2 11/11/1995
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This README file is provided as a courtesy of the CERT Coordination Center,
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Software Engineering Institute, Carnegie Mellon University. This file is
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intended as a supplement to the CERT advisory CA-93:16.sendmail.vulnerability,
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and to the software, smrsh.c, written by Eric Allman.
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The smrsh(8) program is intended as a replacement for /bin/sh in the
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program mailer definition of sendmail(8). This README file describes
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the steps needed to compile and install smrsh.
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smrsh is a restricted shell utility that provides the ability to
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specify, through a configuration, an explicit list of executable
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programs. When used in conjunction with sendmail, smrsh effectively
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limits sendmail's scope of program execution to only those programs
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specified in smrsh's configuration.
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smrsh has been written with portability in mind, and uses traditional
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Unix library utilities. As such, smrsh should compile on most
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Unix C compilers.
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To compile smrsh.c, use the following command:
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host.domain% cc -o smrsh smrsh.c
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For machines that provide dynamic linking, it is advisable to compile
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smrsh without dynamic linking. As an example with the Sun Microsystems
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compiler, you should compile with the -Bstatic option.
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host.domain% cc -Bstatic -o smrsh smrsh.c
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Choose a directory that smrsh will reside in. We will use the traditional
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/usr/local/etc directory for the remainder of this document.
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As root, install smrsh in /usr/local/etc directory, with mode 511.
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host.domain# mv smrsh /usr/local/etc
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host.domain# chmod 511 /usr/local/etc/smrsh
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Next, determine the list of commands that smrsh should allow sendmail
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to run. This list of allowable commands can be determined by:
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1. examining your /etc/aliases file, to indicate what commands
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are being used by the system.
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2. surveying your host's .forward files, to determine what
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commands users have specified.
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See the man page for aliases(5) if you are unfamiliar with the format of
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these specifications. Additionally, you should include in the list,
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popular commands such as /usr/ucb/vacation.
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You should NOT include interpreter programs such as sh(1), csh(1),
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perl(1), uudecode(1) or the stream editor sed(1) in your list of
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acceptable commands.
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You will next need to create the directory /usr/adm/sm.bin and populate
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it with the programs that your site feels are allowable for sendmail
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to execute. This directory is explicitly specified in the source
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code for smrsh, so changing this directory must be accompanied with
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a change in smrsh.c.
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You will have to be root to make these modifications.
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After creating the /usr/adm/sm.bin directory, either copy the programs
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to the directory, or establish links to the allowable programs from
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/usr/adm/sm.bin. Change the file permissions, so that these programs
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can not be modified by non-root users. If you use links, you should
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ensure that the target programs are not modifiable.
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To allow the popular vacation(1) program by creating a link in the
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/usr/adm/sm.bin directory, you should:
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host.domain# cd /usr/adm/sm.bin
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host.domain# ln -s /usr/ucb/vacation vacation
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After populating the /usr/adm/sm.bin directory, you can now configure
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sendmail to use the restricted shell. Save the current sendmail.cf
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file prior to modifying it, as a prudent precaution.
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Typically, the program mailer is defined by a single line in the
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sendmail configuration file, sendmail.cf. This file is traditionally
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found in the /etc, /usr/lib or /etc/mail directories, depending on
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the UNIX vendor.
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If you are unsure of the location of the actual sendmail configuration
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file, a search of the strings(1) output of the sendmail binary, will
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help to locate it.
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In order to configure sendmail to use smrsh, you must modify the Mprog
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definition in the sendmail.cf file, by replacing the /bin/sh specification
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with /usr/local/etc/smrsh.
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As an example:
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In most Sun Microsystems' sendmail.cf files, the line is:
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Mprog, P=/bin/sh, F=lsDFMeuP, S=10, R=20, A=sh -c $u
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which should be changed to:
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Mprog, P=/usr/local/etc/smrsh, F=lsDFMeuP, S=10, R=20, A=sh -c $u
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^^^^^^^^^^^^^^^^^^^^
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A more generic line may be:
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Mprog, P=/bin/sh, F=lsDFM, A=sh -c $u
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and should be changed to;
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Mprog, P=/usr/local/etc/smrsh, F=lsDFM, A=sh -c $u
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After modifying the Mprog definition in the sendmail.cf file, if a frozen
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configuration file is being used, it is essential to create a new one.
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You can determine if you need a frozen configuration by discovering
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if a sendmail.fc file currently exists in either the /etc/, /usr/lib,
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or /etc/mail directories. The specific location can be determined using
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a search of the strings(1) output of the sendmail binary.
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In order to create a new frozen configuration, if it is required:
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host.domain# /usr/lib/sendmail -bz
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Now re-start the sendmail process. An example of how to do this on
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a typical system follows:
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host.domain# /usr/bin/ps aux | /usr/bin/grep sendmail
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root 130 0.0 0.0 168 0 ? IW Oct 2 0:10 /usr/lib/sendmail -bd -q
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host.domain# /bin/kill -9 130
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host.domain# /usr/lib/sendmail -bd -q30m
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