1SMRSH(8) System Manager's Manual SMRSH(8)
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6 smrsh - restricted shell for sendmail
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9 smrsh -c command
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12 The smrsh program is intended as a replacement for sh for use in the
13 ``prog'' mailer in sendmail(8) configuration files. It sharply limits
14 the commands that can be run using the ``|program'' syntax of sendmail
15 in order to improve the over all security of your system. Briefly,
16 even if a ``bad guy'' can get sendmail to run a program without going
17 through an alias or forward file, smrsh limits the set of programs that
18 he or she can execute.
19 Briefly, smrsh limits programs to be in a single directory, by default
21 /etc/smrsh, allowing the system administrator to choose the set of
22 acceptable commands, and to the shell builtin commands ``exec'',
23 ``exit'', and ``echo''. It also rejects any commands with the charac‐
24 ters ``', `<', `>', `;', `$', `(', `)', `\r' (carriage return), or `\n'
25 (newline) on the command line to prevent ``end run'' attacks. It
26 allows ``||'' and ``&&'' to enable commands like: ``"|exec
27 /usr/local/bin/filter || exit 75"''
28 Initial pathnames on programs are stripped, so forwarding to
30 ``/usr/ucb/vacation'', ``/usr/bin/vacation'',
31 ``/home/server/mydir/bin/vacation'', and ``vacation'' all actually for‐
32 ward to ``/etc/smrsh/vacation''.
33 System administrators should be conservative about populating the
35 /etc/smrsh directory. For example, a reasonable additions is vaca‐
36 tion(1), and the like. No matter how brow-beaten you may be, never
37 include any shell or shell-like program (such as perl(1)) in the
38 /etc/smrsh directory. Note that this does not restrict the use of
39 shell or perl scripts in the sm.bin directory (using the ``#!'' syn‐
40 tax); it simply disallows execution of arbitrary programs. Also,
41 including mail filtering programs such as procmail(1) is a very bad
42 idea. procmail(1) allows users to run arbitrary programs in their
43 procmailrc(5).
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46 /etc/smrsh - directory for restricted programs
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49 sendmail(8)
50 $Date: 2004/08/06 03:55:35 $ SMRSH(8)