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Full Discussion: Safely parsing parameters
Top Forums Shell Programming and Scripting Safely parsing parameters Post 302405346 by drewk on Thursday 18th of March 2010 03:07:47 PM
Old 03-18-2010
If you you run your parameters through awk or perl, you can also break them apart if you do not want to use eval.

Code:
perl -nle 'BEGIN {map {print} @ARGV; exit;}' root=/dev/sda3 noacpi foo "Baz mumble" `echo muahahahaha >&2`
muahahahaha
root=/dev/sda3
noacpi
foo
Baz mumble

Note that the backticks you used in your example are being executed prior to being fed to the perl script. Whatever the user can execute using backticks on your command line, he would have the privileges to execute directly before he executed your script.

The security "hole" only exists if you elevate privileges in your script and then have a way to execute arbitrary code, no?

If you are still concerned, perl or awk can split arbitrary strings just like the shell inside of the interpreter, but this is not entirely trivial.

You would just need to decide which expansions you would want to support and which not:

Shell Expansions - Bash Reference Manual
 

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SETUID(1)						      General Commands Manual							 SETUID(1)

NAME
setuid - run a command with a different uid. SYNOPSIS
setuid username|uid command [ args ] DESCRIPTION
Setuid changes user id, then executes the specified command. Unlike some versions of su(1), this program doesn't ever ask for a password when executed with effective uid=root. This program doesn't change the environment; it only changes the uid and then uses execvp() to find the command in the path, and execute it. (If the command is a script, execvp() passes the command name to /bin/sh for processing.) For example, setuid some_user $SHELL can be used to start a shell running as another user. Setuid is useful inside scripts that are being run by a setuid-root user -- such as a script invoked with super, so that the script can execute some commands using the uid of the original user, instead of root. This allows unsafe commands (such as editors and pagers) to be used in a non-root mode inside a super script. For example, an operator with permission to modify a certain protected_file could use a super command that simply does: cp protected_file temp_file setuid $ORIG_USER ${EDITOR:-/bin/vi} temp_file cp temp_file protected_file (Note: don't use this example directly. If the temp_file can somehow be replaced by another user, as might be the case if it's kept in a temporary directory, there will be a race condition in the time between editing the temporary file and copying it back to the protected file.) AUTHOR
Will Deich local SETUID(1)
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