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Re: *SAFE* use of string in system command

by jakobi (Pilgrim)
on Oct 18, 2009 at 09:28 UTC ( #801848=note: print w/replies, xml ) Need Help??

in reply to string in system command

Don't forget that you need to protect some special characters from shell interpretation in the general case when you cannot guarantee your file naming at exactly 100%. This includes files created by nice little programming or commandline errors starting with simple oversights such as a command creating "-" instead of understanding it as the intended /dev/stdout alias within a pipe.

The special characters to protect include quotes, dollar signs or whitespace and semicolons. Just placing double quotes around a string only takes care of whitespace, semicolons and single quotes.

$file='a" b ; echo "$c '; $output=`cp "$file" newfile 2>&1`; # will fail despite of double-quote quoting

You can of course, ignore this issue. Which is safe to do, IFF you have 100% control of your filenames and know that they contain spaces at worst: You trust all your code, all the code is error-free, there's no bitrot in DRAM or disk, etc. pp. But in reality none of these preconditions is valid, so we better have quality & well-tested backups. Or we just use ...

Some reasonably safe work-arounds on Unix (and most of cygwin) are, in my subjective order of preference:

  1. place the files in shell variables using %ENV:
    ($ENV{f1},$ENV{f2})=("$DIR/$FILE", "$DIR2/${FILE}_devl"); $output=`cp "\$f1" "\$f2" 2>&1`;
  2. use the list form of exec/system to run the command directly without going through the shell:
    system("cp","$DIR/$FILE", "$DIR2/${FILE}_devl").

    For `` aka qx!!, use the 4+ argument form of open(FH,"-|","cmd","arg1",...).

  3. if unavoidable: quote or escape offending characters for use in either bare-word, single or double quoted shell string.

    First check cpan, as there might be an os- and shell-agnnostic module for this. If you need to do it yourself, consider e.g.
    s/["`\\\$]/\\$&/g (escaping for double quotes) or
    s/'/'"'"'/g (shell string concatenation for single quotes; this is more or less what the String::ShellQuote does, just w/o adding single-quotes around the string. Another example module using such regexes ARGV::readonly for treating the secure filename issue when using the magic-and-intentionally--broken <> - both of which work on Unix/Bourne-Shell, and both of which do not seem to be very OS-agnostic to me).

  • The above is valid for Unix and most of cygwin. In windows, you also need to account for whatever quoting and escaping cmd.exe supports.
  • In all 3 cases, the variables are expanded by Perl. Case 1 does variable expansion in the invoked shell (seeing e.g. "$f1" in the example). Case 3 applies the full set of shell interpolations (thus the need for to escape all characters that might be special to the shell). Case 2 bypasses the shell and invokes the command directly.
  • You may also need to prefix a leading - with ./, otherwise a command may interpret the filename as a command option. If the command supports a special option of -- as end-of-options option, use that instead.
  • quoting-s/hell: The line-noise generated in case 3 is what you normally see in shell scripts. Not to mention magic nested double-quote shell quoting as in echo "$(echo "$i")" or echo "`echo "1 +  2" 3; echo x`". Perl's ability to switch quoting delimeters with q and qq offers a welcome escape to readability and sanity. Albeit with one little ...
  • ... Perlish Caveat: You also need to use shell-safe quoting for some 2- and 3-argument forms of open (e.g. open(FH,"-|",$cmd); open(FH,$cmd . "|")). This also affects the 2-argument open implied by the (implied) magic <> using @ARGV as filenames (consider this unsafe clone of the cat command: perl -0ne: *)

Related issues to keep in mind:

  • ANSI or Terminal Control sequences / breaking line-based reporting: The securely quoted/escaped filenames still make a mess when used e.g. in die("cannot open $file\n") in two regards: 1) theyr're not really readable & you cannot paste them as-is due to quoting/escaping. 2) filenames (both raw & secured) are not safe for direct use on stdout, stderr and /dev/tty (consider esc sequences to break both reporting format and your xterm/vt100 terminal settings; depending on the available terminal capabilities, this ranges from more-or-less killing the terminal, exploitable character-injection-into-input-stream, to command execution at worst). Logging might also profit from filtering at least newlines embedded in filenames (replace with e.g. '?'). So you better use a second regex/function to smash non-printable chars in e.g. error reporting, die, carp, ...

Some of the App:: or Logging:: modules & frameworks might offer some support for these concerns as well. If so: Which of these do you trust and suggest for both security as well as cutting down on boilerplate code?


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