Perl Cookbook

Executing Commands Without Shell Escapes

Problem

You need to use a user’s input as part of a command, but you don’t want to allow the user to make the shell run other commands or look at other files. If you just blindly call the system function or backticks on a single string containing a command line, the shell might be used to run the command. This would be unsafe.

Solution

Unlike its single-argument version, the list form of the system function is safe from shell escapes. When the command’s arguments involve user input from a form, never use this:

system("command $input @files");            # UNSAFE

Write it this way instead:

system("command", $input, @files);          # safer

Discussion

Because Perl was designed as a glue language, it’s easy to use it to call other programs—too easy, in some cases.

If you’re merely trying to run a shell command but don’t need to capture its output, it’s easy enough to call system using its multiple argument form. But what happens if you’re using the command in backticks or as part of a piped open? Now you have a real problem, because those don’t permit the multiple argument form that system does. The solution is to manually fork and exec the child processes on your own. It’s more work, but at least stray shell escapes won’t be ruining your day.

It’s safe to use backticks in a CGI script only if the arguments you give the program are purely internally generated, as in:

chomp($now = `date`);

But if the command within the backticks contains user-supplied input, perhaps ...