Programming Perl, 4th Edition

Cleaning Up Your Environment

When you execute another program from within your Perl script, no matter how, Perl checks to make sure your PATH environment variable is secure. Since it came from your environment, your PATH starts out tainted; so, if you try to run another program, Perl raises an “Insecure $ENV{PATH}” exception. When you set it to a known, untainted value, Perl makes sure that each directory in that path is nonwritable by anyone other than the directory’s owner and group; otherwise, it raises an “Insecure directory” exception.

You may be surprised to find that Perl cares about your PATH even when you specify the full pathname of the command you want to execute. It’s true that with an absolute filename, the PATH isn’t used to find the executable to run. But there’s no reason to trust the program you’re running not to turn right around and execute some other program and get into trouble because of the insecure PATH. So Perl forces you to set a secure PATH before you call any program, no matter how you say to call it.

The PATH isn’t the only environment variable that can bring grief. Because some shells use the variables IFS, CDPATH, ENV, and BASH_ENV, Perl makes sure that those are all either empty or untainted before it will run another command. Either set these variables to something known to be safe or delete them from the environment altogether:

delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};   # Make %ENV safer

Features convenient in a normal environment can become security ...