Beyond the obvious problems that stem from giving special privileges to such flexible systems as scripts, on many operating systems, setuid scripts are inherently insecure right from the start. This is because that between the time that the kernel opens up the file to see what to run, and when the now setuid interpreter it ran turns around and reopens the file so it can interpret it, things may have changed, especially if you have symbolic links on your system.
Fortunately, sometimes this kernel ``feature'' can be disabled. Unfortunately, there are two ways to disable it. The system can simply outlaw scripts with the setuid bit set, which doesn't help much. Alternately, it can simply ignore the setuid bit on scripts. If the latter is true, Perl can emulate the setuid and setgid mechanism when it notices the otherwise useless setuid/gid bits on Perl scripts. It does this via a special executable called suidperl that is automatically invoked for you if it's needed.
If, however, the kernel setuid script feature isn't disabled, Perl will complain loudly that your setuid script is insecure. You'll need to either disable the kernel setuid script feature, or put a C wrapper around the script. See the program wrapsuid in the eg directory of your Perl distribution for how to go about doing this.
There are some systems on which setuid scripts are free of this inherent
security bug. For example, recent releases of Solaris are like this. On
such systems, when the kernel passes the name of the setuid script to open
to the interpreter, rather than using a pathname subject to mettling, it
instead passes /dev/fd/3. This is a special file already opened on the
script, so that there can be no race condition for evil scripts to
exploit. On these systems, Perl should be compiled with
-DSETUID_SCRIPTS_ARE_SECURE_NOW. The Configure program that builds
Perl tries to figure this out for itself.
When executing a setuid script, or when you have turned on taint checking explicitly using the -T flag, Perl takes special precautions to prevent you from falling into any obvious traps. (In some ways, a Perl script is more secure than the corresponding C program.) Any command line argument, environment variable, or input is marked as ``tainted'', and may not be used, directly or indirectly, in any command that invokes a subshell, or in any command that modifies files, directories, or processes. Any variable that is set within an expression that has previously referenced a tainted value also becomes tainted (even if it is logically impossible for the tainted value to influence the variable). For example:
The taintedness is associated with each scalar value, so some elements of an array can be tainted, and others not.
If you try to do something insecure, you will get a fatal error saying something like ``Insecure dependency'' or ``Insecure PATH''. Note that you can still write an insecure system call or exec, but only by explicitly doing something like the last example above. You can also bypass the tainting mechanism by referencing subpatterns--Perl presumes that if you reference a substring using $1, $2, etc, you knew what you were doing when you wrote the pattern:
This is fairly secure since \w+ doesn't match shell metacharacters.
Use of /.+/ would have been insecure, but Perl doesn't check for that,
so you must be careful with your patterns. This is the ONLY mechanism
for untainting user supplied filenames if you want to do file operations
on them (unless you make $> equal to $< ).
For ``Insecure $ENV{PATH}'' messages, you need to set $ENV{'PATH'} to a known
value, and each directory in the path must be non-writable by the world.
A frequently voiced gripe is that you can get this message even
if the pathname to an executable is fully qualified. But Perl can't
know that the executable in question isn't going to execute some other
program depending on the PATH.
It's also possible to get into trouble with other operations that don't
care whether they use tainted values. Make judicious use of the file
tests in dealing with any user-supplied filenames. When possible, do
opens and such after setting $> = $<. (Remember group IDs,
too!) Perl doesn't prevent you from opening tainted filenames for reading,
so be careful what you print out. The tainting mechanism is intended to
prevent stupid mistakes, not to remove the need for thought.
This gives us a reasonably safe way to open a file or pipe: just reset the id set to the original IDs. Here's a way to do backticks reasonably safely. Notice how the exec() is not called with a string that the shell could expand. By the time we get to the exec() , tainting is turned off, however, so be careful what you call and what you pass it.
For those even more concerned about safety, see the Safe and Safe CGI modules at a CPAN site near you. See the perlmod manpage for a list of CPAN sites.