Perl Setuid Scripts

by Geethalakshmi 2010-09-17 12:54:27

Perl Setuid Scripts

Perl is designed to make it easy to write secure setuid and setgid scripts. Unlike shells, which are based on multiple substitution passes on each line of the script, perl uses a more conventional evaluation scheme with fewer hidden "gotchas". Additionally, since the language has more built-in functionality, it has to rely less upon external (and possibly untrustworthy) programs to accomplish its purposes.

In an unpatched 4.2 or 4.3bsd kernel, setuid scripts are intrinsically insecure, but this kernel feature can be disabled. If it is, perl can emulate the setuid and setgid mechanism when it notices the otherwise useless setuid/gid bits on perl scripts. If the kernel 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.

When perl is executing a setuid script, it 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:

$foo = shift; # $foo is tainted
$bar = $foo,'bar'; # $bar is also tainted
$xxx = <>; # Tainted
$path = $ENV{'PATH'}; # Tainted, but see below
$abc = 'abc'; # Not tainted

system "echo $foo"; # Insecure
system "/bin/echo", $foo; # Secure (doesn't use sh)
system "echo $bar"; # Insecure
system "echo $abc"; # Insecure until PATH set

$ENV{'PATH'} = '/bin:/usr/bin';
$ENV{'IFS'} = '' if $ENV{'IFS'} ne '';

$path = $ENV{'PATH'}; # Not tainted
system "echo $abc"; # Is secure now!

open(FOO,"$foo"); # OK
open(FOO,">$foo"); # Not OK

open(FOO,"echo $foo|"); # Not OK, but...
open(FOO,"-|") || exec 'echo', $foo; # OK

$zzz = `echo $foo`; # Insecure, zzz tainted

unlink $abc,$foo; # Insecure
umask $foo; # Insecure

exec "echo $foo"; # Insecure
exec "echo", $foo; # Secure (doesn't use sh)
exec "sh", '-c', $foo; # Considered secure, alas

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:

$ARGV[0] =~ /^-P(\w+)$/;
$printer = $1; # Not tainted

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 `$>' equal to `$<').

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 `$> = $<'. 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.

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