Untainting system calls correctly

c has asked for the wisdom of the Perl Monks concerning the following question:

I'm reading through perldoc perlsec in the hopes of getting a system() call to work with taint checking enabled. In particular, I have been staring at:

           system "echo $arg";         # Insecure
           system "/bin/echo", $arg;   # Secure (doesn't use sh)
           system "echo $hid";         # Insecure
           system "echo $data";        # Insecure until PATH set

           $path = $ENV{'PATH'};       # $path now tainted

           $ENV{'PATH'} = '/bin:/usr/bin';
           delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};

           $path = $ENV{'PATH'};       # $path now NOT tainted
           system "echo $data";        # Is secure now!
[download]

my system call is:

system "/usr/sbin/useradd $cli";
[download]

I have the following in my script which I had hoped would allow for this command to be used:

## taint environmentals
$ENV{'PATH'} = "/bin:/usr/sbin";
delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};

## clean up user's name
$formdata{username} =~ s/ //g;
$formdata{username} =~ s/[^A-Za-z0-9]//g;
$formdata{username} =~ /^([A-Za-z0-9]{1,8}).*$/;
$formdata{username} = $1;

my $cli = "-s $shell -d $home -G $group $formdata{username}";
  $cli =~ /^(\-s \"\/bin\/false\" \-d \"\/home\/clients\/(?:stage\.)?(
+?:[\w\-]+\.)(?:\w{2,3}\.)?(?:\w{2,4})\/[A-Za-z0-9]+\" \-G \"hosting\,
+[\w\-]+\" [A-Za-z0-9]+)$/;
  $cli = $1;

  print "CLI is $cli";

  system("/usr/sbin/useradd $cli");
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I stuck in that print "CLI is $cli" to see just what I was getting out of it. Sure enough, it is as i expected it to be.
In my situation, that turns out to be:

-s "/bin/false" -d "/home/clients/stage.domain.com/testcjm" -G "hostin
+g,domain" testcjm
[download]

In the course of my testing, I've also untainted the $shell $home $group and $formdata{username} variables.

When I dont use $cli and instead just pass the $formdata{username} variable to the system call, it works. It seems like the problem is only when I pass all the command line flags as part of $cli.

this has become my white whale for the day's chasing.

thanks -c

Comment on Untainting system calls correctly Select or Download Code

Replies are listed 'Best First'.
Re: untainting system calls correctly by dws (Chancellor) on Apr 10, 2002 at 16:37 UTC
I'm curious. First, you identify that the single argument form of `system()` is a potential problem, but then you proceed on to use it. If you're going to use `system("/usr/sbin/useradd $cli");` then you need to prevent `$cli` from having as it's trailing substring something day-ruining like `"... ; /bin/rm -rf *"` You don't have this problem with the multiple argument form, since you're bypassing the shell entirely. Here's a simple reworking: `my $username = ... untaint $formdata{'username'}; ... etc. for other form variables my @cli = ("-s", $shell, "-d", $home, "-G", $group $usrname); ... system("/usr/sbin/useradd", @cli);` [download] Edit: s/training/trailing/	[reply] [d/l] [select]
Re: Re: untainting system calls correctly by Fletch (Bishop) on Apr 10, 2002 at 16:43 UTC
Not to meniton preventing someone from slipping in a username of ``-u 0 I0wNj00`'. Update: As a clarification, I mean that by using the multiple arugment form rather than letting the shell split you prevent the user from submitting extra arguments (in the example I gave they could specify that their new account would get a uid of 0).	[reply] [d/l]
Re: Re: untainting system calls correctly by c (Hermit) on Apr 10, 2002 at 17:24 UTC
Sorry if I am just missing the nomenclature you're using, but what do you mean by "training substring"? thanks! -c	[reply]
Re: tainting system calls correctly by Zaxo (Archbishop) on Apr 10, 2002 at 16:39 UTC
As a security measure, I think taint is a little beside the point if you are handing out shell accounts via a web form. Please say you have some layers of approval, authorization and authentication above this! :) After Compline, Zaxo	[reply]
Re: Re: tainting system calls correctly by c (Hermit) on Apr 10, 2002 at 17:18 UTC
Not really shell accounts. If you notice the shell is set to /bin/false. Plus the script is only available to clients that have signed off on a lengthy contract agreement. To boot, its available through a strictly ssl connection that requires user/pass authorization. There are additional checks within the script to restrict touching UID less than 500, passing a group to a GID that is greater than 500. I know that there are plenty of issues that still need to be addressed, but I was just curious to see exactly why i am seeing it fail in my current situation. thanks -c	[reply]


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