getpwxxx() calls), and all file input are marked as "tainted".
Tainted data may not be used directly or indirectly in any command
that invokes a sub-shell, nor in any command that modifies files,
-directories, or processes. (B<Important exception>: If you pass a list
-of arguments to either C<system> or C<exec>, the elements of that list
-are B<NOT> checked for taintedness.) Any variable set to a value
-derived from tainted data will itself be tainted, even if it is
-logically impossible for the tainted data to alter the variable.
+directories, or processes, B<with the following exceptions>:
+
+=over 4
+
+=item *
+
+If you pass more than one argument to either C<system> or C<exec>,
+the arguments are B<not> checked for taintedness.
+
+=item *
+
+Arguments to C<print> and C<syswrite> are B<not> checked for taintedness.
+
+=back
+
+The value of an expression containing tainted data will itself be
+tainted, even if it is logically impossible for the tainted data to
+affect the value.
+
Because taintedness is associated with each scalar value, some
elements of an array can be tainted and others not.
unlink $data, $arg; # Insecure
umask $arg; # Insecure
- exec "echo $arg"; # Insecure
+ exec "echo $arg"; # Insecure (uses the shell)
exec "echo", $arg; # Secure (doesn't use the shell)
exec "sh", '-c', $arg; # Considered secure, alas!
@files = <*.c>; # insecure (uses readdir() or similar)
@files = glob('*.c'); # insecure (uses readdir() or similar)
+ # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would
+ # have used an external program to do the filename expansion; but in
+ # either case the result is tainted since the list of filenames comes
+ # from outside of the program.
+
+ $bad = ($arg, 23); # $bad will be tainted
+ $arg, `true`; # Insecure (although it isn't really)
+
If you try to do something insecure, you will get a fatal error saying
something like "Insecure dependency" or "Insecure $ENV{PATH}". Note that you
can still write an insecure B<system> or B<exec>, but only by explicitly
=head2 Laundering and Detecting Tainted Data
-To test whether a variable contains tainted data, and whose use would thus
-trigger an "Insecure dependency" message, check your nearby CPAN mirror
-for the F<Taint.pm> module, which should become available around November
-1997. Or you may be able to use the following I<is_tainted()> function.
+To test whether a variable contains tainted data, and whose use would
+thus trigger an "Insecure dependency" message, you can use the
+tainted() function of the Scalar::Util module, available in your
+nearby CPAN mirror, and included in Perl starting from the release 5.8.0.
+Or you may be able to use the following I<is_tainted()> function.
sub is_tainted {
return ! eval {
best way to call something that might be subjected to shell escapes: just
never call the shell at all.
- use English;
- die "Can't fork: $!" unless defined $pid = open(KID, "-|");
- if ($pid) { # parent
- while (<KID>) {
- # do something
- }
- close KID;
- } else {
- my @temp = ($EUID, $EGID);
- $EUID = $UID;
- $EGID = $GID; # initgroups() also called!
- # Make sure privs are really gone
- ($EUID, $EGID) = @temp;
- die "Can't drop privileges"
- unless $UID == $EUID && $GID eq $EGID;
- $ENV{PATH} = "/bin:/usr/bin";
- exec 'myprog', 'arg1', 'arg2'
- or die "can't exec myprog: $!";
- }
+ use English;
+ die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
+ if ($pid) { # parent
+ while (<KID>) {
+ # do something
+ }
+ close KID;
+ } else {
+ my @temp = ($EUID, $EGID);
+ my $orig_uid = $UID;
+ my $orig_gid = $GID;
+ $EUID = $UID;
+ $EGID = $GID;
+ # Drop privileges
+ $UID = $orig_uid;
+ $GID = $orig_gid;
+ # Make sure privs are really gone
+ ($EUID, $EGID) = @temp;
+ die "Can't drop privileges"
+ unless $UID == $EUID && $GID eq $EGID;
+ $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH.
+ # Consider sanitizing the environment even more.
+ exec 'myprog', 'arg1', 'arg2'
+ or die "can't exec myprog: $!";
+ }
A similar strategy would work for wildcard expansion via C<glob>, although
you can use C<readdir> instead.
source. Security through obscurity, the name for hiding your bugs
instead of fixing them, is little security indeed.
-You can try using encryption via source filters (Filter::* from CPAN).
-But crackers might be able to decrypt it. You can try using the
-byte code compiler and interpreter described below, but crackers might
-be able to de-compile it. You can try using the native-code compiler
+You can try using encryption via source filters (Filter::* from CPAN,
+or Filter::Util::Call and Filter::Simple since Perl 5.8).
+But crackers might be able to decrypt it. You can try using the byte
+code compiler and interpreter described below, but crackers might be
+able to de-compile it. You can try using the native-code compiler
described below, but crackers might be able to disassemble it. These
pose varying degrees of difficulty to people wanting to get at your
code, but none can definitively conceal it (this is true of every
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