You may not use data derived from outside your program to affect
something else outside your program--at least, not by accident. All
command line arguments, environment variables, locale information (see
-L<perllocale>), results of certain system calls (readdir(),
-readlink(), the variable of shmread(), the messages returned by
-msgrcv(), the password, gcos and shell fields returned by the
-getpwxxx() calls), and all file input are marked as "tainted".
+L<perllocale>), results of certain system calls (C<readdir()>,
+C<readlink()>, the variable of C<shmread()>, the messages returned by
+C<msgrcv()>, the password, gcos and shell fields returned by the
+C<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<with the following exceptions>:
If you try to do something insecure, you will get a fatal error saying
something like "Insecure dependency" or "Insecure $ENV{PATH}".
+The exception to the principle of "one tainted value taints the whole
+expression" is with the ternary conditional operator C<?:>. Since code
+with a ternary conditional
+
+ $result = $tainted_value ? "Untainted" : "Also untainted";
+
+is effectively
+
+ if ( $tainted_value ) {
+ $result = "Untainted";
+ } else {
+ $result = "Also untainted";
+ }
+
+it doesn't make sense for C<$result> to be tainted.
+
=head2 Laundering and Detecting Tainted Data
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
+C<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 C<is_tainted()> function.
untainting dirty data, unless you use the strategy detailed below to fork
a child of lesser privilege.
-The example does not untaint $data if C<use locale> is in effect,
+The example does not untaint C<$data> if C<use locale> is in effect,
because the characters matched by C<\w> are determined by the locale.
Perl considers that locale definitions are untrustworthy because they
contain data from outside the program. If you are writing a
so be careful what you print out. The tainting mechanism is intended to
prevent stupid mistakes, not to remove the need for thought.
-Perl does not call the shell to expand wild cards when you pass B<system>
-and B<exec> explicit parameter lists instead of strings with possible shell
-wildcards in them. Unfortunately, the B<open>, B<glob>, and
+Perl does not call the shell to expand wild cards when you pass C<system>
+and C<exec> explicit parameter lists instead of strings with possible shell
+wildcards in them. Unfortunately, the C<open>, C<glob>, and
backtick functions provide no such alternate calling convention, so more
subterfuge will be required.
Perl provides a reasonably safe way to open a file or pipe from a setuid
or setgid program: just create a child process with reduced privilege who
does the dirty work for you. First, fork a child using the special
-B<open> syntax that connects the parent and child by a pipe. Now the
+C<open> syntax that connects the parent and child by a pipe. Now the
child resets its ID set and any other per-process attributes, like
environment variables, umasks, current working directories, back to the
originals or known safe values. Then the child process, which no longer
-has any special permissions, does the B<open> or other system call.
+has any special permissions, does the C<open> or other system call.
Finally, the child passes the data it managed to access back to the
parent. Because the file or pipe was opened in the child while running
under less privilege than the parent, it's not apt to be tricked into
doing something it shouldn't.
-Here's a way to do backticks reasonably safely. Notice how the B<exec> is
+Here's a way to do backticks reasonably safely. Notice how the C<exec> is
not called with a string that the shell could expand. This is by far the
best way to call something that might be subjected to shell escapes: just
never call the shell at all.
Alternately, it can simply ignore the set-id bits 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 B<suidperl> that is automatically
+this via a special executable called F<suidperl> that is automatically
invoked for you if it's needed.
However, if the kernel set-id script feature isn't disabled, Perl will
pathname subject to meddling, it instead passes I</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 C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure>
+compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F<Configure>
program that builds Perl tries to figure this out for itself, so you
should never have to specify this yourself. Most modern releases of
SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.
-Prior to release 5.6.1 of Perl, bugs in the code of B<suidperl> could
+Prior to release 5.6.1 of Perl, bugs in the code of F<suidperl> could
introduce a security hole.
=head2 Protecting Your Programs
use Config;
use File::Spec::Functions;
+my $total_tests = 236;
my $test = 177;
sub ok ($;$) {
my($ok, $name) = @_;
my $TEST = catfile(curdir(), 'TEST');
-print "1..223\n";
+print "1..$total_tests\n";
# First, let's make sure that Perl is checking the dangerous
# environment variables. Maybe they aren't set yet, so we'll
eval '$^O = $^X';
test 223, $@ =~ /Insecure dependency in/;
}
+
+EFFECTIVELY_CONSTANTS: {
+ my $tainted_number = 12 + $TAINT0;
+ test 224, tainted( $tainted_number );
+
+ # Even though it's always 0, it's still tainted
+ my $tainted_product = $tainted_number * 0;
+ test 225, tainted( $tainted_product );
+ test 226, $tainted_product == 0;
+}
+
+TERNARY_CONDITIONALS: {
+ my $tainted_true = $TAINT . "blah blah blah";
+ my $tainted_false = $TAINT0;
+ test 227, tainted( $tainted_true );
+ test 228, tainted( $tainted_false );
+
+ my $result = $tainted_true ? "True" : "False";
+ test 229, $result eq "True";
+ test 230, !tainted( $result );
+
+ $result = $tainted_false ? "True" : "False";
+ test 231, $result eq "False";
+ test 232, !tainted( $result );
+
+ my $untainted_whatever = "The Fabulous Johnny Cash";
+ my $tainted_whatever = "Soft Cell" . $TAINT;
+
+ $result = $tainted_true ? $tainted_whatever : $untainted_whatever;
+ test 233, $result eq "Soft Cell";
+ test 234, tainted( $result );
+
+ $result = $tainted_false ? $tainted_whatever : $untainted_whatever;
+ test 235, $result eq "The Fabulous Johnny Cash";
+ test 236, !tainted( $result );
+}
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