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 gecos field of getpw* 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. Because taintedness is associated with each
-scalar value, some elements of an array can be tainted and others not.
+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".
+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>:
+
+=over 4
+
+=item *
+
+If you pass more than one argument to either C<system> or C<exec>,
+the arguments are checked for taintedness B<but> the operation will still
+be attempted, emitting an optional warning. This will be fatal in a
+future version of perl so do not rely on it to bypass the tainting
+mechanism.
+
+=item *
+
+Arguments to C<print> and C<syswrite> are B<not> checked for taintedness.
+
+=item *
+
+Symbolic methods
+
+ $obj->$method(@args);
+
+and symbolic sub references
+
+ &{$foo}(@args);
+ $foo->(@args);
+
+are not checked for taintedness. This requires extra carefulness
+unless you want external data to affect your control flow. Unless
+you carefully limit what these symbolic values are, people are able
+to call functions B<outside> your Perl code, such as POSIX::system,
+in which case they are able to run arbitrary external code.
+
+=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.
For example:
$data = 'abc'; # Not tainted
system "echo $arg"; # Insecure
- system "/bin/echo", $arg; # Secure (doesn't use sh)
+ system "/bin/echo", $arg; # Allowed but considered insecure
+ # (Perl doesn't know about /bin/echo)
system "echo $hid"; # Insecure
system "echo $data"; # Insecure until PATH set
open(FOO, "< $arg"); # OK - read-only file
open(FOO, "> $arg"); # Not OK - trying to write
- open(FOO,"echo $arg|"); # Not OK, but...
+ open(FOO,"echo $arg|"); # Not OK
open(FOO,"-|")
- or exec 'echo', $arg; # OK
+ or exec 'echo', $arg; # Allowed but not really OK
$shout = `echo $arg`; # Insecure, $shout now tainted
umask $arg; # Insecure
exec "echo $arg"; # Insecure
- exec "echo", $arg; # Secure (doesn't use the shell)
+ exec "echo", $arg; # Allowed but considered insecure
exec "sh", '-c', $arg; # Considered secure, alas!
- @files = <*.c>; # Always insecure (uses csh)
- @files = glob('*.c'); # Always insecure (uses csh)
+ @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 PATH". Note that you
+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
-doing something like the "considered secure" example above.
+doing something like the "considered secure" example above. This will not
+be possible in a future version of Perl.
=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 {
- join('',@_), kill 0;
- 1;
- };
+ return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
}
This function makes use of the fact that the presence of tainted data
to the shell. Use of C</.+/> would have been insecure in theory because
it lets everything through, but Perl doesn't check for that. The lesson
is that when untainting, you must be exceedingly careful with your patterns.
-Laundering data using regular expression is the I<ONLY> mechanism for
+Laundering data using regular expression is the I<only> mechanism for
untainting dirty data, unless you use the strategy detailed below to fork
a child of lesser privilege.
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 '-no_match_vars';
+ 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.
Compile this wrapper into a binary executable and then make I<it> rather
than your script setuid or setgid.
-See the program B<wrapsuid> in the F<eg> directory of your Perl
-distribution for a convenient way to do this automatically for all your
-setuid Perl programs. It moves setuid scripts into files with the same
-name plus a leading dot, and then compiles a wrapper like the one above
-for each of them.
-
In recent years, vendors have begun to supply systems free of this
inherent security bug. On such systems, when the kernel passes the name
of the set-id script to open to the interpreter, rather than using a
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 5.003 of Perl, a bug in the code of B<suidperl> could
-introduce a security hole in systems compiled with strict POSIX
-compliance.
+Prior to release 5.6.1 of Perl, bugs in the code of B<suidperl> could
+introduce a security hole.
=head2 Protecting Your Programs
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
blah." You should see a lawyer to be sure your licence's wording will
stand up in court.
+=head2 Unicode
+
+Unicode is a new and complex technology and one may easily overlook
+certain security pitfalls. See L<perluniintro> for an overview and
+L<perlunicode> for details, and L<perlunicode/"Security Implications
+of Unicode"> for security implications in particular.
+
=head1 SEE ALSO
L<perlrun> for its description of cleaning up environment variables.
https://perl5.git.perl.org / perl5.git / blobdiff