Perl is designed to make it easy to program securely even when running
with extra privileges, like setuid or setgid programs. Unlike most
-command-line shells, which are based on multiple substitution passes on
+command line shells, which are based on multiple substitution passes on
each line of the script, Perl uses a more conventional evaluation scheme
with fewer hidden snags. Additionally, because the language has more
-built-in functionality, it can rely less upon external (and possibly
+builtin functionality, it can rely less upon external (and possibly
untrustworthy) programs to accomplish its purposes.
Perl automatically enables a set of special security checks, called I<taint
setgid bit mode 02000; either or both may be set. You can also enable taint
mode explicitly by using the B<-T> command line flag. This flag is
I<strongly> suggested for server programs and any program run on behalf of
-someone else, such as a CGI script.
+someone else, such as a CGI script. Once taint mode is on, it's on for
+the remainder of your script.
While in this mode, Perl takes special precautions called I<taint
checks> to prevent both obvious and subtle traps. Some of these checks
are reasonably simple, such as verifying that path directories aren't
writable by others; careful programmers have always used checks like
these. Other checks, however, are best supported by the language itself,
-and it is these checks especially that contribute to making a setuid Perl
+and it is these checks especially that contribute to making a set-id Perl
program more secure than the corresponding C program.
-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>),
-and 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. Any variable set
-within an expression that has previously referenced a tainted value itself
-becomes tainted, even if it is logically impossible for the tainted value
-to influence the variable. Because taintedness is associated with each
-scalar value, some elements of an array can be tainted and others not.
+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".
+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 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.
For example:
$path = $ENV{'PATH'}; # $path now tainted
- $ENV{'PATH'} = '/bin:/usr/bin';
- $ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
+ $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!
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 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 last example above.
+doing something like the "considered secure" example above.
=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 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
But testing for taintedness gets you only so far. Sometimes you have just
to clear your data's taintedness. The only way to bypass the tainting
-mechanism is by referencing sub-patterns from a regular expression match.
+mechanism is by referencing subpatterns from a regular expression match.
Perl presumes that if you reference a substring using $1, $2, etc., that
you knew what you were doing when you wrote the pattern. That means using
a bit of thought--don't just blindly untaint anything, or you defeat the
characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
or a dot.
- if ($data =~ /^([-\@\w.]+)$/) {
+ if ($data =~ /^([-\@\w.]+)$/) {
$data = $1; # $data now untainted
} else {
die "Bad data in $data"; # log this somewhere
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.
When you make a script executable, in order to make it usable as a
command, the system will pass switches to perl from the script's #!
-line. Perl checks that any command-line switches given to a setuid
+line. Perl checks that any command line switches given to a setuid
(or setgid) script actually match the ones set on the #! line. Some
-UNIX and UNIX-like environments impose a one-switch limit on the #!
+Unix and Unix-like environments impose a one-switch limit on the #!
line, so you may need to use something like C<-wU> instead of C<-w -U>
-under such systems. (This issue should arise only in UNIX or
-UNIX-like environments that support #! and setuid or setgid scripts.)
+under such systems. (This issue should arise only in Unix or
+Unix-like environments that support #! and setuid or setgid scripts.)
=head2 Cleaning Up Your Path
you didn't set it to something that was safe. Because Perl can't
guarantee that the executable in question isn't itself going to turn
around and execute some other program that is dependent on your PATH, it
-makes sure you set the PATH.
+makes sure you set the PATH.
+
+The PATH isn't the only environment variable which can cause problems.
+Because some shells may use the variables IFS, CDPATH, ENV, and
+BASH_ENV, Perl checks that those are either empty or untainted when
+starting subprocesses. You may wish to add something like this to your
+setid and taint-checking scripts.
+
+ delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
It's also possible to get into trouble with other operations that don't
care whether they use tainted values. Make judicious use of the file
tests in dealing with any user-supplied filenames. When possible, do
-opens and such after setting C<$E<gt> = $E<lt>>. (Remember group IDs,
-too!) Perl doesn't prevent you from opening tainted filenames for reading,
+opens and such B<after> properly dropping any special user (or group!)
+privileges. Perl doesn't prevent you from opening tainted filenames for reading,
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
-back-tick functions provide no such alternate calling convention, so more
-subterfuge will be required.
+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
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 back-ticks reasonably safely. Notice how the B<exec> is
+Here's a way to do backticks reasonably safely. Notice how the B<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. By the time we get to the B<exec>, tainting
-is turned off, however, so be careful what you call and what you pass it.
-
- use English;
- die unless defined $pid = open(KID, "-|");
- if ($pid) { # parent
- while (<KID>) {
- # do something
- }
- close KID;
- } else {
- $EUID = $UID;
- $EGID = $GID; # XXX: initgroups() not called
- $ENV{PATH} = "/bin:/usr/bin";
- exec 'myprog', 'arg1', 'arg2';
- die "can't exec myprog: $!";
- }
-
-A similar strategy would work for wildcard expansion via C<glob>.
+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);
+ 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.
Taint checking is most useful when although you trust yourself not to have
written a program to give away the farm, you don't necessarily trust those
who end up using it not to try to trick it into doing something bad. This
-is the kind of security checking that's useful for setuid programs and
+is the kind of security checking that's useful for set-id programs and
programs launched on someone else's behalf, like CGI programs.
This is quite different, however, from not even trusting the writer of the
=head2 Security Bugs
Beyond the obvious problems that stem from giving special privileges to
-systems as flexible as scripts, on many versions of Unix, setuid scripts
+systems as flexible as scripts, on many versions of Unix, set-id scripts
are inherently insecure right from the start. The problem is a race
condition in the kernel. Between the time the kernel opens the file to
-see which interpreter to run and when the (now-setuid) interpreter turns
+see which interpreter to run and when the (now-set-id) interpreter turns
around and reopens the file to interpret it, the file in question may have
changed, especially if you have symbolic links on your system.
Fortunately, sometimes this kernel "feature" can be disabled.
Unfortunately, there are two ways to disable it. The system can simply
-outlaw scripts with the setuid bit set, which doesn't help much.
-Alternately, it can simply ignore the setuid bit on scripts. If the
+outlaw scripts with any set-id bit set, which doesn't help much.
+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
-invoked for you if it's needed.
+invoked for you if it's needed.
-However, if the kernel setuid script feature isn't disabled, Perl will
-complain loudly that your setuid script is insecure. You'll need to
-either disable the kernel setuid script feature, or put a C wrapper around
+However, if the kernel set-id script feature isn't disabled, Perl will
+complain loudly that your set-id script is insecure. You'll need to
+either disable the kernel set-id script feature, or put a C wrapper around
the script. A C wrapper is just a compiled program that does nothing
except call your Perl program. Compiled programs are not subject to the
-kernel bug that plagues setuid scripts. Here's a simple wrapper, written
+kernel bug that plagues set-id scripts. Here's a simple wrapper, written
in C:
#define REAL_PATH "/path/to/script"
- main(ac, av)
+ main(ac, av)
char **av;
{
execv(REAL_PATH, av);
- }
-
-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.
+Compile this wrapper into a binary executable and then make I<it> rather
+than your script setuid or setgid.
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 setuid script to open to the interpreter, rather than using a
+of the set-id script to open to the interpreter, rather than using a
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
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
the source code has to be readable in order to be compiled and
interpreted. (That doesn't mean that a CGI script's source is
readable by people on the web, though.) So you have to leave the
-permissions at the socially friendly 0755 level.
+permissions at the socially friendly 0755 level. This lets
+people on your local system only see your source.
-Some people regard this as a security problem. If your program does
+Some people mistakenly regard this as a security problem. If your program does
insecure things, and relies on people not knowing how to exploit those
insecurities, it is not secure. It is often possible for someone to
determine the insecure things and exploit them without viewing the
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
Your access to it does not give you permission to use it blah blah
blah." You should see a lawyer to be sure your licence's wording will
stand up in court.
+
+=head1 SEE ALSO
+
+L<perlrun> for its description of cleaning up environment variables.
https://perl5.git.perl.org / perl5.git / blobdiff