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
+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.
For example:
exec "echo", $arg; # Secure (doesn't use the shell)
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)
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 $ENV{PATH}". Note that you
can still write an insecure B<system> or B<exec>, but only by explicitly
-doing something like the last example above.
+doing something like the "considered secure" example above.
=head2 Laundering and Detecting Tainted Data
To test whether a variable contains tainted data, and whose use would thus
-trigger an "Insecure dependency" message, you can use the following
-I<is_tainted()> function.
+trigger an "Insecure dependency" message, check your nearby CPAN mirror
+for the F<Taint.pm> module, which should become available around November
+1997. Or you may be able to use the following I<is_tainted()> function.
sub is_tainted {
return ! eval {
to the shell. Use of C</.+/> would have been insecure in theory because
it lets everything through, but Perl doesn't check for that. The lesson
is that when untainting, you must be exceedingly careful with your patterns.
-Laundering data using regular expression is the I<ONLY> mechanism for
+Laundering data using regular expression is the I<only> mechanism for
untainting dirty data, unless you use the strategy detailed below to fork
a child of lesser privilege.
around and execute some other program that is dependent on your PATH, it
makes sure you set the PATH.
+The PATH isn't the only environment variable which can cause problems.
+Because some shells may use the variables IFS, CDPATH, ENV, and
+BASH_ENV, Perl checks that those are either empty or untainted when
+starting subprocesses. You may wish to add something like this to your
+setid and taint-checking scripts.
+
+ delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
+
It's also possible to get into trouble with other operations that don't
care whether they use tainted values. Make judicious use of the file
tests in dealing with any user-supplied filenames. When possible, do
-opens and such after setting C<$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.
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.
+never call the shell at all.
use English;
- die unless defined $pid = open(KID, "-|");
+ 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; # XXX: initgroups() not called
+ $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';
- die "can't exec myprog: $!";
+ exec 'myprog', 'arg1', 'arg2'
+ or die "can't exec myprog: $!";
}
-A similar strategy would work for wildcard expansion via C<glob>.
+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.
-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"
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
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
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.