-
=head1 NAME
perlsec - Perl security
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
mode>, when it detects its program running with differing real and effective
user or group IDs. The setuid bit in Unix permissions is mode 04000, the
setgid bit mode 02000; either or both may be set. You can also enable taint
-mode explicitly by using the the B<-T> command line flag. This flag is
+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, and file input are marked as "tainted".
-Tainted data may not be used directly or indirectly in any command that
-invokes a subshell, 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<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:
$arg = shift; # $arg is tainted
$hid = $arg, 'bar'; # $hid is also tainted
$line = <>; # Tainted
+ $line = <STDIN>; # Also tainted
+ open FOO, "/home/me/bar" or die $!;
+ $line = <FOO>; # Still tainted
$path = $ENV{'PATH'}; # Tainted, but see below
$data = 'abc'; # Not tainted
$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!
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)
+
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 {
- join('',@_), kill 0;
- 1;
+ return ! eval {
+ join('',@_), kill 0;
+ 1;
};
}
approach is used that if any tainted value has been accessed within the
same expression, the whole expression is considered tainted.
-But testing for taintedness only gets you so far. Sometimes you just have
+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 subpatterns from a regular expression match.
Perl presumes that if you reference a substring using $1, $2, etc., that
you knew what you were doing when you wrote the pattern. That means using
a bit of thought--don't just blindly untaint anything, or you defeat the
-entire mechanism. It's better to verify that the variable has only
-good characters (for certain values of "good") rather than checking
-whether it has any bad characters. That's because it's far too easy to
-miss bad characters that you never thought of.
+entire mechanism. It's better to verify that the variable has only good
+characters (for certain values of "good") rather than checking whether it
+has any bad characters. That's because it's far too easy to miss bad
+characters that you never thought of.
Here's a test to make sure that the data contains nothing but "word"
characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
or a dot.
- if ($data =~ /^([-\@\w.]+)$/) {
+ if ($data =~ /^([-\@\w.]+)$/) {
$data = $1; # $data now untainted
} else {
die "Bad data in $data"; # log this somewhere
}
-This is fairly secure since C</\w+/> doesn't normally match shell
+This is fairly secure because C</\w+/> doesn't normally match shell
metacharacters, nor are dot, dash, or at going to mean something special
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.
+The example does not untaint $data if C<use locale> is in effect,
+because the characters matched by C<\w> are determined by the locale.
+Perl considers that locale definitions are untrustworthy because they
+contain data from outside the program. If you are writing a
+locale-aware program, and want to launder data with a regular expression
+containing C<\w>, put C<no locale> ahead of the expression in the same
+block. See L<perllocale/SECURITY> for further discussion and examples.
+
+=head2 Switches On the "#!" Line
+
+When you make a script executable, in order to make it usable as a
+command, the system will pass switches to perl from the script's #!
+line. Perl checks that any command line switches given to a setuid
+(or setgid) script actually match the ones set on the #! line. Some
+Unix and Unix-like environments impose a one-switch limit on the #!
+line, so you may need to use something like C<-wU> instead of C<-w -U>
+under such systems. (This issue should arise only in Unix or
+Unix-like environments that support #! and setuid or setgid scripts.)
+
=head2 Cleaning Up Your Path
For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a
you didn't set it to something that was safe. Because Perl can't
guarantee that the executable in question isn't itself going to turn
around and execute some other program that is dependent on your PATH, it
-makes sure you set the PATH.
+makes sure you set the PATH.
+
+The PATH isn't the only environment variable which can cause problems.
+Because some shells may use the variables IFS, CDPATH, ENV, and
+BASH_ENV, Perl checks that those are either empty or untainted when
+starting subprocesses. You may wish to add something like this to your
+setid and taint-checking scripts.
+
+ delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
It's also possible to get into trouble with other operations that don't
care whether they use tainted values. Make judicious use of the file
tests in dealing with any user-supplied filenames. When possible, do
-opens and such after setting C<$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.
and B<exec> explicit parameter lists instead of strings with possible shell
wildcards in them. Unfortunately, the B<open>, B<glob>, and
backtick functions provide no such alternate calling convention, so more
-subterfuge will be required.
+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
originals or known safe values. Then the child process, which no longer
has any special permissions, does the B<open> or other system call.
Finally, the child passes the data it managed to access back to the
-parent. Since the file or pipe was opened in the child while running
+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
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, "-|");
+ 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; # 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.
+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
+
+There are a number of ways to hide the source to your Perl programs,
+with varying levels of "security".
+
+First of all, however, you I<can't> take away read permission, because
+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. This lets
+people on your local system only see your source.
+
+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
+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
+language, not just Perl).
+
+If you're concerned about people profiting from your code, then the
+bottom line is that nothing but a restrictive licence will give you
+legal security. License your software and pepper it with threatening
+statements like "This is unpublished proprietary software of XYZ Corp.
+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