


PERLSEC(1)       Perl Programmers Reference Guide      PERLSEC(1)


NAME
       perlsec - Perl security

DESCRIPTION
       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 each line of the
       script, Perl uses a more conventional evaluation scheme
       with fewer hidden snags.  Additionally, because the
       language has more 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 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 -T
       command line flag. This flag is strongly suggested for
       server programs and any program run on behalf of 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
       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 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 the perllocale
       manpage), 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. (Important exception: If you
       pass a list of arguments to either system or exec, the
       elements of that list are 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:



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           $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

           system "echo $arg";         # Insecure
           system "/bin/echo", $arg;   # Secure (doesn't use sh)
           system "echo $hid";         # Insecure
           system "echo $data";        # Insecure until PATH set

           $path = $ENV{'PATH'};       # $path now tainted

           $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!

           open(FOO, "< $arg");        # OK - read-only file
           open(FOO, "> $arg");        # Not OK - trying to write

           open(FOO,"echo $arg|");     # Not OK, but...
           open(FOO,"-|")
               or exec 'echo', $arg;   # OK

           $shout = `echo $arg`;       # Insecure, $shout now tainted

           unlink $data, $arg;         # Insecure
           umask $arg;                 # Insecure

           exec "echo $arg";           # Insecure
           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)

       If you try to do something insecure, you will get a fatal
       error saying something like "Insecure dependency" or
       "Insecure $ENV{PATH}".  Note that you can still write an
       insecure system or exec, but only by explicitly doing
       something like the "considered secure" example above.

       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 Taint.pm
       module, which should become available around November
       1997.  Or you may be able to use the following



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PERLSEC(1)       Perl Programmers Reference Guide      PERLSEC(1)


       is_tainted() function.

           sub is_tainted {
               return ! eval {
                   join('',@_), kill 0;
                   1;
               };
           }

       This function makes use of the fact that the presence of
       tainted data anywhere within an expression renders the
       entire expression tainted.  It would be inefficient for
       every operator to test every argument for taintedness.
       Instead, the slightly more efficient and conservative
       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 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.

       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 ($data =~ /^([-\@\w.]+)$/) {
               $data = $1;                     # $data now untainted
           } else {
               die "Bad data in $data";        # log this somewhere
           }

       This is fairly secure because /\w+/ doesn't normally match
       shell metacharacters, nor are dot, dash, or at going to
       mean something special to the shell.  Use of /.+/ 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 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 use locale is in



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       effect, because the characters matched by \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 \w, put no locale ahead of the
       expression in the same block.  See the SECURITY entry in
       the perllocale manpage for further discussion and
       examples.

       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 -wU instead of
       -w -U under such systems.  (This issue should arise only
       in Unix or Unix-like environments that support #! and
       setuid or setgid scripts.)

       Cleaning Up Your Path

       For "Insecure $ENV{PATH}" messages, you need to set
       $ENV{'PATH'} to a known value, and each directory in the
       path must be non-writable by others than its owner and
       group.  You may be surprised to get this message even if
       the pathname to your executable is fully qualified.  This
       is not generated because you didn't supply a full path to
       the program; instead, it's generated because you never set
       your PATH environment variable, or 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.

       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 properly dropping any special user (or
       group!)  privileges. Perl doesn't prevent you from opening
       tainted filenames for reading, so be careful what you



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       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 system and exec explicit parameter lists instead of
       strings with possible shell wildcards in them.
       Unfortunately, the open, 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 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 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 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.

           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: $!";
           }

       A similar strategy would work for wildcard expansion via
       glob, although you can use readdir instead.



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       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 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 code not to try to do something evil.
       That's the kind of trust needed when someone hands you a
       program you've never seen before and says, "Here, run
       this."  For that kind of safety, check out the Safe
       module, included standard in the Perl distribution.  This
       module allows the programmer to set up special
       compartments in which all system operations are trapped
       and namespace access is carefully controlled.

       Security Bugs

       Beyond the obvious problems that stem from giving special
       privileges to 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-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 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 suidperl that is automatically invoked for you if
       it's needed.

       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
       set-id scripts.  Here's a simple wrapper, written in C:








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           #define REAL_PATH "/path/to/script"
           main(ac, av)
               char **av;
           {
               execv(REAL_PATH, av);
           }

       Compile this wrapper into a binary executable and then
       make it rather than your script setuid or setgid.

       See the program wrapsuid in the 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 pathname subject to
       meddling, it instead passes /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
       -DSETUID_SCRIPTS_ARE_SECURE_NOW.  The 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 5.003 of Perl, a bug in the code of
       suidperl could introduce a security hole in systems
       compiled with strict POSIX compliance.

       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 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



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       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.

SEE ALSO
       the perlrun manpage for its description of cleaning up
       environment variables.
































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