perlsec



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 possi-
       bly 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 pro-
       gram 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 some-
       thing else outside your program--at least, not by accident.  All com-
       mand line arguments, environment variables, locale information (see
       perllocale), results of certain system calls ("readdir()", "read-
       link()", the variable of "shmread()", the messages returned by
       "msgrcv()", the password, gcos and shell fields returned by the "getp-
       wxxx()" 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, with the following exceptions:

       ·   Arguments to "print" and "syswrite" are not checked for tainted-
           ness.

       ·   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 outside your Perl code, such as POSIX::system, in
           which case they are able to run arbitrary external code.

       For efficiency reasons, Perl takes a conservative view of whether data
       is tainted.  If an expression contains tainted data, any subexpression
       may be considered tainted, even if the value of the subexpression is
       not itself affected by the tainted data.

       Because taintedness is associated with each scalar value, some elements
       of an array or hash can be tainted and others not.  The keys of a hash
       are never tainted.

       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

           system "echo $arg";         # Insecure
           system "/bin/echo", $arg;   # Considered insecure
                                       # (Perl doesn’t know about /bin/echo)
           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
           open(FOO,"-│")
               or exec ’echo’, $arg;   # Also not OK

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

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

           exec "echo $arg";           # Insecure
           exec "echo", $arg;          # Insecure
           exec "sh", ’-c’, $arg;      # Very insecure!

           @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 $ENV{PATH}".

       The exception to the principle of "one tainted value taints the whole
       expression" is with the ternary conditional operator "?:".  Since code
       with a ternary conditional

           $result = $tainted_value ? "Untainted" : "Also untainted";

       is effectively

           if ( $tainted_value ) {
               $result = "Untainted";
           } else {
               $result = "Also untainted";
           }

       it doesn’t make sense for $result to be tainted.

       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
       "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 "is_tainted()" func-
       tion.

           sub is_tainted {
               return ! eval { eval("#" . substr(join("", @_), 0, 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.  Values may be untainted by
       using them as keys in a hash; otherwise the only way to bypass the
       tainting mechanism is by referencing subpatterns from a regular expres-
       sion 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 spe-
       cial 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 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 "SECURITY" in perllocale for further discussion and exam-
       ples.

       Switches On the "#!" Line

       When you make a script executable, in order to make it usable as a com-
       mand, the system will pass switches to perl from the script’s #!  line.
       Perl checks that any command line switches given to a setuid (or set-
       gid) 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 environ-
       ments that support #! and setuid or setgid scripts.)

       Taint mode and @INC

       When the taint mode ("-T") is in effect, the "." directory is removed
       from @INC, and the environment variables "PERL5LIB" and "PERLLIB" are
       ignored by Perl. You can still adjust @INC from outside the program by
       using the "-I" command line option as explained in perlrun. The two
       environment variables are ignored because they are obscured, and a user
       running a program could be unaware that they are set, whereas the "-I"
       option is clearly visible and therefore permitted.

       Another way to modify @INC without modifying the program, is to use the
       "lib" pragma, e.g.:

         perl -Mlib=/foo program

       The benefit of using "-Mlib=/foo" over "-I/foo", is that the former
       will automagically remove any duplicated directories, while the later
       will not.

       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 absolute and 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 qual-
       ified.  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 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 "sys-
       tem" 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 ’-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 "glob",
       although you can use "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 some-
       thing 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 oper-
       ations 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) inter-
       preter 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.  Unfortu-
       nately, 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 sub-
       ject to the kernel bug that plagues set-id scripts.  Here’s a simple
       wrapper, written in C:

           #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.

       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 pro-
       gram 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 condi-
       tion.

       Prior to release 5.6.1 of Perl, bugs in the code of suidperl could
       introduce a security hole.

       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 cant 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 view-
       ing 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,
       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-com-
       pile 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.

       Unicode

       Unicode is a new and complex technology and one may easily overlook
       certain security pitfalls.  See perluniintro for an overview and perlu-
       nicode for details, and "Security Implications of Unicode" in perluni-
       code for security implications in particular.

       Algorithmic Complexity Attacks

       Certain internal algorithms used in the implementation of Perl can be
       attacked by choosing the input carefully to consume large amounts of
       either time or space or both.  This can lead into the so-called Denial
       of Service (DoS) attacks.

       ·   Hash Function - the algorithm used to "order" hash elements has
           been changed several times during the development of Perl, mainly
           to be reasonably fast.  In Perl 5.8.1 also the security aspect was
           taken into account.

           In Perls before 5.8.1 one could rather easily generate data that as
           hash keys would cause Perl to consume large amounts of time because
           internal structure of hashes would badly degenerate.  In Perl 5.8.1
           the hash function is randomly perturbed by a pseudorandom seed
           which makes generating such naughty hash keys harder.  See
           "PERL_HASH_SEED" in perlrun for more information.

           The random perturbation is done by default but if one wants for
           some reason emulate the old behaviour one can set the environment
           variable PERL_HASH_SEED to zero (or any other integer).  One possi-
           ble reason for wanting to emulate the old behaviour is that in the
           new behaviour consecutive runs of Perl will order hash keys differ-
           ently, which may confuse some applications (like Data::Dumper: the
           outputs of two different runs are no more identical).

           Perl has never guaranteed any ordering of the hash keys, and the
           ordering has already changed several times during the lifetime of
           Perl 5.  Also, the ordering of hash keys has always been, and con-
           tinues to be, affected by the insertion order.

           Also note that while the order of the hash elements might be ran-
           domised, this "pseudoordering" should not be used for applications
           like shuffling a list randomly (use List::Util::shuffle() for that,
           see List::Util, a standard core module since Perl 5.8.0; or the
           CPAN module Algorithm::Numerical::Shuffle), or for generating per-
           mutations (use e.g. the CPAN modules Algorithm::Permute or Algo-
           rithm::FastPermute), or for any cryptographic applications.

       ·   Regular expressions - Perl’s regular expression engine is so called
           NFA (Non-Finite Automaton), which among other things means that it
           can rather easily consume large amounts of both time and space if
           the regular expression may match in several ways.  Careful crafting
           of the regular expressions can help but quite often there really
           isn’t much one can do (the book "Mastering Regular Expressions" is
           required reading, see perlfaq2).  Running out of space manifests
           itself by Perl running out of memory.

       ·   Sorting - the quicksort algorithm used in Perls before 5.8.0 to
           implement the sort() function is very easy to trick into misbehav-
           ing so that it consumes a lot of time.  Nothing more is required
           than resorting a list already sorted.  Starting from Perl 5.8.0 a
           different sorting algorithm, mergesort, is used.  Mergesort is
           insensitive to its input data, so it cannot be similarly fooled.

       See <http://www.cs.rice.edu/~scrosby/hash/> for more information, and
       any computer science text book on the algorithmic complexity.


SEE ALSO

       perlrun for its description of cleaning up environment variables.



perl v5.8.6                       2004-11-05                        PERLSEC(1)

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