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

       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
       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.

       If you believe you have found a security vulnerability in Perl, please
       email with details.  This points to a
       closed subscription, unarchived mailing list.  Please only use this
       address for security issues in the Perl core, not for modules
       independently distributed on CPAN.

   Taint mode
       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
       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, with the following exceptions:

       o   Arguments to "print" and "syswrite" are not checked for

       o   Symbolic methods


           and symbolic sub references


           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.

       o   Hash keys are never tainted.

       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 = ;            # Also tainted
           open FOO, "/home/me/bar" or die $!;
           $line = ;              # 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
               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 either case, the results of glob are 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()"

           sub is_tainted {
               local $@;   # Don't pollute caller's value.
               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
       expression match.  Perl presumes that if you reference a substring
       using $1, $2, etc. in a non-tainting pattern, that you knew what you
       were doing when you wrote that 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 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

   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.)

   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 latter
       will not.

       Note that if a tainted string is added to @INC, the following problem
       will be reported:

         Insecure dependency in require while running with -T switch

   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
       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 print out.  The tainting mechanism
       is intended to prevent stupid mistakes, not to remove the need for

       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 ($pid) {           # parent
                   while () {
                       # do something
                   close KID;
               } else {
                   my @temp     = ($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
       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, you might want to
       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.  Safe should not be considered bullet-proof,
       though: it will not prevent the foreign code to set up infinite loops,
       allocate gigabytes of memory, or even abusing perl bugs to make the
       host interpreter crash or behave in unpredictable ways.  In any case
       it's better avoided completely if you're really concerned about

   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.

       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:

           #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
       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

   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
       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-
       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 license 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 license's wording will
       stand up in court.

       Unicode is a new and complex technology and one may easily overlook
       certain security pitfalls.  See perluniintro for an overview and
       perlunicode for details, and "Security Implications of Unicode" in
       perlunicode 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.

       o   Hash Algorithm - Hash algorithms like the one used in Perl are well
           known to be vulnerable to collision attacks on their hash function.
           Such attacks involve constructing a set of keys which collide into
           the same bucket producing inefficient behavior.  Such attacks often
           depend on discovering the seed of the hash function used to map the
           keys to buckets.  That seed is then used to brute-force a key set
           which can be used to mount a denial of service attack.  In Perl
           5.8.1 changes were introduced to harden Perl to such attacks, and
           then later in Perl 5.18.0 these features were enhanced and
           additional protections added.

           At the time of this writing, Perl 5.18.0 is considered to be well-
           hardened against algorithmic complexity attacks on its hash
           implementation.  This is largely owed to the following measures
           mitigate attacks:

           Hash Seed Randomization
               In order to make it impossible to know what seed to generate an
               attack key set for, this seed is randomly initialized at
               process start.  This may be overridden by using the
               PERL_HASH_SEED environment variable, see "PERL_HASH_SEED" in
               perlrun.  This environment variable controls how items are
               actually stored, not how they are presented via "keys",
               "values" and "each".

           Hash Traversal Randomization
               Independent of which seed is used in the hash function, "keys",
               "values", and "each" return items in a per-hash randomized
               order.  Modifying a hash by insertion will change the iteration
               order of that hash.  This behavior can be overridden by using
               "hash_traversal_mask()" from Hash::Util or by using the
               PERL_PERTURB_KEYS environment variable, see "PERL_PERTURB_KEYS"
               in perlrun.  Note that this feature controls the "visible"
               order of the keys, and not the actual order they are stored in.

           Bucket Order Perturbance
               When items collide into a given hash bucket the order they are
               stored in the chain is no longer predictable in Perl 5.18.
               This has the intention to make it harder to observe a
               collision.  This behavior can be overridden by using the
               PERL_PERTURB_KEYS environment variable, see "PERL_PERTURB_KEYS"
               in perlrun.

           New Default Hash Function
               The default hash function has been modified with the intention
               of making it harder to infer the hash seed.

           Alternative Hash Functions
               The source code includes multiple hash algorithms to choose
               from.  While we believe that the default perl hash is robust to
               attack, we have included the hash function Siphash as a fall-
               back option.  At the time of release of Perl 5.18.0 Siphash is
               believed to be of cryptographic strength.  This is not the
               default as it is much slower than the default hash.

           Without compiling a special Perl, there is no way to get the exact
           same behavior of any versions prior to Perl 5.18.0.  The closest
           one can get is by setting PERL_PERTURB_KEYS to 0 and setting the
           PERL_HASH_SEED to a known value.  We do not advise those settings
           for production use due to the above security considerations.

           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
           continues to be, affected by the insertion order and the history of
           changes made to the hash over its lifetime.

           Also note that while the order of the hash elements might be
           randomized, this "pseudo-ordering" 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 permutations
           (use e.g. the CPAN modules "Algorithm::Permute" or
           "Algorithm::FastPermute"), or for any cryptographic applications.

           Tied hashes may have their own ordering and algorithmic complexity

       o   Regular expressions - Perl's regular expression engine is so called
           NFA (Non-deterministic 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.

       o   Sorting - the quicksort algorithm used in Perls before 5.8.0 to
           implement the sort() function is very easy to trick into
           misbehaving so that it consumes a lot of time.  Starting from Perl
           5.8.0 a different sorting algorithm, mergesort, is used by default.
           Mergesort cannot misbehave on any input.

       See  for more information, and
       any computer science textbook on algorithmic complexity.

       perlrun for its description of cleaning up environment variables.

perl v5.20.2                      2015-01-15                        PERLSEC(1)