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 the details to This creates a new
   Request Tracker ticket in a special queue which isn't initially
   publicly accessible. The email will also be copied to a closed
   subscription unarchived mailing list which includes all the core
   committers, who will be able to help assess the impact of issues,
   figure out a resolution, and help co-ordinate the release of patches to
   mitigate or fix the problem across all platforms on which Perl is
   supported. Please only use this address for security issues in the Perl
   core, not for modules independently distributed on CPAN.

   When sending an initial request to the security email address, please
   don't Cc any other parties, because if they reply to all, the reply
   will generate yet another new ticket. Once you have received an initial
   reply with a "[perl #NNNNNN]" ticket number in  the headline, it's okay
   to Cc subsequent replies to third parties: all emails to the
   perl5-security-report address with the ticket number in the subject
   line will be added to the ticket; without it, a new ticket will be


   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:

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

   *   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.

   *   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 = <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
           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 ($pid) {           # parent
               while (<KID>) {
                   # do something
               close KID;
           } else {
               my @temp     = ($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
   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.

   *   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

   *   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.

   *   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.


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