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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.

SECURITY VULNERABILITY CONTACT INFORMATION

       If you believe you have found a security vulnerability in Perl, please email
       perl5-security-report@perl.org 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.

SECURITY MECHANISMS AND CONCERNS

   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 taintedness.

       •   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.

       •   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
           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 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()" function.

           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 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.)

   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 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);
                   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.

   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 condition.

   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
       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 attacks.

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

SEE ALSO

       perlrun for its description of cleaning up environment variables.