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NAME    [Toc]    [Back]

       perlsec - Perl security

DESCRIPTION    [Toc]    [Back]

       Perl is designed to make it easy to program securely even
       when running with extra privileges, like setuid or setgid
       programs.  Unlike most command line shells, which are
       based on multiple substitution passes on each line of the
       script, Perl uses a more conventional evaluation scheme
       with fewer hidden snags.  Additionally, because the language
 has more builtin functionality, it can rely less
       upon external (and possibly untrustworthy) programs to
       accomplish its purposes.

       Perl automatically enables a set of special security
       checks, called taint mode, when it detects its program
       running with differing real and effective user or group
       IDs.  The setuid bit in Unix permissions is mode 04000,
       the setgid bit mode 02000; either or both may be set.  You
       can also enable taint mode explicitly by using the -T command
 line flag. This flag is strongly suggested for server
       programs and any program run on behalf of someone else,
       such as a CGI script. Once taint mode is on, it's on for
       the remainder of your script.

       While in this mode, Perl takes special precautions called
       taint checks to prevent both obvious and subtle traps.
       Some of these checks are reasonably simple, such as verifying
 that path directories aren't writable by others;
       careful programmers have always used checks like these.
       Other checks, however, are best supported by the language
       itself, and it is these checks especially that contribute
       to making a set-id Perl program more secure than the corresponding
 C program.

       You may not use data derived from outside your program to
       affect something else outside your program--at least, not
       by accident.  All command line arguments, environment
       variables, locale information (see 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 fol-
       lowing exceptions:

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

       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.

       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
           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

           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}".

       Laundering and Detecting Tainted Data    [Toc]    [Back]

       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 {
               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., 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 =~ /^([-@192
               $data  =  $1;                      # $data now untainted
           } else {
               die "Bad data in '$data'";      # log  this  somewhere

       This is fairly secure because "/456
       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 "264
       mined 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 "144no 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 later will not.

       Cleaning Up Your Path    [Toc]    [Back]

       For "Insecure $ENV{PATH}" messages, you need to set
       $ENV{'PATH'} to a known value, and each directory in the
       path must be non-writable by others than its owner and
       group.  You may be surprised to get this message even if
       the pathname to your executable is fully qualified.  This
       is not generated because you didn't supply a full path to
       the program; instead, it's generated because you never set
       your PATH environment variable, or you didn't set it to
       something that was safe.  Because Perl can't guarantee
       that the executable in question isn't itself going to turn
       around and execute some other program that is dependent on
       your PATH, it makes sure you set the PATH.

       The PATH isn't the only environment variable which can
       cause problems.  Because some shells may use the variables
       IFS, CDPATH, ENV, and BASH_ENV, Perl checks that those are
       either empty or untainted when starting subprocesses. You
       may wish to add something like this to your setid and
       taint-checking scripts.

           delete  @ENV{qw(IFS  CDPATH  ENV  BASH_ENV)};   # Make
%ENV safer

       It's also possible to get into trouble with other operations
 that don't care whether they use tainted values.
       Make judicious use of the file tests in dealing with any
       user-supplied filenames.  When possible, do opens and such
       after properly dropping any special user (or group!)
       privileges. Perl doesn't prevent you from opening tainted
       filenames for reading, so be careful what you 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

       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
                   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, check out the Safe module,
 included standard in the Perl distribution.  This
       module allows the programmer to set up special compartments
 in which all system operations are trapped and
       namespace access is carefully controlled.

       Security Bugs    [Toc]    [Back]

       Beyond the obvious problems that stem from giving special
       privileges to systems as flexible as scripts, on many versions
 of Unix, set-id scripts are inherently insecure
       right from the start.  The problem is a race condition in
       the kernel.  Between the time the kernel opens the file to
       see which interpreter to run and when the (now-set-id)
       interpreter turns around and reopens the file to interpret
       it, the file in question may have changed, especially if
       you have symbolic links on your system.

       Fortunately, sometimes this kernel "feature" can be disabled.
  Unfortunately, there are two ways to disable it.
       The system can simply outlaw scripts with any set-id bit
       set, which doesn't help much.  Alternately, it can simply
       ignore the set-id bits on scripts.  If the latter is true,
       Perl can emulate the setuid and setgid mechanism when it
       notices the otherwise useless setuid/gid bits on Perl
       scripts.  It does this via a special executable called
       suidperl that is automatically invoked for you if it's

       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.

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

       Protecting Your Programs    [Toc]    [Back]

       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

       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
       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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)

       o   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

           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 possible reason for
           wanting to emulate the old behaviour is that in the
           new behaviour consecutive runs of Perl will order hash
           keys differently, 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 continues to be,
           affected by the insertion order.

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

       o   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.

       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.  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

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

SEE ALSO    [Toc]    [Back]

       perlrun for its description of cleaning up environment

perl v5.8.5                 2002-11-06                         10
[ Back ]
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