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Paul Seamons


Taint::Runtime - Runtime enable taint checking


  ### sample "enable" usage

  #!/usr/bin/perl -w
  use Taint::Runtime qw(enable taint_env);
  # having the keyword enable in the import list starts taint

  ### sample $TAINT usage

  #!/usr/bin/perl -w
  use Taint::Runtime qw($TAINT taint_env);
  $TAINT = 1;

  # taint is now enabled

  if (1) {
    local $TAINT = 0;

    # do something we trust

  # back to an untrustwory area

  ### sample functional usage

  #!/usr/bin/perl -w
  use strict;
  use Taint::Runtime qw(taint_start is_tainted taint_env
                        taint untaint

  ### other operations here

  taint_start(); # taint should become active
  taint_env(); # %ENV was previously untainted

  print taint_enabled() ? "enabled\n" : "not enabled\n";

  my $var = taint("some string");

  print is_tainted($var) ? "tainted\n" : "not tainted\n";

  $var = untaint($var);
  # OR
  untaint \$var;

  print is_tainted($var) ? "tainted\n" : "not tainted\n";


First - you probably shouldn't use this module to control taint. You should probably use the -T switch on the commandline instead. There are a somewhat limited number of legitimate use cases where you should use this module instead of the -T switch. Unless you have a specific and good reason for not using the -T option, you should use the -T option.

Taint is a good thing. However, few people (that I work with or talk to or discuss items with) use taint even though they should. The goal of this module isn't to use taint less, but to actually encourage its use more. This module aims to make using taint as painless as possible (This can be an argument against it - often implementation of security implies pain - so taking away pain might lessen security - sort of).

In general - the more secure your script needs to be - the earlier on in your program that tainting should be enabled. For most setuid scripts, you should enable taint by using the -T switch. Without doing so you allow for a non-root user to override @INC which allows for them to put their own module in the place of trusted modules. This is bad. This is very bad. Use the -T switch.

There are some common places where this module may be useful, and where most people don't use it. One such place is in a web server. The -T switch removes PERL5LIB and PERLLIB and '.' from @INC (or remove them before they can be added). This makes sense under setuid. The use of the -T switch in a CGI environment may cause a bit of a headache. For new development, CGI scripts it may be possible to use the -T switch and for mod_perl environments there is the PerlTaint variable. Both of these methods will enable taint and from that point on development should be done with taint.

However, many (possibly most) perl web server implentations add their own paths to the PERL5LIB. All CGI's and mod_perl scripts can then have access. Using the -T switch throws a wrench into the works as suddenly PERL5LIB disappears (mod_perl can easily have the extra directories added again using <perl>push @INC, '/our/lib/dir';</perl>). The company I work for has 200 plus user visible scripts mixed with some mod_perl. Currently none of the scripts use taint. We would like for them all to, but it is not feasible to make the change all at once. Taint::Runtime allows for moving legacy scripts over one at a time.

Again, if you are using setuid - don't use this script.

If you are not using setuid and have reasons not to use the -T and are using this module, make sure that taint is enabled before processing any user data. Also remember that BECAUSE THE -T SWITCH WAS NOT USED %ENV IS INITIALLY NOT MARKED AS TAINTED. Call taint_env() to mark it as tainted (especially important in CGI scripts which all read from $ENV{'QUERY_STRING'}).

If you are not using the -T switch, you most likely should use the following at the very top of your script:

  #!/usr/bin/perl -w

  use strict;
  use Taint::Runtime qw(enable taint_env);

Though this module allows for you to turn taint off - you probably shouldn't. This module is more for you to turn taint on - and once it is on it probably ought to stay on.


The following very basic functions provide the base functionality.


Sets PL_tainting


Sets PL_tainting


View of PL_tainting


Returns a zero length tainted string.


The variable $TAINT is tied to the current state of taint. If $TAINT is set to 0 taint mode is off. When it is set to 1 taint mode is enabled.

  if (1) {
    local $TAINT = 1;

    # taint is enabled



Not really functions. If these keywords are in the import list, taint will be either enabled or disabled.


Start taint mode. $TAINT will equal 1.


Stop taint mode. $TAINT will equal 0.


Convenience function that taints the keys and values of %ENV. If the -T switch was not used - you most likely should call this as soon as taint mode is enabled.


Taints the passed in variable. Only works on writeable scalar values. If a scalar ref is passed in - it is modified. If a scalar is passed in (non ref) it is copied, modified and returned. If a value was undefined, it becomes a zero length defined and tainted string.


  my $tainted_copy = taint($some_var);

For a stronger taint, see the Taint module by Dan Sulgalski which is capable of tainting most types of data.


Untaints the passed in variable. Only works on writeable scalar values. If a scalar ref is passed in - it is modified. If a scalar is passed in (non ref) it is copied, modified and returned. If a value was undefined it becomes an untainted undefined value.

Note: Just because the variable is untainted, doesn't mean that it is safe. You really should use CGI::Ex::Validate, or Data::FormValidator or any of the Untaint:: modules. If you are doing your own validation, and once you have put the user data through very strict checks, then you can use untaint.

  if ($var_to_be_untainted =~ /^[\w\.\-]{0,100}$/) {

  my $untainted_copy = untaint($some_var);

Boolean - Is taint on.


Returns a zero length tainted string.


Boolean - True if the passed value is tainted.


Convenience function that attempts to deply recurse a structure and mark it as tainted. Takes a hashref, arrayref, scalar ref, or scalar and recursively untaints the structure.

For a stronger taint, see the Taint module by Dan Sulgalski which is capable of tainting most types of data.


(Be sure to call taint_env() after turning taint on the first time)

  #!/usr/bin/perl -T

  use Taint::Runtime qw(enable);
  # this does not create a function called enable - just starts taint

  use Taint::Runtime qw($TAINT);
  $TAINT = 1;

  use Taint::Runtime qw(taint_start);


  use Taint::Runtime qw(disable);
  # this does not create a function called disable - just stops taint

  use Taint::Runtime qw($TAINT);
  $TAINT = 0;

  use Taint::Runtime qw(taint_stop);


C code was provided by "hv" on perlmonks. This module wouldn't really be possible without insight into the internals that "hv" provided. His post with the code was shown in this node on perlmonks:


The basic premise in that node was the following code:

  use Inline C => 'void _start_taint() { PL_tainting = 1; }';
  use Inline C => 'SV* _tainted() { PL_tainted = 1; return newSVpvn("", 0); }';

In this module, these two lines have instead been turned into XS for runtime speed (and so you won't need Inline and Parse::RecDescent).

Note: even though "hv" provided the base code example, that doesn't mean that he necessarily endorses the idea. If there are disagreements, quirks, annoyances or any other negative side effects with this module - blame me - not "hv."


Thanks to Alexey A. Kiritchun for pointing out untaint failure on multiline strings.


Paul Seamons (2005)

C stub functions by "hv" on perlmonks.org


This module may be used and distributed under the same terms as Perl itself.