builtin functionality, it can rely less upon external (and possibly
untrustworthy) programs to accomplish its purposes.
+=head1 SECURITY VULNERABILITY CONTACT INFORMATION
+
+If you believe you have found a security vulnerability in Perl, please
+email the details to perl5-security-report@perl.org. 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 C<[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 created.
+
+=head1 SECURITY MECHANISMS AND CONCERNS
+
+=head2 Taint mode
+
Perl automatically enables a set of special security checks, called I<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 B<-T> command line flag. This flag is
+mode explicitly by using the B<-T> command line flag. This flag is
I<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
+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 I<taint
For example:
$arg = shift; # $arg is tainted
- $hid = $arg, 'bar'; # $hid is also tainted
+ $hid = $arg . 'bar'; # $hid is also tainted
$line = <>; # Tainted
$line = <STDIN>; # Also tainted
open FOO, "/home/me/bar" or die $!;
@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.
+ # 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)
Or you may be able to use the following C<is_tainted()> function.
sub is_tainted {
+ local $@; # Don't pollute caller's value.
return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
}
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
+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
When the taint mode (C<-T>) is in effect, the "." directory is removed
from C<@INC>, and the environment variables C<PERL5LIB> and C<PERLLIB>
-are ignored by Perl. You can still adjust C<@INC> from outside the
+are ignored by Perl. You can still adjust C<@INC> from outside the
program by using the C<-I> command line option as explained in
-L<perlrun>. The two environment variables are ignored because
+L<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 C<-I> option is clearly visible and
therefore permitted.
perl -Mlib=/foo program
The benefit of using C<-Mlib=/foo> over C<-I/foo>, is that the former
-will automagically remove any duplicated directories, while the later
+will automagically remove any duplicated directories, while the latter
will not.
Note that if a tainted string is added to C<@INC>, the following
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
+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
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 B<after> properly dropping any special user (or group!)
-privileges. Perl doesn't prevent you from opening tainted filenames for reading,
+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.
best way to call something that might be subjected to shell escapes: just
never call the shell at all.
- use English '-no_match_vars';
+ use English;
die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
if ($pid) { # parent
while (<KID>) {
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
+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.
+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.
=head2 Security Bugs
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 F<suidperl> that is automatically
-invoked for you if it's needed.
+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
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 F<suidperl> could
-introduce a security hole.
-
=head2 Protecting Your Programs
There are a number of ways to hide the source to your Perl programs,
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
+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 licence's wording will
+blah." You should see a lawyer to be sure your license's wording will
stand up in court.
=head2 Unicode
=item *
-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 account.
-
-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 L<perlrun/PERL_HASH_SEED> 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).
-
-B<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.
+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:
+
+=over 4
+
+=item 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
+L<perlrun/PERL_HASH_SEED>. This environment variable controls how
+items are actually stored, not how they are presented via
+C<keys>, C<values> and C<each>.
+
+=item Hash Traversal Randomization
+
+Independent of which seed is used in the hash function, C<keys>,
+C<values>, and C<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 C<hash_traversal_mask()> from
+L<Hash::Util> or by using the PERL_PERTURB_KEYS environment variable,
+see L<perlrun/PERL_PERTURB_KEYS>. Note that this feature controls the
+"visible" order of the keys, and not the actual order they are stored in.
+
+=item 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 L<perlrun/PERL_PERTURB_KEYS>.
+
+=item New Default Hash Function
+
+The default hash function has been modified with the intention of making
+it harder to infer the hash seed.
+
+=item 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.
+
+=back
+
+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.
+
+B<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
-randomised, this "pseudoordering" should B<not> be used for
-applications like shuffling a list randomly (use List::Util::shuffle()
+randomized, this "pseudo-ordering" should B<not> be used for
+applications like shuffling a list randomly (use C<List::Util::shuffle()>
for that, see L<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.
+or the CPAN module C<Algorithm::Numerical::Shuffle>), or for generating
+permutations (use e.g. the CPAN modules C<Algorithm::Permute> or
+C<Algorithm::FastPermute>), or for any cryptographic applications.
+
+Tied hashes may have their own ordering and algorithmic complexity
+attacks.
=item *
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 fooled.
+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.
=back
-See L<http://www.cs.rice.edu/~scrosby/hash/> for more information,
-and any computer science textbook on the algorithmic complexity.
+See L<https://www.usenix.org/legacy/events/sec03/tech/full_papers/crosby/crosby.pdf> for more information,
+and any computer science textbook on algorithmic complexity.
=head1 SEE ALSO
https://perl5.git.perl.org / perl5.git / blobdiff