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
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
-L<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".
+L<perllocale>), results of certain system calls (C<readdir()>,
+C<readlink()>, the variable of C<shmread()>, the messages returned by
+C<msgrcv()>, the password, gcos and shell fields returned by the
+C<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, B<with the following exceptions>:
=item *
-If you pass more than one argument to either C<system> or C<exec>,
-the arguments are checked for taintedness B<but> the operation will still
-be attempted, emitting an optional warning. This will be fatal in a
-future version of perl so do not rely on it to bypass the tainting
-mechanism.
-
-=item *
-
Arguments to C<print> and C<syswrite> are B<not> checked for taintedness.
=item *
to call functions B<outside> your Perl code, such as POSIX::system,
in which case they are able to run arbitrary external code.
+=item *
+
+Hash keys are B<never> tainted.
+
=back
-The value of an expression containing tainted data will itself be
-tainted, even if it is logically impossible for the tainted data to
-affect the value.
+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 can be tainted and others not.
+elements of an array or hash can be tainted and others not.
+The keys of a hash are B<never> tainted.
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 $!;
$data = 'abc'; # Not tainted
system "echo $arg"; # Insecure
- system "/bin/echo", $arg; # Allowed but considered 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
open(FOO,"echo $arg|"); # Not OK
open(FOO,"-|")
- or exec 'echo', $arg; # Allowed but not really OK
+ or exec 'echo', $arg; # Also not OK
$shout = `echo $arg`; # Insecure, $shout now tainted
umask $arg; # Insecure
exec "echo $arg"; # Insecure
- exec "echo", $arg; # Allowed but considered insecure
- exec "sh", '-c', $arg; # Considered secure, alas!
+ 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.
+ # 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}". Note that you
-can still write an insecure B<system> or B<exec>, but only by explicitly
-doing something like the "considered secure" example above. This will not
-be possible in a future version of Perl.
+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 C<?:>. Since code
+with a ternary conditional
+
+ $result = $tainted_value ? "Untainted" : "Also untainted";
+
+is effectively
+
+ if ( $tainted_value ) {
+ $result = "Untainted";
+ } else {
+ $result = "Also untainted";
+ }
+
+it doesn't make sense for C<$result> to be tainted.
=head2 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
+C<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 I<is_tainted()> function.
+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 };
}
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. The only way to bypass the tainting
+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, $2, etc. in a
+non-tainting pattern, that
+you knew what you were doing when you wrote that pattern. That means using
a bit of thought--don't just blindly untaint anything, or you defeat the
entire mechanism. It's better to verify that the variable has only good
characters (for certain values of "good") rather than checking whether it
if ($data =~ /^([-\@\w.]+)$/) {
$data = $1; # $data now untainted
} else {
- die "Bad data in $data"; # log this somewhere
+ die "Bad data in '$data'"; # log this somewhere
}
This is fairly secure because C</\w+/> doesn't normally match shell
untainting dirty data, unless you use the strategy detailed below to fork
a child of lesser privilege.
-The example does not untaint $data if C<use locale> is in effect,
+The example does not untaint C<$data> if C<use locale> is in effect,
because the characters matched by C<\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
under such systems. (This issue should arise only in Unix or
Unix-like environments that support #! and setuid or setgid scripts.)
+=head2 Taint mode and @INC
+
+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
+program by using the C<-I> command line option as explained in
+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.
+
+Another way to modify C<@INC> without modifying the program, is to use
+the C<lib> pragma, e.g.:
+
+ 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 latter
+will not.
+
+Note that if a tainted string is added to C<@INC>, the following
+problem will be reported:
+
+ Insecure dependency in require while running with -T switch
+
=head2 Cleaning Up Your Path
-For "Insecure C<$ENV{PATH}>" messages, you need to set C<$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 I<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.
+For "Insecure C<$ENV{PATH}>" messages, you need to set C<$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 I<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
+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.
-Perl does not call the shell to expand wild cards when you pass B<system>
-and B<exec> explicit parameter lists instead of strings with possible shell
-wildcards in them. Unfortunately, the B<open>, B<glob>, and
+Perl does not call the shell to expand wild cards when you pass C<system>
+and C<exec> explicit parameter lists instead of strings with possible shell
+wildcards in them. Unfortunately, the C<open>, C<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
-B<open> syntax that connects the parent and child by a pipe. Now the
+C<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 B<open> or other system call.
+has any special permissions, does the C<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 B<exec> is
+Here's a way to do backticks reasonably safely. Notice how the C<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';
+ 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
+=head2 Shebang Race Condition
Beyond the obvious problems that stem from giving special privileges to
systems as flexible as scripts, on many versions of Unix, set-id scripts
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 B<suidperl> that is automatically
-invoked for you if it's needed.
-
-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
+Some Unixes, especially more recent ones, are 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 I</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 C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F<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.
+
+If you don't have the safe version of set-id scripts, all is not lost.
+Sometimes this kernel "feature" can be disabled, so that the kernel
+either doesn't run set-id scripts with the set-id or doesn't run them
+at all. Either way avoids the exploitability of the race condition,
+but doesn't help in actually running scripts set-id.
+
+If the kernel set-id script feature isn't disabled, then any set-id
+script provides an exploitable vulnerability. Perl can't avoid being
+exploitable, but will point out vulnerable scripts where it can. If Perl
+detects that it is being applied to a set-id script then it will complain
+loudly that your set-id script is insecure, and won't run it. When Perl
+complains, you need to remove the set-id bit from the script to eliminate
+the vulnerability. Refusing to run the script doesn't in itself close
+the vulnerability; it is just Perl's way of encouraging you to do this.
+
+To actually run a script set-id, if you don't have the safe version of
+set-id scripts, you'll need to 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:
+ #include <unistd.h>
+ #include <stdio.h>
+ #include <string.h>
+ #include <errno.h>
+
#define REAL_PATH "/path/to/script"
- main(ac, av)
- char **av;
+
+ int main(int argc, char **argv)
{
- execv(REAL_PATH, av);
+ execv(REAL_PATH, argv);
+ fprintf(stderr, "%s: %s: %s\n",
+ argv[0], REAL_PATH, strerror(errno));
+ return 127;
}
Compile this wrapper into a binary executable and then make I<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 I</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 C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<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 B<suidperl> could
-introduce a security hole.
+than your script setuid or setgid. Note that this wrapper isn't doing
+anything to sanitise the execution environment other than ensuring
+that a safe path to the script is used. It only avoids the shebang
+race condition. It relies on Perl's own features, and on the script
+itself being careful, to make it safe enough to run the script set-id.
=head2 Protecting Your 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
L<perlunicode> for details, and L<perlunicode/"Security Implications
of Unicode"> for security implications in particular.
+=head2 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
+I<Denial of Service> (DoS) attacks.
+
+=over 4
+
+=item *
+
+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
+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 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 *
+
+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 L<perlfaq2>). Running out of space manifests itself by
+Perl running out of memory.
+
+=item *
+
+Sorting - the quicksort algorithm used in Perls before 5.8.0 to
+implement the sort() function was 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.
+
+=back
+
+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.
+
+=head2 Using Sudo
+
+The popular tool C<sudo> provides a controlled way for users to be able
+to run programs as other users. It sanitises the execution environment
+to some extent, and will avoid the L<shebang race condition|/"Shebang
+Race Condition">. If you don't have the safe version of set-id scripts,
+then C<sudo> may be a more convenient way of executing a script as
+another user than writing a C wrapper would be.
+
+However, C<sudo> sets the real user or group ID to that of the target
+identity, not just the effective ID as set-id bits do. As a result, Perl
+can't detect that it is running under C<sudo>, and so won't automatically
+take its own security precautions such as turning on taint mode. Where
+C<sudo> configuration dictates exactly which command can be run, the
+approved command may include a C<-T> option to perl to enable taint mode.
+
+In general, it is necessary to evaluate the suitaility of a script to
+run under C<sudo> specifically with that kind of execution environment
+in mind. It is neither necessary nor sufficient for the same script to
+be suitable to run in a traditional set-id arrangement, though many of
+the issues overlap.
+
=head1 SEE ALSO
L<perlrun> for its description of cleaning up environment variables.
https://perl5.git.perl.org / perl5.git / blobdiff