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
+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.
+
+=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
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>:
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:
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 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
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 later
+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
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.
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 B<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
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>
+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.
-Prior to release 5.6.1 of Perl, bugs in the code of B<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
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).
+In Perl 5.8.1 the random perturbation was done by default, but as of
+5.8.2 it is only used on individual hashes if the internals detect the
+insertion of pathological data. If one wants for some reason emulate the
+old behaviour (and expose oneself to DoS attacks) one can set the
+environment variable PERL_HASH_SEED to zero to disable the protection
+(or any other integer to force a known perturbation, rather than random).
+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 longer identical).
B<Perl has never guaranteed any ordering of the hash keys>, and the
ordering has already changed several times during the lifetime of
=item *
-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 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
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 text book on the algorithmic complexity.
+and any computer science textbook on algorithmic complexity.
=head1 SEE ALSO
https://perl5.git.perl.org / perl5.git / blobdiff