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
+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
-built-in functionality, it can rely less upon external (and possibly
+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 I<taint
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
I<strongly> suggested for server programs and any program run on behalf of
-someone else, such as a CGI script.
+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
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 setuid Perl
+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 L<perllocale>),
-and 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. Any variable set
-within an expression that has previously referenced a tainted value itself
-becomes tainted, even if it is logically impossible for the tainted value
-to influence the variable. Because taintedness is associated with each
-scalar value, some elements of an array can be tainted and others not.
+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 (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>:
+
+=over 4
+
+=item *
+
+Arguments to C<print> and C<syswrite> are B<not> checked for taintedness.
+
+=item *
+
+Symbolic methods
+
+ $obj->$method(@args);
+
+and symbolic sub references
+
+ &{$foo}(@args);
+ $foo->(@args);
+
+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 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
+
+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 B<never> tainted.
For example:
$data = 'abc'; # Not tainted
system "echo $arg"; # Insecure
- system "/bin/echo", $arg; # Secure (doesn't use sh)
+ system "/bin/echo", $arg; # Considered insecure
+ # (Perl doesn't know about /bin/echo)
system "echo $hid"; # Insecure
system "echo $data"; # Insecure until PATH set
$path = $ENV{'PATH'}; # $path now tainted
- $ENV{'PATH'} = '/bin:/usr/bin';
- $ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
+ $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, but...
+ open(FOO,"echo $arg|"); # Not OK
open(FOO,"-|")
- or exec 'echo', $arg; # 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; # Secure (doesn't use the shell)
- 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.
+
+ $bad = ($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 PATH". Note that you
-can still write an insecure B<system> or B<exec>, but only by explicitly
-doing something like the last example above.
+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 following
-I<is_tainted()> function.
+To test whether a variable contains tainted data, and whose use would
+thus trigger an "Insecure dependency" message, you can use the
+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 C<is_tainted()> function.
sub is_tainted {
- return ! eval {
- join('',@_), kill 0;
- 1;
- };
+ return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
}
This function makes use of the fact that the presence of tainted data
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
-mechanism is by referencing sub-patterns from a regular expression match.
+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, $2, etc., that
you knew what you were doing when you wrote the pattern. That means using
a bit of thought--don't just blindly untaint anything, or you defeat the
characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
or a dot.
- if ($data =~ /^([-\@\w.]+)$/) {
+ if ($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
to the shell. Use of C</.+/> 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 I<ONLY> mechanism for
+Laundering data using regular expression is the I<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 $data if C<use locale> is in effect,
+The example does not untaint C<$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
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
+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 #!
+Unix and Unix-like environments impose a one-switch limit on the #!
line, so you may need to use something like C<-wU> instead of C<-w -U>
-under such systems. (This issue should arise only in UNIX or
-UNIX-like environments that support #! and setuid or setgid scripts.)
+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
+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 setting C<$E<gt> = $E<lt>>. (Remember group IDs,
-too!) Perl doesn't prevent you from opening tainted filenames for reading,
+opens and such B<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 B<system>
-and B<exec> explicit parameter lists instead of strings with possible shell
-wildcards in them. Unfortunately, the B<open>, B<glob>, and
-back-tick functions provide no such alternate calling convention, so more
-subterfuge will be required.
+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 back-ticks 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. By the time we get to the B<exec>, tainting
-is turned off, however, so be careful what you call and what you pass it.
-
- use English;
- die unless defined $pid = open(KID, "-|");
- if ($pid) { # parent
- while (<KID>) {
- # do something
- }
- close KID;
- } else {
- $EUID = $UID;
- $EGID = $GID; # XXX: initgroups() not called
- $ENV{PATH} = "/bin:/usr/bin";
- exec 'myprog', 'arg1', 'arg2';
- die "can't exec myprog: $!";
- }
-
-A similar strategy would work for wildcard expansion via C<glob>.
+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 more.
+ exec 'myprog', 'arg1', 'arg2'
+ or die "can't exec myprog: $!";
+ }
+
+A similar strategy would work for wildcard expansion via C<glob>, although
+you can use C<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 setuid programs and
+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
=head2 Security Bugs
Beyond the obvious problems that stem from giving special privileges to
-systems as flexible as scripts, on many versions of Unix, setuid scripts
+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-setuid) interpreter turns
+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 the setuid bit set, which doesn't help much.
-Alternately, it can simply ignore the setuid bit 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 setuid script feature isn't disabled, Perl will
-complain loudly that your setuid script is insecure. You'll need to
-either disable the kernel setuid script feature, or put a C wrapper around
+outlaw scripts with any set-id bit set, which doesn't help much.
+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
+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 setuid scripts. Here's a simple wrapper, written
+kernel bug that plagues set-id scripts. Here's a simple wrapper, written
in C:
#define REAL_PATH "/path/to/script"
- main(ac, av)
+ main(ac, av)
char **av;
{
execv(REAL_PATH, av);
- }
-
-Compile this wrapper into a binary executable and then make I<it> rather
-than your script setuid or setgid.
+ }
-See the program B<wrapsuid> in the F<eg> directory of your Perl
-distribution for a convenient way to do this automatically for all your
-setuid Perl programs. It moves setuid scripts into files with the same
-name plus a leading dot, and then compiles a wrapper like the one above
-for each of them.
+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 setuid script to open to the interpreter, rather than using a
+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>
+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 5.003 of Perl, a bug in the code of B<suidperl> could
-introduce a security hole in systems compiled with strict POSIX
-compliance.
+=head2 Protecting Your Programs
+
+There are a number of ways to hide the source to your Perl programs,
+with varying levels of "security".
+
+First of all, however, you I<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 indeed.
+
+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 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 license's wording will
+stand up in court.
+
+=head2 Unicode
+
+Unicode is a new and complex technology and one may easily overlook
+certain security pitfalls. See L<perluniintro> for an overview and
+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 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.
+
+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
+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 B<not> be used for
+applications like shuffling a list randomly (use 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.
+
+=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 is 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<http://www.cs.rice.edu/~scrosby/hash/> for more information,
+and any computer science textbook on algorithmic complexity.
+
+=head1 SEE ALSO
+
+L<perlrun> for its description of cleaning up environment variables.
https://perl5.git.perl.org / perl5.git / blobdiff