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PERLSEC(1) Perl Programmers Reference Guide PERLSEC(1)
NAME
perlsec - Perl security
DESCRIPTION
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 each line of the
script, Perl uses a more conventional evaluation scheme
with fewer hidden snags. Additionally, because the lan-
guage has more 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 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 -T com-
mand line flag. This flag is 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
the remainder of your script.
While in this mode, Perl takes special precautions called
taint checks to prevent both obvious and subtle traps.
Some of these checks are reasonably simple, such as veri-
fying 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 set-id Perl program more secure than the cor-
responding 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 perllocale), results of
certain system calls ("readdir()", "readlink()", the vari-
able of "shmread()", the messages returned by "msgrcv()",
the password, gcos and shell fields returned by the "getp-
wxxx()" calls), and all file input are marked as
"tainted". Tainted data may not be used directly or indi-
rectly in any command that invokes a sub-shell, nor in any
command that modifies files, directories, or processes,
with the following exceptions:
o Arguments to "print" and "syswrite" are not checked
for taintedness.
o 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 outside your Perl code, such as POSIX::sys-
tem, in which case they are able to run arbitrary
external code.
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 oth-
ers not. The keys of a hash are never tainted.
For example:
$arg = shift; # $arg is tainted
$hid = $arg, 'bar'; # $hid is also tainted
$line = <>; # Tainted
$line = ; # Also tainted
open FOO, "/home/me/bar" or die $!;
$line = ; # Still tainted
$path = $ENV{'PATH'}; # Tainted, but see below
$data = 'abc'; # Not tainted
system "echo $arg"; # 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
$path = $ENV{'PATH'}; # $path now tainted
$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
open(FOO,"-|")
or exec 'echo', $arg; # Also not OK
$shout = `echo $arg`; # Insecure, $shout now tainted
unlink $data, $arg; # Insecure
umask $arg; # Insecure
exec "echo $arg"; # Insecure
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 $ENV{PATH}".
The exception to the principle of "one tainted value
taints the whole expression" is with the ternary condi-
tional operator "?:". Since code with a ternary condi-
tional
$result = $tainted_value ? "Untainted" : "Also untainted";
is effectively
if ( $tainted_value ) {
$result = "Untainted";
} else {
$result = "Also untainted";
}
it doesn't make sense for $result to be tainted.
Laundering and Detecting Tainted Data
To test whether a variable contains tainted data, and
whose use would thus trigger an "Insecure dependency" mes-
sage, you can use the "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 "is_tainted()" func-
tion.
sub is_tainted {
return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
}
This function makes use of the fact that the presence of
tainted data anywhere within an expression renders the
entire expression tainted. It would be inefficient for
every operator to test every argument for taintedness.
Instead, the slightly more efficient and conservative
approach is used that if any tainted value has been
accessed within the same expression, the whole expression
is considered tainted.
But testing for taintedness gets you only so far. Some-
times you have just 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, $2, etc., that you knew what you were doing when
you wrote the pattern. That means using a bit of
thought--don't just blindly untaint anything, or you
defeat the entire mechanism. It's better to verify that
the variable has only good characters (for certain values
of "good") rather than checking whether it has any bad
characters. That's because it's far too easy to miss bad
characters that you never thought of.
Here's a test to make sure that the data contains nothing
but "word" characters (alphabetics, numerics, and under-
scores), a hyphen, an at sign, or a dot.
if ($data =~ /^([-\@\w.]+)$/) {
$data = $1; # $data now untainted
} else {
die "Bad data in '$data'"; # log this somewhere
}
This is fairly secure because "/\w+/" doesn't normally
match shell metacharacters, nor are dot, dash, or at going
to mean something special to the shell. Use of "/.+/"
would have been insecure in theory because it lets every-
thing through, but Perl doesn't check for that. The les-
son is that when untainting, you must be exceedingly care-
ful with your patterns. Laundering data using regular
expression is the 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 "use locale" is in
effect, because the characters matched by "\w" are deter-
mined by the locale. Perl considers that locale defini-
tions are untrustworthy because they contain data from
outside the program. If you are writing a locale-aware
program, and want to launder data with a regular expres-
sion containing "\w", put "no locale" ahead of the expres-
sion in the same block. See "SECURITY" in perllocale for
further discussion and examples.
Switches On the "#!" Line
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 (or setgid) script actu-
ally match the ones set on the #! line. Some Unix and
Unix-like environments impose a one-switch limit on the #!
line, so you may need to use something like "-wU" instead
of "-w -U" under such systems. (This issue should arise
only in Unix or Unix-like environments that support #! and
setuid or setgid scripts.)
Taint mode and @INC
When the taint mode ("-T") is in effect, the "." directory
is removed from @INC, and the environment variables
"PERL5LIB" and "PERLLIB" are ignored by Perl. You can
still adjust @INC from outside the program by using the
"-I" command line option as explained in 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 "-I" option is clearly vis-
ible and therefore permitted.
Another way to modify @INC without modifying the program,
is to use the "lib" pragma, e.g.:
perl -Mlib=/foo program
The benefit of using "-Mlib=/foo" over "-I/foo", is that
the former will automagically remove any duplicated direc-
tories, while the later will not.
Note that if a tainted string is added to @INC, the fol-
lowing problem will be reported:
Insecure dependency in require while running with -T switch
Cleaning Up Your Path
For "Insecure $ENV{PATH}" messages, you need to set
$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 quali-
fied. This is 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 opera-
tions 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 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 mis-
takes, not to remove the need for thought.
Perl does not call the shell to expand wild cards when you
pass "system" and "exec" explicit parameter lists instead
of strings with possible shell wildcards in them. Unfor-
tunately, the "open", "glob", and backtick functions pro-
vide no such alternate calling convention, so more sub-
terfuge 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 pro-
cess with reduced privilege who does the dirty work for
you. First, fork a child using the special "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 "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 "exec" is not called with a string that the shell
could expand. This is by far the best way to call some-
thing that might be subjected to shell escapes: just never
call the shell at all.
use English '-no_match_vars';
die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
if ($pid) { # parent
while () {
# do something
}
close KID;
} else {
my @temp = ($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
"glob", although you can use "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 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 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 mod-
ule, included standard in the Perl distribution. This
module allows the programmer to set up special compart-
ments in which all system operations are trapped and
namespace access is carefully controlled.
Security Bugs
Beyond the obvious problems that stem from giving special
privileges to systems as flexible as scripts, on many ver-
sions 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-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 dis-
abled. 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
suidperl that is automatically invoked for you if it's
needed.
However, if the kernel set-id script feature isn't dis-
abled, 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 set-id
scripts. Here's a simple wrapper, written in C:
#define REAL_PATH "/path/to/script"
main(ac, av)
char **av;
{
execv(REAL_PATH, av);
}
Compile this wrapper into a binary executable and then
make 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 med-
dling, it instead passes /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 sys-
tems, Perl should be compiled with "-DSE-
TUID_SCRIPTS_ARE_SECURE_NOW". The 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 suid-
perl could introduce a security hole.
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 can't take away read permis-
sion, 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
licence will give you legal security. License your soft-
ware 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 stand up in court.
Unicode
Unicode is a new and complex technology and one may easily
overlook certain security pitfalls. See perluniintro for
an overview and perlunicode for details, and "Security
Implications of Unicode" in perlunicode for security
implications in particular.
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 Denial of Service (DoS)
attacks.
o 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 "PERL_HASH_SEED" in perlrun 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).
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 order-
ing 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 not
be used for applications like shuffling a list ran-
domly (use List::Util::shuffle() for that, see
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.
o 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
expression may match in several ways. Careful craft-
ing 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 perlfaq2). Running out of space manifests itself
by Perl running out of memory.
o 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 simi-
larly fooled.
See for more
information, and any computer science text book on the
algorithmic complexity.
SEE ALSO
perlrun for its description of cleaning up environment
variables.
perl v5.8.8 2006-01-07 PERLSEC(1)