PERLSEC(1)
NNAAMMEE
perlsec - Perl security
DDEESSCCRRIIPPTTIIOONN
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
language 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 --TT
command 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
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 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 the perllocale
manpage), results of certain system calls (readdir,
readlink, the gecos field of getpw* 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. Any variable set to a value
derived from tainted data will itself be tainted, even if
it is logically impossible for the tainted data to alter
the variable. Because taintedness is associated with each
scalar value, some elements of an array can be tainted and
others not.
For example:
$arg = shift; # $arg is tainted
$hid = $arg, 'bar'; # $hid is also tainted
$line = <>; # Tainted
$line = <STDIN>; # Also tainted
open FOO, "/home/me/bar" or die $!;
$line = <FOO>; # Still tainted
$path = $ENV{'PATH'}; # Tainted, but see below
$data = 'abc'; # Not tainted
system "echo $arg"; # Insecure
system "/bin/echo", $arg; # Secure (doesn't use sh)
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, but...
open(FOO,"-|")
or exec 'echo', $arg; # OK
$shout = `echo $arg`; # Insecure, $shout now tainted
unlink $data, $arg; # Insecure
umask $arg; # Insecure
exec "echo $arg"; # Insecure
exec "echo", $arg; # Secure (doesn't use the shell)
exec "sh", '-c', $arg; # Considered secure, alas!
@files = <*.c>; # Always insecure (uses csh)
@files = glob('*.c'); # Always insecure (uses csh)
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 ssyysstteemm or eexxeecc, but only by explicitly doing
something like the last example above.
LLaauunnddeerriinngg aanndd DDeetteeccttiinngg TTaaiinntteedd DDaattaa
To test whether a variable contains tainted data, and
whose use would thus trigger an "Insecure dependency"
message, check your nearby CPAN mirror for the Taint.pm
module, which should become available around November
1997. Or you may be able to use the following
is_tainted() function.
sub is_tainted {
return ! eval {
join('',@_), kill 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.
Sometimes you have just to clear your data's taintedness.
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
underscores), 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 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 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
determined by the locale. Perl considers that locale
definitions 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
expression containing \w, put no locale ahead of the
expression in the same block. See the SECURITY entry in
the perllocale manpage for further discussion and
examples.
SSwwiittcchheess OOnn tthhee """"##!!"""" LLiinnee
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
actually 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.)
CClleeaanniinngg UUpp YYoouurr PPaatthh
For "Insecure $ENV{PATH}" messages, you need to set
$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 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.
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 aafftteerr 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 ssyysstteemm and eexxeecc explicit parameter lists instead of
strings with possible shell wildcards in them.
Unfortunately, the ooppeenn, gglloobb, 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 ooppeenn 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 ooppeenn 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 eexxeecc 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;
die "Can't fork: $!" unless defined $pid = open(KID, "-|");
if ($pid) { # parent
while (<KID>) {
# do something
}
close KID;
} else {
my @temp = ($EUID, $EGID);
$EUID = $UID;
$EGID = $GID; # XXX: initgroups() not called
# Make sure privs are really gone
($EUID, $EGID) = @temp;
die "Can't drop privileges" unless
$UID == $EUID and
$GID eq $EGID; # String test
$ENV{PATH} = "/bin:/usr/bin";
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
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.
SSeeccuurriittyy BBuuggss
Beyond the obvious problems that stem from giving special
privileges to 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-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 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 ssuuiiddppeerrll 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
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.
See the program wwrraappssuuiidd in the 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.
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 /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
-DSETUID_SCRIPTS_ARE_SECURE_NOW. The CCoonnffiigguurree 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
ssuuiiddppeerrll could introduce a security hole in systems
compiled with strict POSIX compliance.
PPrrootteeccttiinngg YYoouurr PPrrooggrraammss
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
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.
Some people 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). 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
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 stand up in court.