PERLSEC(1) Perl Programmers Reference Guide PERLSEC(1)NAMEperlsec - 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 the perllocale man
page), results of certain system calls (readdir(), read_
link(), 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". 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:
If you pass a list of arguments to either "system" or
"exec", the elements of that list are not checked for
taintedness.
Arguments to "print" and "syswrite" are not checked
for taintedness.
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 tainted
ness 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>; # insecure (uses readdir() or similar)
@files = glob('*.c'); # insecure (uses readdir() or similar)
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 system or exec, but only by explicitly doing
something like the "considered secure" example above.
Laundering and Detecting Tainted Data
To test whether a variable contains tainted data, and
whose use would thus trigger an "Insecure dependency" mes
sage, check your nearby CPAN mirror for the Taint.pm mod
ule, which should become available around November 1997.
Or you may be able to use the following is_tainted() func
tion.
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. Some
times you have just to clear your data's taintedness. The
only way to bypass the tainting mechanism is by referenc
ing subpatterns from a regular expression match. Perl
presumes that if you reference a substring using $1, $2,
etc., that you knew what you were doing when you wrote the
pattern. That means using a bit of thought--don't just
blindly untaint anything, or you defeat the entire mecha
nism. It's better to verify that the variable has only
good characters (for certain values of "good") rather than
checking whether it has any bad characters. That's
because it's far too easy to miss bad characters that you
never thought of.
Here's a test to make sure that the data contains nothing
but "word" characters (alphabetics, numerics, and under
scores), a hyphen, an at sign, or a dot.
if ($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 $data if "use locale" is in
effect, because the characters matched by "\w" are deter
mined by the locale. Perl considers that locale defini
tions are untrustworthy because they contain data from
outside the program. If you are writing a locale-aware
program, and want to launder data with a regular expres
sion containing "\w", put "no locale" ahead of the expres
sion in the same block. See the SECURITY entry in the
perllocale manpage for further discussion and examples.
Switches On the "#!" Line
When you make a script executable, in order to make it
usable as a command, the system will pass switches to perl
from the script's #! line. Perl checks that any command
line switches given to a setuid (or setgid) script actu
ally match the ones set on the #! line. Some Unix and
Unix-like environments impose a one-switch limit on the #!
line, so you may need to use something like "-wU" instead
of "-w -U" under such systems. (This issue should arise
only in Unix or Unix-like environments that support #! and
setuid or setgid scripts.)
Cleaning Up Your Path
For "Insecure "$ENV{PATH}"" messages, you need to set
"$ENV{'PATH'}" to a known value, and each directory in the
path must be 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.
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. Unfortu
nately, the open, 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 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;
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
"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). 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.
SEE ALSO
the perlrun manpage for its description of cleaning up
environment variables.
2001-03-18 perl v5.6.1 PERLSEC(1)
