PERLSEC(1) 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 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 -T 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
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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 system or exec, but only by explicitly doing
something like the last example above.
Laundering and Detecting Tainted Data
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;
};
}
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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.
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 actually match the ones set on the #! line. Some Unix and
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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.
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 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 system
and exec explicit parameter lists instead of strings with possible shell
wildcards in them. Unfortunately, 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 process 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 something that might be subjected to shell escapes: just
never call the shell at all.
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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.
Security Bugs
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.
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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 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 wrapsuid 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 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 suidperl could
introduce a security hole in systems compiled with strict POSIX
compliance.
Protecting Your Programs
There are a number of ways to hide the source to your Perl programs, with
varying levels of "security".
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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.
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