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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 perllocale),
results of certain system calls (readdir(), readlink(), 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 indirectly in any
command that invokes a sub-shell, nor in any command that modifies files,
directories, or processes, with the following exceptions:
· Arguments to "print" and "syswrite" are not checked for taintedness.
· 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::system, 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 others 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 = <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; # 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}".
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 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()" function.
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. Sometimes 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, $2, etc., that you knew what
you were doing when you wrote the pattern. That means using a bit of
thought--don't just blindly untaint anything, or you defeat the entire
mechanism. It's better to verify that the variable has only good
characters (for certain values of "good") rather than checking whether it
has any bad characters. That's because it's far too easy to miss bad
characters that you never thought of.
Here's a test to make sure that the data contains nothing but "word"
characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
or a dot.
if ($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 "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
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.)
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 visible
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 directories, while the later will not.
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 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.
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 "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.
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.
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 5.6.1 of Perl, bugs in the code of suidperl 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 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 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.
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.
· 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 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 not be used for applications
like shuffling a list randomly (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.
· 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 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 perlfaq2). Running out of space manifests itself by Perl
running out of memory.
· 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 similarly fooled.
See <http://www.cs.rice.edu/~scrosby/hash/> for more information, and any
computer science text book on the algorithmic complexity.
SEE ALSO
perlrun for its description of cleaning up environment variables.
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