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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
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:
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::system, in which case they are able to run arbitrary external code.
o Hash keys are never tainted.
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}".
The exception to the principle of "one tainted value taints the whole expression" is with
the ternary conditional operator "?:". Since code with a ternary conditional
$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" 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.
Note that if a tainted string is added to @INC, the following 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 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.
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 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.
o Regular expressions - Perl's regular expression engine is so called NFA (Non-
deterministic 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.
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 similarly fooled.
See <http://www.cs.rice.edu/~scrosby/hash/> for more information, and any computer science
textbook on the algorithmic complexity.
SEE ALSO
perlrun for its description of cleaning up environment variables.
perl v5.10.0 2011-06-30 PERLSEC(1)
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On Apache
Under GNU General Public License
2012-05-26 01:51 @38.107.179.240 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)