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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 possi-
bly 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 pro-
gram 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 some-
thing else outside your program--at least, not by accident. All com-
mand line arguments, environment variables, locale information (see
perllocale), 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 "getp-
wxxx()" 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 tainted-
ness.
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.
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 = ; # Also tainted
open FOO, "/home/me/bar" or die $!;
$line = ; # 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()" func-
tion.
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 expres-
sion 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 spe-
cial 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 con-
taining "\w", put "no locale" ahead of the expression in the same
block. See "SECURITY" in perllocale for further discussion and exam-
ples.
Switches On the "#!" Line
When you make a script executable, in order to make it usable as a com-
mand, the system will pass switches to perl from the script's #! line.
Perl checks that any command line switches given to a setuid (or set-
gid) 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 environ-
ments 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 qual-
ified. 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 "sys-
tem" and "exec" explicit parameter lists instead of strings with possi-
ble 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 ($pid) { # parent
while () {
# do something
}
close KID;
} else {
my @temp = ($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 some-
thing 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 oper-
ations 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) inter-
preter 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. Unfortu-
nately, 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 sub-
ject 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 pro-
gram 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 condi-
tion.
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 pro-
gram 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 view-
ing 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-com-
pile 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 perlu-
nicode for details, and "Security Implications of Unicode" in perluni-
code 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 possi-
ble reason for wanting to emulate the old behaviour is that in the
new behaviour consecutive runs of Perl will order hash keys differ-
ently, 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 con-
tinues to be, affected by the insertion order.
Also note that while the order of the hash elements might be ran-
domised, 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 per-
mutations (use e.g. the CPAN modules Algorithm::Permute or Algo-
rithm::FastPermute), or for any cryptographic applications.
o 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.
o Sorting - the quicksort algorithm used in Perls before 5.8.0 to
implement the sort() function is very easy to trick into misbehav-
ing 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 for more information, and
any computer science text book on the algorithmic complexity.
SEE ALSO
perlrun for its description of cleaning up environment variables.
perl v5.8.8 2006-01-07 PERLSEC(1)
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