| perlsec - Perl security |
perlsec - Perl security
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.
If you believe you have found a security vulnerability in Perl, please email the details to perl5-security-report@perl.org. This creates a new Request Tracker ticket in a special queue which isn't initially publicly accessible. The email will also be copied to a closed subscription unarchived mailing list which includes all the core committers, who will be able to help assess the impact of issues, figure out a resolution, and help co-ordinate the release of patches to mitigate or fix the problem across all platforms on which Perl is supported. Please only use this address for security issues in the Perl core, not for modules independently distributed on CPAN.
When sending an initial request to the security email address, please
don't Cc any other parties, because if they reply to all, the reply will
generate yet another new ticket. Once you have received an initial reply
with a [perl #NNNNNN] ticket number in the headline, it's okay to Cc
subsequent replies to third parties: all emails to the
perl5-security-report address with the ticket number in the subject line
will be added to the ticket; without it, a new ticket will be created.
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 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:
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.
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 either case, the results of glob are 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.
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 {
local $@; # Don't pollute caller's value.
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, $2, etc. in a non-tainting pattern, that you knew what you were doing when you wrote that pattern. That means using a bit of thought--don't just blindly untaint anything, or you defeat the entire mechanism. It's better to verify that the variable has only good characters (for certain values of ``good'') rather than checking whether it has any bad characters. That's because it's far too easy to miss bad characters that you never thought of.
Here's a test to make sure that the data contains nothing but ``word'' characters (alphabetics, numerics, and underscores), a hyphen, an at sign, or a dot.
if ($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 the perllocale manpage for further discussion and examples.
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.)
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
the perlrun manpage. 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 latter
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
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;
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, you might want to 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. Safe should not be considered bullet-proof, though: it will not prevent the foreign code to set up infinite loops, allocate gigabytes of memory, or even abusing perl bugs to make the host interpreter crash or behave in unpredictable ways. In any case it's better avoided completely if you're really concerned about security.
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.
Some Unixes, especially more recent ones, are 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.
If you don't have the safe version of set-id scripts, all is not lost. Sometimes this kernel ``feature'' can be disabled, so that the kernel either doesn't run set-id scripts with the set-id or doesn't run them at all. Either way avoids the exploitability of the race condition, but doesn't help in actually running scripts set-id.
If the kernel set-id script feature isn't disabled, then any set-id script provides an exploitable vulnerability. Perl can't avoid being exploitable, but will point out vulnerable scripts where it can. If Perl detects that it is being applied to a set-id script then it will complain loudly that your set-id script is insecure, and won't run it. When Perl complains, you need to remove the set-id bit from the script to eliminate the vulnerability. Refusing to run the script doesn't in itself close the vulnerability; it is just Perl's way of encouraging you to do this.
To actually run a script set-id, if you don't have the safe version of set-id scripts, you'll need to 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:
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#define REAL_PATH "/path/to/script"
int main(int argc, char **argv)
{
execv(REAL_PATH, argv);
fprintf(stderr, "%s: %s: %s\n",
argv[0], REAL_PATH, strerror(errno));
return 127;
}
Compile this wrapper into a binary executable and then make it rather than your script setuid or setgid. Note that this wrapper isn't doing anything to sanitise the execution environment other than ensuring that a safe path to the script is used. It only avoids the shebang race condition. It relies on Perl's own features, and on the script itself being careful, to make it safe enough to run the script set-id.
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 license 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 license's wording will stand up in court.
Unicode is a new and complex technology and one may easily overlook certain security pitfalls. See the perluniintro manpage for an overview and the perlunicode manpage for details, and Security Implications of Unicode in the perlunicode manpage for security implications in particular.
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.
At the time of this writing, Perl 5.18.0 is considered to be well-hardened against algorithmic complexity attacks on its hash implementation. This is largely owed to the following measures mitigate attacks:
keys, values and each.
keys,
values, and each return items in a per-hash randomized order.
Modifying a hash by insertion will change the iteration order of that hash.
This behavior can be overridden by using hash_traversal_mask() from
the Hash::Util manpage or by using the PERL_PERTURB_KEYS environment variable,
see PERL_PERTURB_KEYS in the perlrun manpage. Note that this feature controls the
``visible'' order of the keys, and not the actual order they are stored in.
Without compiling a special Perl, there is no way to get the exact same behavior of any versions prior to Perl 5.18.0. The closest one can get is by setting PERL_PERTURB_KEYS to 0 and setting the PERL_HASH_SEED to a known value. We do not advise those settings for production use due to the above security considerations.
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 and the history of changes made to the hash over its lifetime.
Also note that while the order of the hash elements might be
randomized, this ``pseudo-ordering'' should not be used for
applications like shuffling a list randomly (use List::Util::shuffle()
for that, see the List::Util manpage, 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.
Tied hashes may have their own ordering and algorithmic complexity attacks.
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 the perlfaq2 manpage). 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 thesort() function was very easy to trick into misbehaving
so that it consumes a lot of time. Starting from Perl 5.8.0 a different
sorting algorithm, mergesort, is used by default. Mergesort cannot
misbehave on any input.
See https://www.usenix.org/legacy/events/sec03/tech/full_papers/crosby/crosby.pdf for more information, and any computer science textbook on algorithmic complexity.
The popular tool sudo provides a controlled way for users to be able
to run programs as other users. It sanitises the execution environment
to some extent, and will avoid the shebang race condition. If you don't have the safe version of set-id scripts,
then sudo may be a more convenient way of executing a script as
another user than writing a C wrapper would be.
However, sudo sets the real user or group ID to that of the target
identity, not just the effective ID as set-id bits do. As a result, Perl
can't detect that it is running under sudo, and so won't automatically
take its own security precautions such as turning on taint mode. Where
sudo configuration dictates exactly which command can be run, the
approved command may include a -T option to perl to enable taint mode.
In general, it is necessary to evaluate the suitaility of a script to
run under sudo specifically with that kind of execution environment
in mind. It is neither necessary nor sufficient for the same script to
be suitable to run in a traditional set-id arrangement, though many of
the issues overlap.
the perlrun manpage for its description of cleaning up environment variables.
| perlsec - Perl security |