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.
SECURITY VULNERABILITY CONTACT INFORMATION
If you believe you have found a security vulnerability in the Perl
interpreter or modules maintained in the core Perl codebase, email the
details to perl-security@perl.org <mailto:perl-security@perl.org>. This
address is a closed membership mailing list monitored by the Perl
security team.
See perlsecpolicy for additional information.
SECURITY MECHANISMS AND CONCERNS
Taint mode
By default, 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.
Support for taint checks adds an overhead to all Perl programs, whether
or not you're using the taint features. Perl 5.18 introduced C
preprocessor symbols that can be used to disable the taint features when
building perl.
When taint is enabled, 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.
• 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.
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 {
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 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 taint mode ("-T") is in effect, the environment variables
"PERL5LIB", "PERLLIB", and "PERL_USE_UNSAFE_INC" 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 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
On versions of Perl before 5.26, activating taint mode will also remove
the current directory (".") from the default value of @INC. Since
version 5.26, the current directory isn't included in @INC by default.
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;
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.
Shebang Race Condition
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.
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 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
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 Algorithm - Hash algorithms like the one used in Perl are well
known to be vulnerable to collision attacks on their hash function.
Such attacks involve constructing a set of keys which collide into
the same bucket producing inefficient behavior. Such attacks often
depend on discovering the seed of the hash function used to map the
keys to buckets. That seed is then used to brute-force a key set
which can be used to mount a denial of service attack. In Perl
5.8.1 changes were introduced to harden Perl to such attacks, and
then later in Perl 5.18.0 these features were enhanced and
additional protections added.
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:
Hash Seed Randomization
In order to make it impossible to know what seed to generate an
attack key set for, this seed is randomly initialized at process
start. This may be overridden by using the PERL_HASH_SEED
environment variable, see "PERL_HASH_SEED" in perlrun. This
environment variable controls how items are actually stored, not
how they are presented via "keys", "values" and "each".
Hash Traversal Randomization
Independent of which seed is used in the hash function, "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 Hash::Util or by using the
PERL_PERTURB_KEYS environment variable, see "PERL_PERTURB_KEYS"
in perlrun. Note that this feature controls the "visible" order
of the keys, and not the actual order they are stored in.
Bucket Order Perturbance
When items collide into a given hash bucket the order they are
stored in the chain is no longer predictable in Perl 5.18. This
has the intention to make it harder to observe a collision.
This behavior can be overridden by using the PERL_PERTURB_KEYS
environment variable, see "PERL_PERTURB_KEYS" in perlrun.
New Default Hash Function
The default hash function has been modified with the intention
of making it harder to infer the hash seed.
Alternative Hash Functions
The source code includes multiple hash algorithms to choose
from. While we believe that the default perl hash is robust to
attack, we have included the hash function Siphash as a fall-
back option. At the time of release of Perl 5.18.0 Siphash is
believed to be of cryptographic strength. This is not the
default as it is much slower than the default hash.
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 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.
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 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 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.
Using Sudo
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 suitability 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.
SEE ALSO
"ENVIRONMENT" in perlrun for its description of cleaning up environment
variables.
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