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 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.
SECURITY MECHANISMS AND CONCERNS
Taint mode
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 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 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 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
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.
o 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.
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 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 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.
SEE ALSO
perlrun for its description of cleaning up environment variables.
perl v5.30.3 2020-06-07 PERLSEC(1)