PERLSEC(1) Perl Programmers Reference Guide PERLSEC(1) #
PERLSEC(1) Perl Programmers Reference Guide PERLSEC(1)
NNAAMMEE #
perlsec - Perl security
DDEESSCCRRIIPPTTIIOONN #
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.
SSEECCUURRIITTYY VVUULLNNEERRAABBIILLIITTYY CCOONNTTAACCTT IINNFFOORRMMAATTIIOONN #
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.
SSEECCUURRIITTYY MMEECCHHAANNIISSMMSS AANNDD CCOONNCCEERRNNSS #
TTaaiinntt mmooddee By default, Perl automatically enables a set of special security checks, called _t_a_i_n_t _m_o_d_e, 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 --TT command line flag. This flag is _s_t_r_o_n_g_l_y 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 _t_a_i_n_t _c_h_e_c_k_s 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, wwiitthh tthhee
ffoolllloowwiinngg eexxcceeppttiioonnss:
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.
• Arguments to "print" and "syswrite" are nnoott 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 oouuttssiiddee your Perl code, such as POSIX::system, in
which case they are able to run arbitrary external code.
• Hash keys are nneevveerr 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 nneevveerr 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.
LLaauunnddeerriinngg aanndd DDeetteeccttiinngg TTaaiinntteedd DDaattaa 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 _o_n_l_y 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.
SSwwiittcchheess OOnn tthhee “”##!!“” LLiinnee 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.)
TTaaiinntt mmooddee aanndd @@IINNCC +When the 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.
CClleeaanniinngg UUpp YYoouurr PPaatthh 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 _n_o_t 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 aafftteerr 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.
SShheebbaanngg RRaaccee CCoonnddiittiioonn 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 _/_d_e_v_/_f_d_/_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 _C_o_n_f_i_g_u_r_e 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 _i_t 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.
PPrrootteeccttiinngg YYoouurr PPrrooggrraammss There are a number of ways to hide the source to your Perl programs, with varying levels of “security”.
First of all, however, you _c_a_n_'_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.
UUnniiccooddee 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.
AAllggoorriitthhmmiicc CCoommpplleexxiittyy AAttttaacckkss 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 _D_e_n_i_a_l _o_f _S_e_r_v_i_c_e (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.
PPeerrll hhaass nneevveerr gguuaarraanntteeeedd aannyy oorrddeerriinngg ooff tthhee hhaasshh kkeeyyss, 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 nnoott 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 ssoorrtt(()) 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.
UUssiinngg SSuuddoo 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.
SSEEEE AALLSSOO #
"ENVIRONMENT" in perlrun for its description of cleaning up environment
variables.
perl v5.36.3 2023-02-15 PERLSEC(1)