PERLSEC
Section: Perl Programmers Reference Guide (1)
Updated: 2017-06-30
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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:
- *
-
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 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.
Security Bugs
Beyond the obvious problems that stem from giving special privileges to
systems as flexible as scripts, on many versions of Unix, set-id scripts
are inherently insecure right from the start. The problem is a race
condition in the kernel. Between the time the kernel opens the file to
see which interpreter to run and when the (now-set-id) 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.
Fortunately, sometimes this kernel ``feature'' can be disabled.
Unfortunately, there are two ways to disable it. The system can simply
outlaw scripts with any set-id bit set, which doesn't help much.
Alternately, it can simply ignore the set-id bits on scripts.
However, if the kernel set-id script feature isn't disabled, Perl will
complain loudly that your set-id script is insecure. You'll need to
either disable the kernel set-id script feature, or put a C wrapper around
the script. A C wrapper is just a compiled program that does nothing
except call your Perl program. Compiled programs are not subject to the
kernel bug that plagues set-id scripts. Here's a simple wrapper, written
in C:
#define REAL_PATH "/path/to/script"
main(ac, av)
char **av;
{
execv(REAL_PATH, av);
}
Compile this wrapper into a binary executable and then make it rather
than your script setuid or setgid.
In recent years, vendors have begun to supply systems free of this
inherent security bug. On such systems, when the kernel passes the name
of the set-id script to open to the interpreter, rather than using a
pathname subject to meddling, it instead passes /dev/fd/3. This is a
special file already opened on the script, so that there can be no race
condition for evil scripts to exploit. On these systems, Perl should be
compiled with "-DSETUID_SCRIPTS_ARE_SECURE_NOW". The Configure
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.
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 is 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 <http://www.cs.rice.edu/~scrosby/hash/> for more information,
and any computer science textbook on algorithmic complexity.
SEE ALSO
perlrun for its description of cleaning up environment variables.
Index
- NAME
-
- DESCRIPTION
-
- SECURITY VULNERABILITY CONTACT INFORMATION
-
- SECURITY MECHANISMS AND CONCERNS
-
- Taint mode
-
- Laundering and Detecting Tainted Data
-
- Switches On the #! Line
-
- Taint mode and @INC
-
- Cleaning Up Your Path
-
- Security Bugs
-
- Protecting Your Programs
-
- Unicode
-
- Algorithmic Complexity Attacks
-
- SEE ALSO
-
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