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 [email protected]
with details. This points to a closed subscription, unarchived mailing list. Please only use this address
for security issues in the Perl core, not for modules independently distributed on CPAN.
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
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 Perl releases older than 5.6.0 the <*.c> and glob('*.c') would
# have used an external program to do the filename expansion; but in
# either case the result is 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";
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, $2, etc., that you knew what you were doing when you wrote the pattern. That means
using a bit of thought--don't just blindly untaint anything, or you defeat the entire mechanism. It's better
to verify that the variable has only good characters (for certain values of "good") rather than checking
whether it has any bad characters. That's because it's far too easy to miss bad characters that you never
thought of.
Here's a test to make sure that the data contains nothing but "word" characters (alphabetics, numerics, and
underscores), a hyphen, an at sign, or a dot.
if ($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
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 '-no_match_vars';
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;
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".
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 Function - the algorithm used to "order" hash elements has been changed several times during the
development of Perl, mainly to be reasonably fast. In Perl 5.8.1 also the security aspect was taken into
account.
In Perls before 5.8.1 one could rather easily generate data that as hash keys would cause Perl to consume
large amounts of time because internal structure of hashes would badly degenerate. In Perl 5.8.1 the hash
function is randomly perturbed by a pseudorandom seed which makes generating such naughty hash keys
harder. See "PERL_HASH_SEED" in perlrun for more information.
In Perl 5.8.1 the random perturbation was done by default, but as of 5.8.2 it is only used on individual
hashes if the internals detect the insertion of pathological data. If one wants for some reason emulate
the old behaviour (and expose oneself to DoS attacks) one can set the environment variable PERL_HASH_SEED
to zero to disable the protection (or any other integer to force a known perturbation, rather than
random). One possible reason for wanting to emulate the old behaviour is that in the new behaviour
consecutive runs of Perl will order hash keys differently, which may confuse some applications (like
Data::Dumper: the outputs of two different runs are no longer identical).
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.
Also note that while the order of the hash elements might be randomised, this "pseudoordering" 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.
· 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.
perl v5.16.3 2013-03-04 PERLSEC(1)
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