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.
If you believe you have found a security vulnerability in Perl, please email
perl5-security-report@perl.org 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 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 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";
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 {
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 "-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 later
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 '-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;
# 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 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.
- •
- 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.