PERLSEC(1) Perl Programmers Reference Guide PERLSEC(1)NAMEperlsec - 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.
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 vari-
ables, locale information (see perllocale), results of cer-
tain 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 excep-
tions:
+ Arguments to "print" and "syswrite" are not checked for
taintedness.
+ Symbolic methods
$obj->$method(@args);
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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.
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
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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 opera-
tor "?:". 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.
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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 con-
sidered 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, $2, etc., that
you knew what you were doing when you wrote the pattern.
That means using a bit of thought--don't just blindly
untaint anything, or you defeat the entire mechanism. It's
better to verify that the variable has only good characters
(for certain values of "good") rather than checking whether
it has any bad characters. That's because it's far too easy
to miss bad characters that you never thought of.
Here's a test to make sure that the data contains nothing
but "word" characters (alphabetics, numerics, and under-
scores), a hyphen, an at sign, or a dot.
if ($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 pat-
terns. 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 deter-
mined 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
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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 set-
gid 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" com-
mand line option as explained in perlrun. The two environ-
ment 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 follow-
ing 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
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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 judi-
cious 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. Unfor-
tunately, 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
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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 your-
self 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 pro-
grams.
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, check out the Safe module, included
standard in the Perl distribution. This module allows the
programmer to set up special compartments in which all sys-
tem operations are trapped and namespace access is carefully
controlled.
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Security Bugs
Beyond the obvious problems that stem from giving special
privileges to systems as flexible as scripts, on many ver-
sions of Unix, set-id scripts are inherently insecure right
from the start. The problem is a race condition in the ker-
nel. Between the time the kernel opens the file to see
which interpreter to run and when the (now-set-id) inter-
preter 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 dis-
abled. 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. If the latter is true, Perl can
emulate the setuid and setgid mechanism when it notices the
otherwise useless setuid/gid bits on Perl scripts. It does
this via a special executable called suidperl that is
automatically invoked for you if it's needed.
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 med-
dling, 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
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releases of SysVr4 and BSD 4.4 use this approach to avoid
the kernel race condition.
Prior to release 5.6.1 of Perl, bugs in the code of suidperl
could introduce a security hole.
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
licence 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 licence'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
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Implications of Unicode" in perlunicode for security impli-
cations in particular.
Algorithmic Complexity Attacks
Certain internal algorithms used in the implementation of
Perl can be attacked by choosing the input carefully to con-
sume 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 ele-
ments has been changed several times during the develop-
ment 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.
The random perturbation is done by default but if one
wants for some reason emulate the old behaviour one can
set the environment variable PERL_HASH_SEED to zero (or
any other integer). One possible reason for wanting to
emulate the old behaviour is that in the new behaviour
consecutive runs of Perl will order hash keys dif-
ferently, which may confuse some applications (like
Data::Dumper: the outputs of two different runs are no
more identical).
Perl has never guaranteed any ordering of the hash keys,
and the ordering has already changed several times dur-
ing 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-Finite Automaton), which among
other things means that it can rather easily consume
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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. Nothing more is required than resorting a list
already sorted. Starting from Perl 5.8.0 a different
sorting algorithm, mergesort, is used. Mergesort is
insensitive to its input data, so it cannot be similarly
fooled.
See <http://www.cs.rice.edu/~scrosby/hash/> for more infor-
mation, and any computer science text book on the algo-
rithmic complexity.
SEE ALSO
perlrun for its description of cleaning up environment vari-
ables.
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