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perlsec - Perl security
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 variables, locale information (see
the perllocale manpage), 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;
$hid = $arg, 'bar';
$line = <>;
$line = <STDIN>;
open FOO, "/home/me/bar" or die $!;
$line = <FOO>;
$path = $ENV{'PATH'};
$data = 'abc';
system "echo $arg";
system "/bin/echo", $arg;
system "echo $hid";
system "echo $data";
$path = $ENV{'PATH'};
$ENV{'PATH'} = '/bin:/usr/bin';
delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
$path = $ENV{'PATH'};
system "echo $data";
open(FOO, "< $arg");
open(FOO, "> $arg");
open(FOO,"echo $arg|");
open(FOO,"-|")
or exec 'echo', $arg;
$shout = `echo $arg`;
unlink $data, $arg;
umask $arg;
exec "echo $arg";
exec "echo", $arg;
exec "sh", '-c', $arg;
@files = <*.c>;
@files = glob('*.c');
$bad = ($arg, 23);
$arg, `true`;
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.
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 = $1;
} else {
die "Bad data in '$data'";
}
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 the perllocale manpage for further discussion and examples.
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.)
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
the perlrun manpage. 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
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)};
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 ($pid) {
while (<KID>) {
}
close KID;
} else {
my @temp = ($EUID, $EGID);
my $orig_uid = $UID;
my $orig_gid = $GID;
$EUID = $UID;
$EGID = $GID;
$UID = $orig_uid;
$GID = $orig_gid;
($EUID, $EGID) = @temp;
die "Can't drop privileges"
unless $UID == $EUID && $GID eq $EGID | 
