+
=head1 NAME
perlsec - Perl security
=head1 DESCRIPTION
-Perl is designed to make it easy to write secure setuid and setgid
-scripts. Unlike shells, which are based on multiple substitution
-passes on each line of the script, Perl uses a more conventional
-evaluation scheme with fewer hidden "gotchas". Additionally, since the
-language has more built-in functionality, it has to rely less upon
-external (and possibly untrustworthy) programs to accomplish its
-purposes.
+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
+built-in functionality, it can rely less upon external (and possibly
+untrustworthy) programs to accomplish its purposes.
-Beyond the obvious problems that stem from giving special privileges to
-such flexible systems as scripts, on many operating systems, setuid
-scripts are inherently insecure right from the start. This is because
-that between the time that the kernel opens up the file to see what to
-run, and when the now setuid interpreter it ran turns around and reopens
-the file so it can interpret it, things may have changed, especially if
-you have symbolic links on your system.
+Perl automatically enables a set of special security checks, called I<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 the B<-T> command line flag. This flag is
+I<strongly> suggested for server programs and any program run on behalf of
+someone else, such as a CGI script.
-Fortunately, sometimes this kernel "feature" can be disabled.
-Unfortunately, there are two ways to disable it. The system can simply
-outlaw scripts with the setuid bit set, which doesn't help much.
-Alternately, it can simply ignore the setuid bit 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 B<suidperl> that is automatically
-invoked for you if it's needed.
+While in this mode, Perl takes special precautions called I<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 setuid 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, and file input are marked as "tainted".
+Tainted data may not be used directly or indirectly in any command that
+invokes a subshell, nor in any command that modifies files, directories,
+or processes. Any variable set within an expression that has previously
+referenced a tainted value itself becomes tainted, even if it is logically
+impossible for the tainted value to influence the variable. Because
+taintedness is associated with each scalar value, some elements of an
+array can be tainted and others not.
-If, however, the kernel setuid script feature isn't disabled, Perl will
-complain loudly that your setuid script is insecure. You'll need to
-either disable the kernel setuid script feature, or put a C wrapper around
-the script. See the program B<wrapsuid> in the F<eg> directory of your
-Perl distribution for how to go about doing this.
-
-There are some systems on which setuid scripts are free of this inherent
-security bug. For example, recent releases of Solaris are like this. On
-such systems, when the kernel passes the name of the setuid script to open
-to the interpreter, rather than using a pathname subject to mettling, 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
-C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure> program that builds
-Perl tries to figure this out for itself.
-
-When executing a setuid script, or when you have turned on taint checking
-explicitly using the B<-T> flag, Perl takes special precautions to
-prevent you from falling into any obvious traps. (In some ways, a Perl
-script is more secure than the corresponding C program.) Any command line
-argument, environment variable, or input is marked as "tainted", and may
-not be used, directly or indirectly, in any command that invokes a
-subshell, or in any command that modifies files, directories, or
-processes. Any variable that is set within an expression that has
-previously referenced a tainted value also becomes tainted (even if it is
-logically impossible for the tainted value to influence the variable).
For example:
- $foo = shift; # $foo is tainted
- $bar = $foo,'bar'; # $bar is also tainted
- $xxx = <>; # Tainted
+ $arg = shift; # $arg is tainted
+ $hid = $arg, 'bar'; # $hid is also tainted
+ $line = <>; # Tainted
$path = $ENV{'PATH'}; # Tainted, but see below
- $abc = 'abc'; # Not tainted
+ $data = 'abc'; # Not tainted
- system "echo $foo"; # Insecure
- system "/bin/echo", $foo; # Secure (doesn't use sh)
- system "echo $bar"; # Insecure
- system "echo $abc"; # Insecure until PATH set
+ system "echo $arg"; # Insecure
+ system "/bin/echo", $arg; # Secure (doesn't use sh)
+ system "echo $hid"; # Insecure
+ system "echo $data"; # Insecure until PATH set
- $ENV{'PATH'} = '/bin:/usr/bin';
- $ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
+ $path = $ENV{'PATH'}; # $path now tainted
- $path = $ENV{'PATH'}; # Not tainted
- system "echo $abc"; # Is secure now!
+ $ENV{'PATH'} = '/bin:/usr/bin';
+ $ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
- open(FOO,"$foo"); # OK
- open(FOO,">$foo"); # Not OK
+ $path = $ENV{'PATH'}; # $path now NOT tainted
+ system "echo $data"; # Is secure now!
- open(FOO,"echo $foo|"); # Not OK, but...
- open(FOO,"-|") || exec 'echo', $foo; # OK
+ open(FOO, "< $arg"); # OK - read-only file
+ open(FOO, "> $arg"); # Not OK - trying to write
- $zzz = `echo $foo`; # Insecure, zzz tainted
+ open(FOO,"echo $arg|"); # Not OK, but...
+ open(FOO,"-|")
+ or exec 'echo', $arg; # OK
- unlink $abc,$foo; # Insecure
- umask $foo; # Insecure
+ $shout = `echo $arg`; # Insecure, $shout now tainted
- exec "echo $foo"; # Insecure
- exec "echo", $foo; # Secure (doesn't use sh)
- exec "sh", '-c', $foo; # Considered secure, alas
+ unlink $data, $arg; # Insecure
+ umask $arg; # Insecure
-The taintedness is associated with each scalar value, so some elements
-of an array can be tainted, and others not.
+ exec "echo $arg"; # Insecure
+ exec "echo", $arg; # Secure (doesn't use the shell)
+ exec "sh", '-c', $arg; # Considered secure, alas!
If you try to do something insecure, you will get a fatal error saying
something like "Insecure dependency" or "Insecure PATH". Note that you
-can still write an insecure system call or exec, but only by explicitly
-doing something like the last example above. You can also bypass the
-tainting mechanism by referencing subpatterns--Perl presumes that if
-you reference a substring using $1, $2, etc, you knew what you were
-doing when you wrote the pattern:
-
- $ARGV[0] =~ /^-P(\w+)$/;
- $printer = $1; # Not tainted
-
-This is fairly secure since C<\w+> doesn't match shell metacharacters.
-Use of C</.+/> would have been insecure, but Perl doesn't check for that,
-so you must be careful with your patterns. This is the I<ONLY> mechanism
-for untainting user supplied filenames if you want to do file operations
-on them (unless you make C<$E<gt>> equal to C<$E<lt>> ).
-
-For "Insecure $ENV{PATH}" messages, you need to set C<$ENV{'PATH'}> to a known
-value, and each directory in the path must be non-writable by the world.
-A frequently voiced gripe is that you can get this message even
-if the pathname to an executable is fully qualified. But Perl can't
-know that the executable in question isn't going to execute some other
-program depending on the PATH.
+can still write an insecure B<system> or B<exec>, but only by explicitly
+doing something like the last example above.
+
+=head2 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 following
+I<is_tainted()> function.
+
+ sub is_tainted {
+ return ! eval {
+ join('',@_), kill 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 only gets you so far. Sometimes you just have
+to clear your data's taintedness. 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 since C</\w+/> doesn't normally match shell
+metacharacters, nor are dot, dash, or at going to mean something special
+to the shell. Use of C</.+/> 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 I<ONLY> mechanism for
+untainting dirty data, unless you use the strategy detailed below to fork
+a child of lesser privilege.
+
+=head2 Cleaning Up Your Path
+
+For "Insecure $ENV{PATH}" messages, you need to set C<$ENV{'PATH'}> to a
+known value, and each directory in the path must be 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 I<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.
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 setting C<$E<gt> = $E<lt>>. (Remember group IDs,
-too!) Perl doesn't prevent you from opening tainted filenames for reading,
+too!) 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.
-This gives us a reasonably safe way to open a file or pipe: just reset the
-id set to the original IDs. Here's a way to do backticks reasonably
-safely. Notice how the exec() is not called with a string that the shell
-could expand. By the time we get to the exec(), tainting is turned off,
-however, so be careful what you call and what you pass it.
+Perl does not call the shell to expand wild cards when you pass B<system>
+and B<exec> explicit parameter lists instead of strings with possible shell
+wildcards in them. Unfortunately, the B<open>, B<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
+B<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 B<open> or other system call.
+Finally, the child passes the data it managed to access back to the
+parent. Since 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 B<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. By the time we get to the B<exec>, tainting
+is turned off, however, so be careful what you call and what you pass it.
+ use English;
die unless defined $pid = open(KID, "-|");
if ($pid) { # parent
while (<KID>) {
# do something
- }
+ }
close KID;
} else {
- $> = $<;
- $) = $(; # BUG: initgroups() not called
- exec 'program', 'arg1', 'arg2';
- die "can't exec program: $!";
+ $EUID = $UID;
+ $EGID = $GID; # XXX: initgroups() not called
+ $ENV{PATH} = "/bin:/usr/bin";
+ exec 'myprog', 'arg1', 'arg2';
+ die "can't exec myprog: $!";
+ }
+
+A similar strategy would work for wildcard expansion via C<glob>.
+
+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 setuid 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, 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.
+
+=head2 Security Bugs
+
+Beyond the obvious problems that stem from giving special privileges to
+systems as flexible as scripts, on many versions of Unix, setuid 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-setuid) 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 the setuid bit set, which doesn't help much.
+Alternately, it can simply ignore the setuid bit 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 B<suidperl> that is automatically
+invoked for you if it's needed.
+
+However, if the kernel setuid script feature isn't disabled, Perl will
+complain loudly that your setuid script is insecure. You'll need to
+either disable the kernel setuid 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 setuid scripts. Here's a simple wrapper, written
+in C:
+
+ #define REAL_PATH "/path/to/script"
+ main(ac, av)
+ char **av;
+ {
+ execv(REAL_PATH, av);
}
-For those even more concerned about safety, see the I<Safe> and I<Safe CGI>
-modules at a CPAN site near you. See L<perlmod> for a list of CPAN sites.
+Compile this wrapper into a binary executable and then make I<it> rather
+than your script setuid or setgid.
+
+See the program B<wrapsuid> in the F<eg> directory of your Perl
+distribution for a convenient way to do this automatically for all your
+setuid Perl programs. It moves setuid scripts into files with the same
+name plus a leading dot, and then compiles a wrapper like the one above
+for each of them.
+
+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 setuid script to open to the interpreter, rather than using a
+pathname subject to meddling, it instead passes I</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 C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<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.
+
+Prior to release 5.003 of Perl, a bug in the code of B<suidperl> could
+introduce a security hole in systems compiled with strict POSIX
+compliance.
projects / p5sagit/p5-mst-13.2.git / commitdiff