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1=head1 NAME
3perlsec - Perl security
425e5e39 7Perl is designed to make it easy to program securely even when running
8with extra privileges, like setuid or setgid programs. Unlike most
54310121 9command line shells, which are based on multiple substitution passes on
425e5e39 10each line of the script, Perl uses a more conventional evaluation scheme
11with fewer hidden snags. Additionally, because the language has more
54310121 12builtin functionality, it can rely less upon external (and possibly
425e5e39 13untrustworthy) programs to accomplish its purposes.
a0d0e21e 14
425e5e39 15Perl automatically enables a set of special security checks, called I<taint
16mode>, when it detects its program running with differing real and effective
17user or group IDs. The setuid bit in Unix permissions is mode 04000, the
18setgid bit mode 02000; either or both may be set. You can also enable taint
5f05dabc 19mode explicitly by using the B<-T> command line flag. This flag is
425e5e39 20I<strongly> suggested for server programs and any program run on behalf of
fb73857a 21someone else, such as a CGI script. Once taint mode is on, it's on for
22the remainder of your script.
a0d0e21e 23
1e422769 24While in this mode, Perl takes special precautions called I<taint
25checks> to prevent both obvious and subtle traps. Some of these checks
26are reasonably simple, such as verifying that path directories aren't
27writable by others; careful programmers have always used checks like
28these. Other checks, however, are best supported by the language itself,
fb73857a 29and it is these checks especially that contribute to making a set-id Perl
425e5e39 30program more secure than the corresponding C program.
fb73857a 32You may not use data derived from outside your program to affect
33something else outside your program--at least, not by accident. All
34command line arguments, environment variables, locale information (see
35L<perllocale>), results of certain system calls (readdir(),
36readlink(), the variable of() shmread, the password, gcos and shell
37fields of the getpwxxx() calls), and all file input are marked as
38"tainted". Tainted data may not be used directly or indirectly in any
39command that invokes a sub-shell, nor in any command that modifies
40files, directories, or processes. (B<Important exception>: If you pass
41a list of arguments to either C<system> or C<exec>, the elements of
42that list are B<NOT> checked for taintedness.) Any variable set to a
43value derived from tainted data will itself be tainted, even if it is
44logically impossible for the tainted data to alter the variable.
45Because taintedness is associated with each scalar value, some
46elements of an array can be tainted and others not.
a0d0e21e 47
48For example:
425e5e39 50 $arg = shift; # $arg is tainted
51 $hid = $arg, 'bar'; # $hid is also tainted
52 $line = <>; # Tainted
8ebc5c01 53 $line = <STDIN>; # Also tainted
54 open FOO, "/home/me/bar" or die $!;
55 $line = <FOO>; # Still tainted
a0d0e21e 56 $path = $ENV{'PATH'}; # Tainted, but see below
425e5e39 57 $data = 'abc'; # Not tainted
a0d0e21e 58
425e5e39 59 system "echo $arg"; # Insecure
60 system "/bin/echo", $arg; # Secure (doesn't use sh)
61 system "echo $hid"; # Insecure
62 system "echo $data"; # Insecure until PATH set
a0d0e21e 63
425e5e39 64 $path = $ENV{'PATH'}; # $path now tainted
a0d0e21e 65
54310121 66 $ENV{'PATH'} = '/bin:/usr/bin';
c90c0ff4 67 delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
a0d0e21e 68
425e5e39 69 $path = $ENV{'PATH'}; # $path now NOT tainted
70 system "echo $data"; # Is secure now!
a0d0e21e 71
425e5e39 72 open(FOO, "< $arg"); # OK - read-only file
73 open(FOO, "> $arg"); # Not OK - trying to write
a0d0e21e 74
425e5e39 75 open(FOO,"echo $arg|"); # Not OK, but...
76 open(FOO,"-|")
77 or exec 'echo', $arg; # OK
a0d0e21e 78
425e5e39 79 $shout = `echo $arg`; # Insecure, $shout now tainted
a0d0e21e 80
425e5e39 81 unlink $data, $arg; # Insecure
82 umask $arg; # Insecure
a0d0e21e 83
425e5e39 84 exec "echo $arg"; # Insecure
85 exec "echo", $arg; # Secure (doesn't use the shell)
86 exec "sh", '-c', $arg; # Considered secure, alas!
a0d0e21e 87
88 @files = <*.c>; # insecure (uses readdir() or similar)
89 @files = glob('*.c'); # insecure (uses readdir() or similar)
7bac28a0 90
a0d0e21e 91If you try to do something insecure, you will get a fatal error saying
62f468fc 92something like "Insecure dependency" or "Insecure $ENV{PATH}". Note that you
425e5e39 93can still write an insecure B<system> or B<exec>, but only by explicitly
a3cb178b 94doing something like the "considered secure" example above.
425e5e39 95
96=head2 Laundering and Detecting Tainted Data
98To test whether a variable contains tainted data, and whose use would thus
fb73857a 99trigger an "Insecure dependency" message, check your nearby CPAN mirror
100for the F<> module, which should become available around November
1011997. Or you may be able to use the following I<is_tainted()> function.
425e5e39 102
103 sub is_tainted {
54310121 104 return ! eval {
105 join('',@_), kill 0;
106 1;
425e5e39 107 };
108 }
110This function makes use of the fact that the presence of tainted data
111anywhere within an expression renders the entire expression tainted. It
112would be inefficient for every operator to test every argument for
113taintedness. Instead, the slightly more efficient and conservative
114approach is used that if any tainted value has been accessed within the
115same expression, the whole expression is considered tainted.
5f05dabc 117But testing for taintedness gets you only so far. Sometimes you have just
425e5e39 118to clear your data's taintedness. The only way to bypass the tainting
54310121 119mechanism is by referencing subpatterns from a regular expression match.
425e5e39 120Perl presumes that if you reference a substring using $1, $2, etc., that
121you knew what you were doing when you wrote the pattern. That means using
122a bit of thought--don't just blindly untaint anything, or you defeat the
123entire mechanism. It's better to verify that the variable has only good
124characters (for certain values of "good") rather than checking whether it
125has any bad characters. That's because it's far too easy to miss bad
126characters that you never thought of.
425e5e39 127
128Here's a test to make sure that the data contains nothing but "word"
129characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
130or a dot.
54310121 132 if ($data =~ /^([-\@\w.]+)$/) {
425e5e39 133 $data = $1; # $data now untainted
134 } else {
135 die "Bad data in $data"; # log this somewhere
136 }
5f05dabc 138This is fairly secure because C</\w+/> doesn't normally match shell
425e5e39 139metacharacters, nor are dot, dash, or at going to mean something special
140to the shell. Use of C</.+/> would have been insecure in theory because
141it lets everything through, but Perl doesn't check for that. The lesson
142is that when untainting, you must be exceedingly careful with your patterns.
19799a22 143Laundering data using regular expression is the I<only> mechanism for
425e5e39 144untainting dirty data, unless you use the strategy detailed below to fork
145a child of lesser privilege.
147The example does not untaint $data if C<use locale> is in effect,
148because the characters matched by C<\w> are determined by the locale.
149Perl considers that locale definitions are untrustworthy because they
150contain data from outside the program. If you are writing a
151locale-aware program, and want to launder data with a regular expression
152containing C<\w>, put C<no locale> ahead of the expression in the same
153block. See L<perllocale/SECURITY> for further discussion and examples.
155=head2 Switches On the "#!" Line
157When you make a script executable, in order to make it usable as a
158command, the system will pass switches to perl from the script's #!
54310121 159line. Perl checks that any command line switches given to a setuid
3a52c276 160(or setgid) script actually match the ones set on the #! line. Some
54310121 161Unix and Unix-like environments impose a one-switch limit on the #!
3a52c276 162line, so you may need to use something like C<-wU> instead of C<-w -U>
54310121 163under such systems. (This issue should arise only in Unix or
164Unix-like environments that support #! and setuid or setgid scripts.)
3a52c276 165
425e5e39 166=head2 Cleaning Up Your Path
1fef88e7 168For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a
1e422769 169known value, and each directory in the path must be non-writable by others
170than its owner and group. You may be surprised to get this message even
171if the pathname to your executable is fully qualified. This is I<not>
172generated because you didn't supply a full path to the program; instead,
173it's generated because you never set your PATH environment variable, or
174you didn't set it to something that was safe. Because Perl can't
175guarantee that the executable in question isn't itself going to turn
176around and execute some other program that is dependent on your PATH, it
54310121 177makes sure you set the PATH.
a0d0e21e 178
179The PATH isn't the only environment variable which can cause problems.
180Because some shells may use the variables IFS, CDPATH, ENV, and
181BASH_ENV, Perl checks that those are either empty or untainted when
182starting subprocesses. You may wish to add something like this to your
183setid and taint-checking scripts.
185 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
187It's also possible to get into trouble with other operations that don't
188care whether they use tainted values. Make judicious use of the file
189tests in dealing with any user-supplied filenames. When possible, do
fb73857a 190opens and such B<after> properly dropping any special user (or group!)
191privileges. Perl doesn't prevent you from opening tainted filenames for reading,
192so be careful what you print out. The tainting mechanism is intended to
193prevent stupid mistakes, not to remove the need for thought.
425e5e39 195Perl does not call the shell to expand wild cards when you pass B<system>
196and B<exec> explicit parameter lists instead of strings with possible shell
197wildcards in them. Unfortunately, the B<open>, B<glob>, and
54310121 198backtick functions provide no such alternate calling convention, so more
199subterfuge will be required.
425e5e39 200
201Perl provides a reasonably safe way to open a file or pipe from a setuid
202or setgid program: just create a child process with reduced privilege who
203does the dirty work for you. First, fork a child using the special
204B<open> syntax that connects the parent and child by a pipe. Now the
205child resets its ID set and any other per-process attributes, like
206environment variables, umasks, current working directories, back to the
207originals or known safe values. Then the child process, which no longer
208has any special permissions, does the B<open> or other system call.
209Finally, the child passes the data it managed to access back to the
5f05dabc 210parent. Because the file or pipe was opened in the child while running
425e5e39 211under less privilege than the parent, it's not apt to be tricked into
212doing something it shouldn't.
54310121 214Here's a way to do backticks reasonably safely. Notice how the B<exec> is
425e5e39 215not called with a string that the shell could expand. This is by far the
216best way to call something that might be subjected to shell escapes: just
fb73857a 217never call the shell at all.
cb1a09d0 218
54310121 219 use English;
fb73857a 220 die "Can't fork: $!" unless defined $pid = open(KID, "-|");
221 if ($pid) { # parent
222 while (<KID>) {
223 # do something
425e5e39 224 }
225 close KID;
226 } else {
fb73857a 227 my @temp = ($EUID, $EGID);
425e5e39 228 $EUID = $UID;
5a964f20 229 $EGID = $GID; # initgroups() also called!
fb73857a 230 # Make sure privs are really gone
231 ($EUID, $EGID) = @temp;
232 die "Can't drop privileges"
233 unless $UID == $EUID && $GID eq $EGID;
425e5e39 234 $ENV{PATH} = "/bin:/usr/bin";
235 exec 'myprog', 'arg1', 'arg2'
236 or die "can't exec myprog: $!";
425e5e39 237 }
fb73857a 239A similar strategy would work for wildcard expansion via C<glob>, although
240you can use C<readdir> instead.
425e5e39 241
242Taint checking is most useful when although you trust yourself not to have
243written a program to give away the farm, you don't necessarily trust those
244who end up using it not to try to trick it into doing something bad. This
fb73857a 245is the kind of security checking that's useful for set-id programs and
425e5e39 246programs launched on someone else's behalf, like CGI programs.
248This is quite different, however, from not even trusting the writer of the
249code not to try to do something evil. That's the kind of trust needed
250when someone hands you a program you've never seen before and says, "Here,
251run this." For that kind of safety, check out the Safe module,
252included standard in the Perl distribution. This module allows the
253programmer to set up special compartments in which all system operations
254are trapped and namespace access is carefully controlled.
256=head2 Security Bugs
258Beyond the obvious problems that stem from giving special privileges to
fb73857a 259systems as flexible as scripts, on many versions of Unix, set-id scripts
425e5e39 260are inherently insecure right from the start. The problem is a race
261condition in the kernel. Between the time the kernel opens the file to
fb73857a 262see which interpreter to run and when the (now-set-id) interpreter turns
425e5e39 263around and reopens the file to interpret it, the file in question may have
264changed, especially if you have symbolic links on your system.
266Fortunately, sometimes this kernel "feature" can be disabled.
267Unfortunately, there are two ways to disable it. The system can simply
fb73857a 268outlaw scripts with any set-id bit set, which doesn't help much.
269Alternately, it can simply ignore the set-id bits on scripts. If the
425e5e39 270latter is true, Perl can emulate the setuid and setgid mechanism when it
271notices the otherwise useless setuid/gid bits on Perl scripts. It does
272this via a special executable called B<suidperl> that is automatically
54310121 273invoked for you if it's needed.
425e5e39 274
fb73857a 275However, if the kernel set-id script feature isn't disabled, Perl will
276complain loudly that your set-id script is insecure. You'll need to
277either disable the kernel set-id script feature, or put a C wrapper around
425e5e39 278the script. A C wrapper is just a compiled program that does nothing
279except call your Perl program. Compiled programs are not subject to the
fb73857a 280kernel bug that plagues set-id scripts. Here's a simple wrapper, written
425e5e39 281in C:
283 #define REAL_PATH "/path/to/script"
54310121 284 main(ac, av)
425e5e39 285 char **av;
286 {
287 execv(REAL_PATH, av);
54310121 288 }
cb1a09d0 289
54310121 290Compile this wrapper into a binary executable and then make I<it> rather
291than your script setuid or setgid.
425e5e39 292
293See the program B<wrapsuid> in the F<eg> directory of your Perl
294distribution for a convenient way to do this automatically for all your
295setuid Perl programs. It moves setuid scripts into files with the same
296name plus a leading dot, and then compiles a wrapper like the one above
297for each of them.
299In recent years, vendors have begun to supply systems free of this
300inherent security bug. On such systems, when the kernel passes the name
fb73857a 301of the set-id script to open to the interpreter, rather than using a
425e5e39 302pathname subject to meddling, it instead passes I</dev/fd/3>. This is a
303special file already opened on the script, so that there can be no race
304condition for evil scripts to exploit. On these systems, Perl should be
305compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure>
306program that builds Perl tries to figure this out for itself, so you
307should never have to specify this yourself. Most modern releases of
308SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.
310Prior to release 5.003 of Perl, a bug in the code of B<suidperl> could
311introduce a security hole in systems compiled with strict POSIX
68dc0745 313
314=head2 Protecting Your Programs
316There are a number of ways to hide the source to your Perl programs,
317with varying levels of "security".
319First of all, however, you I<can't> take away read permission, because
320the source code has to be readable in order to be compiled and
321interpreted. (That doesn't mean that a CGI script's source is
322readable by people on the web, though.) So you have to leave the
323permissions at the socially friendly 0755 level. This lets
324people on your local system only see your source.
68dc0745 325
5a964f20 326Some people mistakenly regard this as a security problem. If your program does
68dc0745 327insecure things, and relies on people not knowing how to exploit those
328insecurities, it is not secure. It is often possible for someone to
329determine the insecure things and exploit them without viewing the
330source. Security through obscurity, the name for hiding your bugs
331instead of fixing them, is little security indeed.
333You can try using encryption via source filters (Filter::* from CPAN).
334But crackers might be able to decrypt it. You can try using the
54310121 335byte code compiler and interpreter described below, but crackers might
68dc0745 336be able to de-compile it. You can try using the native-code compiler
337described below, but crackers might be able to disassemble it. These
338pose varying degrees of difficulty to people wanting to get at your
339code, but none can definitively conceal it (this is true of every
340language, not just Perl).
342If you're concerned about people profiting from your code, then the
343bottom line is that nothing but a restrictive licence will give you
344legal security. License your software and pepper it with threatening
345statements like "This is unpublished proprietary software of XYZ Corp.
346Your access to it does not give you permission to use it blah blah
347blah." You should see a lawyer to be sure your licence's wording will
348stand up in court.
350=head1 SEE ALSO
352L<perlrun> for its description of cleaning up environment variables.