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1=head1 NAME
2
3perlsec - Perl security
4
5=head1 DESCRIPTION
6
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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
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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
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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
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21someone else, such as a CGI script. Once taint mode is on, it's on for
22the remainder of your script.
a0d0e21e 23
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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
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30program more secure than the corresponding C program.
31
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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
d929ce6f 35L<perllocale>), results of certain system calls (readdir(),
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36readlink(), the variable of shmread(), the messages returned by
37msgrcv(), the password, gcos and shell fields returned by the
38getpwxxx() calls), and all file input are marked as "tainted".
39Tainted data may not be used directly or indirectly in any command
40that invokes a sub-shell, nor in any command that modifies files,
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41directories, or processes, B<with the following exceptions>:
42
43=over 4
44
45=item *
46
47If you pass a list of arguments to either C<system> or C<exec>,
48the elements of that list are B<not> checked for taintedness.
49
50=item *
51
52Arguments to C<print> and C<syswrite> are B<not> checked for taintedness.
53
54=back
55
56Any variable set to a value
41d6edb2 57derived from tainted data will itself be tainted, even if it is
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58logically impossible for the tainted data to alter the variable.
59Because taintedness is associated with each scalar value, some
60elements of an array can be tainted and others not.
a0d0e21e 61
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62For example:
63
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64 $arg = shift; # $arg is tainted
65 $hid = $arg, 'bar'; # $hid is also tainted
66 $line = <>; # Tainted
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67 $line = <STDIN>; # Also tainted
68 open FOO, "/home/me/bar" or die $!;
69 $line = <FOO>; # Still tainted
a0d0e21e 70 $path = $ENV{'PATH'}; # Tainted, but see below
425e5e39 71 $data = 'abc'; # Not tainted
a0d0e21e 72
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73 system "echo $arg"; # Insecure
74 system "/bin/echo", $arg; # Secure (doesn't use sh)
75 system "echo $hid"; # Insecure
76 system "echo $data"; # Insecure until PATH set
a0d0e21e 77
425e5e39 78 $path = $ENV{'PATH'}; # $path now tainted
a0d0e21e 79
54310121 80 $ENV{'PATH'} = '/bin:/usr/bin';
c90c0ff4 81 delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
a0d0e21e 82
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83 $path = $ENV{'PATH'}; # $path now NOT tainted
84 system "echo $data"; # Is secure now!
a0d0e21e 85
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86 open(FOO, "< $arg"); # OK - read-only file
87 open(FOO, "> $arg"); # Not OK - trying to write
a0d0e21e 88
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89 open(FOO,"echo $arg|"); # Not OK, but...
90 open(FOO,"-|")
91 or exec 'echo', $arg; # OK
a0d0e21e 92
425e5e39 93 $shout = `echo $arg`; # Insecure, $shout now tainted
a0d0e21e 94
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95 unlink $data, $arg; # Insecure
96 umask $arg; # Insecure
a0d0e21e 97
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98 exec "echo $arg"; # Insecure
99 exec "echo", $arg; # Secure (doesn't use the shell)
100 exec "sh", '-c', $arg; # Considered secure, alas!
a0d0e21e 101
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102 @files = <*.c>; # insecure (uses readdir() or similar)
103 @files = glob('*.c'); # insecure (uses readdir() or similar)
7bac28a0 104
a0d0e21e 105If you try to do something insecure, you will get a fatal error saying
62f468fc 106something like "Insecure dependency" or "Insecure $ENV{PATH}". Note that you
425e5e39 107can still write an insecure B<system> or B<exec>, but only by explicitly
a3cb178b 108doing something like the "considered secure" example above.
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109
110=head2 Laundering and Detecting Tainted Data
111
112To test whether a variable contains tainted data, and whose use would thus
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113trigger an "Insecure dependency" message, check your nearby CPAN mirror
114for the F<Taint.pm> module, which should become available around November
1151997. Or you may be able to use the following I<is_tainted()> function.
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116
117 sub is_tainted {
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118 return ! eval {
119 join('',@_), kill 0;
120 1;
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121 };
122 }
123
124This function makes use of the fact that the presence of tainted data
125anywhere within an expression renders the entire expression tainted. It
126would be inefficient for every operator to test every argument for
127taintedness. Instead, the slightly more efficient and conservative
128approach is used that if any tainted value has been accessed within the
129same expression, the whole expression is considered tainted.
130
5f05dabc 131But testing for taintedness gets you only so far. Sometimes you have just
425e5e39 132to clear your data's taintedness. The only way to bypass the tainting
54310121 133mechanism is by referencing subpatterns from a regular expression match.
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134Perl presumes that if you reference a substring using $1, $2, etc., that
135you knew what you were doing when you wrote the pattern. That means using
136a bit of thought--don't just blindly untaint anything, or you defeat the
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137entire mechanism. It's better to verify that the variable has only good
138characters (for certain values of "good") rather than checking whether it
139has any bad characters. That's because it's far too easy to miss bad
140characters that you never thought of.
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141
142Here's a test to make sure that the data contains nothing but "word"
143characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
144or a dot.
145
54310121 146 if ($data =~ /^([-\@\w.]+)$/) {
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147 $data = $1; # $data now untainted
148 } else {
149 die "Bad data in $data"; # log this somewhere
150 }
151
5f05dabc 152This is fairly secure because C</\w+/> doesn't normally match shell
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153metacharacters, nor are dot, dash, or at going to mean something special
154to the shell. Use of C</.+/> would have been insecure in theory because
155it lets everything through, but Perl doesn't check for that. The lesson
156is that when untainting, you must be exceedingly careful with your patterns.
19799a22 157Laundering data using regular expression is the I<only> mechanism for
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158untainting dirty data, unless you use the strategy detailed below to fork
159a child of lesser privilege.
160
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161The example does not untaint $data if C<use locale> is in effect,
162because the characters matched by C<\w> are determined by the locale.
163Perl considers that locale definitions are untrustworthy because they
164contain data from outside the program. If you are writing a
165locale-aware program, and want to launder data with a regular expression
166containing C<\w>, put C<no locale> ahead of the expression in the same
167block. See L<perllocale/SECURITY> for further discussion and examples.
168
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169=head2 Switches On the "#!" Line
170
171When you make a script executable, in order to make it usable as a
172command, the system will pass switches to perl from the script's #!
54310121 173line. Perl checks that any command line switches given to a setuid
3a52c276 174(or setgid) script actually match the ones set on the #! line. Some
54310121 175Unix and Unix-like environments impose a one-switch limit on the #!
3a52c276 176line, so you may need to use something like C<-wU> instead of C<-w -U>
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177under such systems. (This issue should arise only in Unix or
178Unix-like environments that support #! and setuid or setgid scripts.)
3a52c276 179
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180=head2 Cleaning Up Your Path
181
1fef88e7 182For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a
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183known value, and each directory in the path must be non-writable by others
184than its owner and group. You may be surprised to get this message even
185if the pathname to your executable is fully qualified. This is I<not>
186generated because you didn't supply a full path to the program; instead,
187it's generated because you never set your PATH environment variable, or
188you didn't set it to something that was safe. Because Perl can't
189guarantee that the executable in question isn't itself going to turn
190around and execute some other program that is dependent on your PATH, it
54310121 191makes sure you set the PATH.
a0d0e21e 192
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193The PATH isn't the only environment variable which can cause problems.
194Because some shells may use the variables IFS, CDPATH, ENV, and
195BASH_ENV, Perl checks that those are either empty or untainted when
196starting subprocesses. You may wish to add something like this to your
197setid and taint-checking scripts.
198
199 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
200
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201It's also possible to get into trouble with other operations that don't
202care whether they use tainted values. Make judicious use of the file
203tests in dealing with any user-supplied filenames. When possible, do
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204opens and such B<after> properly dropping any special user (or group!)
205privileges. Perl doesn't prevent you from opening tainted filenames for reading,
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206so be careful what you print out. The tainting mechanism is intended to
207prevent stupid mistakes, not to remove the need for thought.
208
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209Perl does not call the shell to expand wild cards when you pass B<system>
210and B<exec> explicit parameter lists instead of strings with possible shell
211wildcards in them. Unfortunately, the B<open>, B<glob>, and
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212backtick functions provide no such alternate calling convention, so more
213subterfuge will be required.
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214
215Perl provides a reasonably safe way to open a file or pipe from a setuid
216or setgid program: just create a child process with reduced privilege who
217does the dirty work for you. First, fork a child using the special
218B<open> syntax that connects the parent and child by a pipe. Now the
219child resets its ID set and any other per-process attributes, like
220environment variables, umasks, current working directories, back to the
221originals or known safe values. Then the child process, which no longer
222has any special permissions, does the B<open> or other system call.
223Finally, the child passes the data it managed to access back to the
5f05dabc 224parent. Because the file or pipe was opened in the child while running
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225under less privilege than the parent, it's not apt to be tricked into
226doing something it shouldn't.
227
54310121 228Here's a way to do backticks reasonably safely. Notice how the B<exec> is
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229not called with a string that the shell could expand. This is by far the
230best way to call something that might be subjected to shell escapes: just
fb73857a 231never call the shell at all.
cb1a09d0 232
54310121 233 use English;
fb73857a 234 die "Can't fork: $!" unless defined $pid = open(KID, "-|");
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235 if ($pid) { # parent
236 while (<KID>) {
237 # do something
425e5e39 238 }
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239 close KID;
240 } else {
fb73857a 241 my @temp = ($EUID, $EGID);
425e5e39 242 $EUID = $UID;
5a964f20 243 $EGID = $GID; # initgroups() also called!
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244 # Make sure privs are really gone
245 ($EUID, $EGID) = @temp;
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246 die "Can't drop privileges"
247 unless $UID == $EUID && $GID eq $EGID;
425e5e39 248 $ENV{PATH} = "/bin:/usr/bin";
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249 exec 'myprog', 'arg1', 'arg2'
250 or die "can't exec myprog: $!";
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251 }
252
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253A similar strategy would work for wildcard expansion via C<glob>, although
254you can use C<readdir> instead.
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255
256Taint checking is most useful when although you trust yourself not to have
257written a program to give away the farm, you don't necessarily trust those
258who end up using it not to try to trick it into doing something bad. This
fb73857a 259is the kind of security checking that's useful for set-id programs and
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260programs launched on someone else's behalf, like CGI programs.
261
262This is quite different, however, from not even trusting the writer of the
263code not to try to do something evil. That's the kind of trust needed
264when someone hands you a program you've never seen before and says, "Here,
265run this." For that kind of safety, check out the Safe module,
266included standard in the Perl distribution. This module allows the
267programmer to set up special compartments in which all system operations
268are trapped and namespace access is carefully controlled.
269
270=head2 Security Bugs
271
272Beyond the obvious problems that stem from giving special privileges to
fb73857a 273systems as flexible as scripts, on many versions of Unix, set-id scripts
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274are inherently insecure right from the start. The problem is a race
275condition in the kernel. Between the time the kernel opens the file to
fb73857a 276see which interpreter to run and when the (now-set-id) interpreter turns
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277around and reopens the file to interpret it, the file in question may have
278changed, especially if you have symbolic links on your system.
279
280Fortunately, sometimes this kernel "feature" can be disabled.
281Unfortunately, there are two ways to disable it. The system can simply
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282outlaw scripts with any set-id bit set, which doesn't help much.
283Alternately, it can simply ignore the set-id bits on scripts. If the
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284latter is true, Perl can emulate the setuid and setgid mechanism when it
285notices the otherwise useless setuid/gid bits on Perl scripts. It does
286this via a special executable called B<suidperl> that is automatically
54310121 287invoked for you if it's needed.
425e5e39 288
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289However, if the kernel set-id script feature isn't disabled, Perl will
290complain loudly that your set-id script is insecure. You'll need to
291either disable the kernel set-id script feature, or put a C wrapper around
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292the script. A C wrapper is just a compiled program that does nothing
293except call your Perl program. Compiled programs are not subject to the
fb73857a 294kernel bug that plagues set-id scripts. Here's a simple wrapper, written
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295in C:
296
297 #define REAL_PATH "/path/to/script"
54310121 298 main(ac, av)
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299 char **av;
300 {
301 execv(REAL_PATH, av);
54310121 302 }
cb1a09d0 303
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304Compile this wrapper into a binary executable and then make I<it> rather
305than your script setuid or setgid.
425e5e39 306
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307In recent years, vendors have begun to supply systems free of this
308inherent security bug. On such systems, when the kernel passes the name
fb73857a 309of the set-id script to open to the interpreter, rather than using a
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310pathname subject to meddling, it instead passes I</dev/fd/3>. This is a
311special file already opened on the script, so that there can be no race
312condition for evil scripts to exploit. On these systems, Perl should be
313compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure>
314program that builds Perl tries to figure this out for itself, so you
315should never have to specify this yourself. Most modern releases of
316SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.
317
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318Prior to release 5.6.1 of Perl, bugs in the code of B<suidperl> could
319introduce a security hole.
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320
321=head2 Protecting Your Programs
322
323There are a number of ways to hide the source to your Perl programs,
324with varying levels of "security".
325
326First of all, however, you I<can't> take away read permission, because
327the source code has to be readable in order to be compiled and
328interpreted. (That doesn't mean that a CGI script's source is
329readable by people on the web, though.) So you have to leave the
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330permissions at the socially friendly 0755 level. This lets
331people on your local system only see your source.
68dc0745 332
5a964f20 333Some people mistakenly regard this as a security problem. If your program does
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334insecure things, and relies on people not knowing how to exploit those
335insecurities, it is not secure. It is often possible for someone to
336determine the insecure things and exploit them without viewing the
337source. Security through obscurity, the name for hiding your bugs
338instead of fixing them, is little security indeed.
339
340You can try using encryption via source filters (Filter::* from CPAN).
341But crackers might be able to decrypt it. You can try using the
54310121 342byte code compiler and interpreter described below, but crackers might
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343be able to de-compile it. You can try using the native-code compiler
344described below, but crackers might be able to disassemble it. These
345pose varying degrees of difficulty to people wanting to get at your
346code, but none can definitively conceal it (this is true of every
347language, not just Perl).
348
349If you're concerned about people profiting from your code, then the
350bottom line is that nothing but a restrictive licence will give you
351legal security. License your software and pepper it with threatening
352statements like "This is unpublished proprietary software of XYZ Corp.
353Your access to it does not give you permission to use it blah blah
354blah." You should see a lawyer to be sure your licence's wording will
355stand up in court.
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356
357=head1 SEE ALSO
358
359L<perlrun> for its description of cleaning up environment variables.