3 perlsec - Perl security
7 Perl is designed to make it easy to program securely even when running
8 with extra privileges, like setuid or setgid programs. Unlike most
9 command line shells, which are based on multiple substitution passes on
10 each line of the script, Perl uses a more conventional evaluation scheme
11 with fewer hidden snags. Additionally, because the language has more
12 builtin functionality, it can rely less upon external (and possibly
13 untrustworthy) programs to accomplish its purposes.
15 Perl automatically enables a set of special security checks, called I<taint
16 mode>, when it detects its program running with differing real and effective
17 user or group IDs. The setuid bit in Unix permissions is mode 04000, the
18 setgid bit mode 02000; either or both may be set. You can also enable taint
19 mode explicitly by using the B<-T> command line flag. This flag is
20 I<strongly> suggested for server programs and any program run on behalf of
21 someone else, such as a CGI script. Once taint mode is on, it's on for
22 the remainder of your script.
24 While in this mode, Perl takes special precautions called I<taint
25 checks> to prevent both obvious and subtle traps. Some of these checks
26 are reasonably simple, such as verifying that path directories aren't
27 writable by others; careful programmers have always used checks like
28 these. Other checks, however, are best supported by the language itself,
29 and it is these checks especially that contribute to making a set-id Perl
30 program more secure than the corresponding C program.
32 You may not use data derived from outside your program to affect
33 something else outside your program--at least, not by accident. All
34 command line arguments, environment variables, locale information (see
35 L<perllocale>), results of certain system calls (readdir(),
36 readlink(), the variable of shmread(), the messages returned by
37 msgrcv(), the password, gcos and shell fields returned by the
38 getpwxxx() calls), and all file input are marked as "tainted".
39 Tainted data may not be used directly or indirectly in any command
40 that invokes a sub-shell, nor in any command that modifies files,
41 directories, or processes, B<with the following exceptions>:
47 If you pass more than one argument to either C<system> or C<exec>,
48 the arguments are B<not> checked for taintedness.
52 Arguments to C<print> and C<syswrite> are B<not> checked for taintedness.
56 The value of an expression containing tainted data will itself be
57 tainted, even if it is logically impossible for the tainted data to
60 Because taintedness is associated with each scalar value, some
61 elements of an array can be tainted and others not.
65 $arg = shift; # $arg is tainted
66 $hid = $arg, 'bar'; # $hid is also tainted
68 $line = <STDIN>; # Also tainted
69 open FOO, "/home/me/bar" or die $!;
70 $line = <FOO>; # Still tainted
71 $path = $ENV{'PATH'}; # Tainted, but see below
72 $data = 'abc'; # Not tainted
74 system "echo $arg"; # Insecure
75 system "/bin/echo", $arg; # Secure (doesn't use sh)
76 system "echo $hid"; # Insecure
77 system "echo $data"; # Insecure until PATH set
79 $path = $ENV{'PATH'}; # $path now tainted
81 $ENV{'PATH'} = '/bin:/usr/bin';
82 delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
84 $path = $ENV{'PATH'}; # $path now NOT tainted
85 system "echo $data"; # Is secure now!
87 open(FOO, "< $arg"); # OK - read-only file
88 open(FOO, "> $arg"); # Not OK - trying to write
90 open(FOO,"echo $arg|"); # Not OK, but...
92 or exec 'echo', $arg; # OK
94 $shout = `echo $arg`; # Insecure, $shout now tainted
96 unlink $data, $arg; # Insecure
97 umask $arg; # Insecure
99 exec "echo $arg"; # Insecure (uses the shell)
100 exec "echo", $arg; # Secure (doesn't use the shell)
101 exec "sh", '-c', $arg; # Considered secure, alas!
103 @files = <*.c>; # insecure (uses readdir() or similar)
104 @files = glob('*.c'); # insecure (uses readdir() or similar)
106 # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would
107 # have used an external program to do the filename expansion; but in
108 # either case the result is tainted since the list of filenames comes
109 # from outside of the program.
111 $bad = ($arg, 23); # $bad will be tainted
112 $arg, `true`; # Insecure (although it isn't really)
114 If you try to do something insecure, you will get a fatal error saying
115 something like "Insecure dependency" or "Insecure $ENV{PATH}". Note that you
116 can still write an insecure B<system> or B<exec>, but only by explicitly
117 doing something like the "considered secure" example above.
119 =head2 Laundering and Detecting Tainted Data
121 To test whether a variable contains tainted data, and whose use would
122 thus trigger an "Insecure dependency" message, you can use the
123 tainted() function of the Scalar::Util module, available in your
124 nearby CPAN mirror, and included in Perl starting from the release 5.8.0.
125 Or you may be able to use the following I<is_tainted()> function.
128 return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
131 This function makes use of the fact that the presence of tainted data
132 anywhere within an expression renders the entire expression tainted. It
133 would be inefficient for every operator to test every argument for
134 taintedness. Instead, the slightly more efficient and conservative
135 approach is used that if any tainted value has been accessed within the
136 same expression, the whole expression is considered tainted.
138 But testing for taintedness gets you only so far. Sometimes you have just
139 to clear your data's taintedness. The only way to bypass the tainting
140 mechanism is by referencing subpatterns from a regular expression match.
141 Perl presumes that if you reference a substring using $1, $2, etc., that
142 you knew what you were doing when you wrote the pattern. That means using
143 a bit of thought--don't just blindly untaint anything, or you defeat the
144 entire mechanism. It's better to verify that the variable has only good
145 characters (for certain values of "good") rather than checking whether it
146 has any bad characters. That's because it's far too easy to miss bad
147 characters that you never thought of.
149 Here's a test to make sure that the data contains nothing but "word"
150 characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
153 if ($data =~ /^([-\@\w.]+)$/) {
154 $data = $1; # $data now untainted
156 die "Bad data in $data"; # log this somewhere
159 This is fairly secure because C</\w+/> doesn't normally match shell
160 metacharacters, nor are dot, dash, or at going to mean something special
161 to the shell. Use of C</.+/> would have been insecure in theory because
162 it lets everything through, but Perl doesn't check for that. The lesson
163 is that when untainting, you must be exceedingly careful with your patterns.
164 Laundering data using regular expression is the I<only> mechanism for
165 untainting dirty data, unless you use the strategy detailed below to fork
166 a child of lesser privilege.
168 The example does not untaint $data if C<use locale> is in effect,
169 because the characters matched by C<\w> are determined by the locale.
170 Perl considers that locale definitions are untrustworthy because they
171 contain data from outside the program. If you are writing a
172 locale-aware program, and want to launder data with a regular expression
173 containing C<\w>, put C<no locale> ahead of the expression in the same
174 block. See L<perllocale/SECURITY> for further discussion and examples.
176 =head2 Switches On the "#!" Line
178 When you make a script executable, in order to make it usable as a
179 command, the system will pass switches to perl from the script's #!
180 line. Perl checks that any command line switches given to a setuid
181 (or setgid) script actually match the ones set on the #! line. Some
182 Unix and Unix-like environments impose a one-switch limit on the #!
183 line, so you may need to use something like C<-wU> instead of C<-w -U>
184 under such systems. (This issue should arise only in Unix or
185 Unix-like environments that support #! and setuid or setgid scripts.)
187 =head2 Cleaning Up Your Path
189 For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a
190 known value, and each directory in the path must be non-writable by others
191 than its owner and group. You may be surprised to get this message even
192 if the pathname to your executable is fully qualified. This is I<not>
193 generated because you didn't supply a full path to the program; instead,
194 it's generated because you never set your PATH environment variable, or
195 you didn't set it to something that was safe. Because Perl can't
196 guarantee that the executable in question isn't itself going to turn
197 around and execute some other program that is dependent on your PATH, it
198 makes sure you set the PATH.
200 The PATH isn't the only environment variable which can cause problems.
201 Because some shells may use the variables IFS, CDPATH, ENV, and
202 BASH_ENV, Perl checks that those are either empty or untainted when
203 starting subprocesses. You may wish to add something like this to your
204 setid and taint-checking scripts.
206 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
208 It's also possible to get into trouble with other operations that don't
209 care whether they use tainted values. Make judicious use of the file
210 tests in dealing with any user-supplied filenames. When possible, do
211 opens and such B<after> properly dropping any special user (or group!)
212 privileges. Perl doesn't prevent you from opening tainted filenames for reading,
213 so be careful what you print out. The tainting mechanism is intended to
214 prevent stupid mistakes, not to remove the need for thought.
216 Perl does not call the shell to expand wild cards when you pass B<system>
217 and B<exec> explicit parameter lists instead of strings with possible shell
218 wildcards in them. Unfortunately, the B<open>, B<glob>, and
219 backtick functions provide no such alternate calling convention, so more
220 subterfuge will be required.
222 Perl provides a reasonably safe way to open a file or pipe from a setuid
223 or setgid program: just create a child process with reduced privilege who
224 does the dirty work for you. First, fork a child using the special
225 B<open> syntax that connects the parent and child by a pipe. Now the
226 child resets its ID set and any other per-process attributes, like
227 environment variables, umasks, current working directories, back to the
228 originals or known safe values. Then the child process, which no longer
229 has any special permissions, does the B<open> or other system call.
230 Finally, the child passes the data it managed to access back to the
231 parent. Because the file or pipe was opened in the child while running
232 under less privilege than the parent, it's not apt to be tricked into
233 doing something it shouldn't.
235 Here's a way to do backticks reasonably safely. Notice how the B<exec> is
236 not called with a string that the shell could expand. This is by far the
237 best way to call something that might be subjected to shell escapes: just
238 never call the shell at all.
241 die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
248 my @temp = ($EUID, $EGID);
256 # Make sure privs are really gone
257 ($EUID, $EGID) = @temp;
258 die "Can't drop privileges"
259 unless $UID == $EUID && $GID eq $EGID;
260 $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH.
261 # Consider sanitizing the environment even more.
262 exec 'myprog', 'arg1', 'arg2'
263 or die "can't exec myprog: $!";
266 A similar strategy would work for wildcard expansion via C<glob>, although
267 you can use C<readdir> instead.
269 Taint checking is most useful when although you trust yourself not to have
270 written a program to give away the farm, you don't necessarily trust those
271 who end up using it not to try to trick it into doing something bad. This
272 is the kind of security checking that's useful for set-id programs and
273 programs launched on someone else's behalf, like CGI programs.
275 This is quite different, however, from not even trusting the writer of the
276 code not to try to do something evil. That's the kind of trust needed
277 when someone hands you a program you've never seen before and says, "Here,
278 run this." For that kind of safety, check out the Safe module,
279 included standard in the Perl distribution. This module allows the
280 programmer to set up special compartments in which all system operations
281 are trapped and namespace access is carefully controlled.
285 Beyond the obvious problems that stem from giving special privileges to
286 systems as flexible as scripts, on many versions of Unix, set-id scripts
287 are inherently insecure right from the start. The problem is a race
288 condition in the kernel. Between the time the kernel opens the file to
289 see which interpreter to run and when the (now-set-id) interpreter turns
290 around and reopens the file to interpret it, the file in question may have
291 changed, especially if you have symbolic links on your system.
293 Fortunately, sometimes this kernel "feature" can be disabled.
294 Unfortunately, there are two ways to disable it. The system can simply
295 outlaw scripts with any set-id bit set, which doesn't help much.
296 Alternately, it can simply ignore the set-id bits on scripts. If the
297 latter is true, Perl can emulate the setuid and setgid mechanism when it
298 notices the otherwise useless setuid/gid bits on Perl scripts. It does
299 this via a special executable called B<suidperl> that is automatically
300 invoked for you if it's needed.
302 However, if the kernel set-id script feature isn't disabled, Perl will
303 complain loudly that your set-id script is insecure. You'll need to
304 either disable the kernel set-id script feature, or put a C wrapper around
305 the script. A C wrapper is just a compiled program that does nothing
306 except call your Perl program. Compiled programs are not subject to the
307 kernel bug that plagues set-id scripts. Here's a simple wrapper, written
310 #define REAL_PATH "/path/to/script"
314 execv(REAL_PATH, av);
317 Compile this wrapper into a binary executable and then make I<it> rather
318 than your script setuid or setgid.
320 In recent years, vendors have begun to supply systems free of this
321 inherent security bug. On such systems, when the kernel passes the name
322 of the set-id script to open to the interpreter, rather than using a
323 pathname subject to meddling, it instead passes I</dev/fd/3>. This is a
324 special file already opened on the script, so that there can be no race
325 condition for evil scripts to exploit. On these systems, Perl should be
326 compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure>
327 program that builds Perl tries to figure this out for itself, so you
328 should never have to specify this yourself. Most modern releases of
329 SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.
331 Prior to release 5.6.1 of Perl, bugs in the code of B<suidperl> could
332 introduce a security hole.
334 =head2 Protecting Your Programs
336 There are a number of ways to hide the source to your Perl programs,
337 with varying levels of "security".
339 First of all, however, you I<can't> take away read permission, because
340 the source code has to be readable in order to be compiled and
341 interpreted. (That doesn't mean that a CGI script's source is
342 readable by people on the web, though.) So you have to leave the
343 permissions at the socially friendly 0755 level. This lets
344 people on your local system only see your source.
346 Some people mistakenly regard this as a security problem. If your program does
347 insecure things, and relies on people not knowing how to exploit those
348 insecurities, it is not secure. It is often possible for someone to
349 determine the insecure things and exploit them without viewing the
350 source. Security through obscurity, the name for hiding your bugs
351 instead of fixing them, is little security indeed.
353 You can try using encryption via source filters (Filter::* from CPAN,
354 or Filter::Util::Call and Filter::Simple since Perl 5.8).
355 But crackers might be able to decrypt it. You can try using the byte
356 code compiler and interpreter described below, but crackers might be
357 able to de-compile it. You can try using the native-code compiler
358 described below, but crackers might be able to disassemble it. These
359 pose varying degrees of difficulty to people wanting to get at your
360 code, but none can definitively conceal it (this is true of every
361 language, not just Perl).
363 If you're concerned about people profiting from your code, then the
364 bottom line is that nothing but a restrictive licence will give you
365 legal security. License your software and pepper it with threatening
366 statements like "This is unpublished proprietary software of XYZ Corp.
367 Your access to it does not give you permission to use it blah blah
368 blah." You should see a lawyer to be sure your licence's wording will
373 L<perlrun> for its description of cleaning up environment variables.