1PERLSEC(1) Perl Programmers Reference Guide PERLSEC(1)
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6 perlsec - Perl security
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9 Perl is designed to make it easy to program securely even when running
10 with extra privileges, like setuid or setgid programs. Unlike most
11 command line shells, which are based on multiple substitution passes on
12 each line of the script, Perl uses a more conventional evaluation
13 scheme with fewer hidden snags. Additionally, because the language has
14 more builtin functionality, it can rely less upon external (and
15 possibly untrustworthy) programs to accomplish its purposes.
16
18 If you believe you have found a security vulnerability in Perl, please
19 email the details to perl5-security-report@perl.org. This creates a new
20 Request Tracker ticket in a special queue which isn't initially
21 publicly accessible. The email will also be copied to a closed
22 subscription unarchived mailing list which includes all the core
23 committers, who will be able to help assess the impact of issues,
24 figure out a resolution, and help co-ordinate the release of patches to
25 mitigate or fix the problem across all platforms on which Perl is
26 supported. Please only use this address for security issues in the Perl
27 core, not for modules independently distributed on CPAN.
28
29 When sending an initial request to the security email address, please
30 don't Cc any other parties, because if they reply to all, the reply
31 will generate yet another new ticket. Once you have received an initial
32 reply with a "[perl #NNNNNN]" ticket number in the headline, it's okay
33 to Cc subsequent replies to third parties: all emails to the
34 perl5-security-report address with the ticket number in the subject
35 line will be added to the ticket; without it, a new ticket will be
36 created.
37
39 Taint mode
40 Perl automatically enables a set of special security checks, called
41 taint mode, when it detects its program running with differing real and
42 effective user or group IDs. The setuid bit in Unix permissions is
43 mode 04000, the setgid bit mode 02000; either or both may be set. You
44 can also enable taint mode explicitly by using the -T command line
45 flag. This flag is strongly suggested for server programs and any
46 program run on behalf of someone else, such as a CGI script. Once
47 taint mode is on, it's on for the remainder of your script.
48
49 While in this mode, Perl takes special precautions called taint checks
50 to prevent both obvious and subtle traps. Some of these checks are
51 reasonably simple, such as verifying that path directories aren't
52 writable by others; careful programmers have always used checks like
53 these. Other checks, however, are best supported by the language
54 itself, and it is these checks especially that contribute to making a
55 set-id Perl program more secure than the corresponding C program.
56
57 You may not use data derived from outside your program to affect
58 something else outside your program--at least, not by accident. All
59 command line arguments, environment variables, locale information (see
60 perllocale), results of certain system calls ("readdir()",
61 "readlink()", the variable of "shmread()", the messages returned by
62 "msgrcv()", the password, gcos and shell fields returned by the
63 "getpwxxx()" calls), and all file input are marked as "tainted".
64 Tainted data may not be used directly or indirectly in any command that
65 invokes a sub-shell, nor in any command that modifies files,
66 directories, or processes, with the following exceptions:
67
68 · Arguments to "print" and "syswrite" are not checked for
69 taintedness.
70
71 · Symbolic methods
72
73 $obj->$method(@args);
74
75 and symbolic sub references
76
77 &{$foo}(@args);
78 $foo->(@args);
79
80 are not checked for taintedness. This requires extra carefulness
81 unless you want external data to affect your control flow. Unless
82 you carefully limit what these symbolic values are, people are able
83 to call functions outside your Perl code, such as POSIX::system, in
84 which case they are able to run arbitrary external code.
85
86 · Hash keys are never tainted.
87
88 For efficiency reasons, Perl takes a conservative view of whether data
89 is tainted. If an expression contains tainted data, any subexpression
90 may be considered tainted, even if the value of the subexpression is
91 not itself affected by the tainted data.
92
93 Because taintedness is associated with each scalar value, some elements
94 of an array or hash can be tainted and others not. The keys of a hash
95 are never tainted.
96
97 For example:
98
99 $arg = shift; # $arg is tainted
100 $hid = $arg . 'bar'; # $hid is also tainted
101 $line = <>; # Tainted
102 $line = <STDIN>; # Also tainted
103 open FOO, "/home/me/bar" or die $!;
104 $line = <FOO>; # Still tainted
105 $path = $ENV{'PATH'}; # Tainted, but see below
106 $data = 'abc'; # Not tainted
107
108 system "echo $arg"; # Insecure
109 system "/bin/echo", $arg; # Considered insecure
110 # (Perl doesn't know about /bin/echo)
111 system "echo $hid"; # Insecure
112 system "echo $data"; # Insecure until PATH set
113
114 $path = $ENV{'PATH'}; # $path now tainted
115
116 $ENV{'PATH'} = '/bin:/usr/bin';
117 delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
118
119 $path = $ENV{'PATH'}; # $path now NOT tainted
120 system "echo $data"; # Is secure now!
121
122 open(FOO, "< $arg"); # OK - read-only file
123 open(FOO, "> $arg"); # Not OK - trying to write
124
125 open(FOO,"echo $arg|"); # Not OK
126 open(FOO,"-|")
127 or exec 'echo', $arg; # Also not OK
128
129 $shout = `echo $arg`; # Insecure, $shout now tainted
130
131 unlink $data, $arg; # Insecure
132 umask $arg; # Insecure
133
134 exec "echo $arg"; # Insecure
135 exec "echo", $arg; # Insecure
136 exec "sh", '-c', $arg; # Very insecure!
137
138 @files = <*.c>; # insecure (uses readdir() or similar)
139 @files = glob('*.c'); # insecure (uses readdir() or similar)
140
141 # In either case, the results of glob are tainted, since the list of
142 # filenames comes from outside of the program.
143
144 $bad = ($arg, 23); # $bad will be tainted
145 $arg, `true`; # Insecure (although it isn't really)
146
147 If you try to do something insecure, you will get a fatal error saying
148 something like "Insecure dependency" or "Insecure $ENV{PATH}".
149
150 The exception to the principle of "one tainted value taints the whole
151 expression" is with the ternary conditional operator "?:". Since code
152 with a ternary conditional
153
154 $result = $tainted_value ? "Untainted" : "Also untainted";
155
156 is effectively
157
158 if ( $tainted_value ) {
159 $result = "Untainted";
160 } else {
161 $result = "Also untainted";
162 }
163
164 it doesn't make sense for $result to be tainted.
165
166 Laundering and Detecting Tainted Data
167 To test whether a variable contains tainted data, and whose use would
168 thus trigger an "Insecure dependency" message, you can use the
169 "tainted()" function of the Scalar::Util module, available in your
170 nearby CPAN mirror, and included in Perl starting from the release
171 5.8.0. Or you may be able to use the following "is_tainted()"
172 function.
173
174 sub is_tainted {
175 local $@; # Don't pollute caller's value.
176 return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
177 }
178
179 This function makes use of the fact that the presence of tainted data
180 anywhere within an expression renders the entire expression tainted.
181 It would be inefficient for every operator to test every argument for
182 taintedness. Instead, the slightly more efficient and conservative
183 approach is used that if any tainted value has been accessed within the
184 same expression, the whole expression is considered tainted.
185
186 But testing for taintedness gets you only so far. Sometimes you have
187 just to clear your data's taintedness. Values may be untainted by
188 using them as keys in a hash; otherwise the only way to bypass the
189 tainting mechanism is by referencing subpatterns from a regular
190 expression match. Perl presumes that if you reference a substring
191 using $1, $2, etc. in a non-tainting pattern, that you knew what you
192 were doing when you wrote that pattern. That means using a bit of
193 thought--don't just blindly untaint anything, or you defeat the entire
194 mechanism. It's better to verify that the variable has only good
195 characters (for certain values of "good") rather than checking whether
196 it has any bad characters. That's because it's far too easy to miss
197 bad characters that you never thought of.
198
199 Here's a test to make sure that the data contains nothing but "word"
200 characters (alphabetics, numerics, and underscores), a hyphen, an at
201 sign, or a dot.
202
203 if ($data =~ /^([-\@\w.]+)$/) {
204 $data = $1; # $data now untainted
205 } else {
206 die "Bad data in '$data'"; # log this somewhere
207 }
208
209 This is fairly secure because "/\w+/" doesn't normally match shell
210 metacharacters, nor are dot, dash, or at going to mean something
211 special to the shell. Use of "/.+/" would have been insecure in theory
212 because it lets everything through, but Perl doesn't check for that.
213 The lesson is that when untainting, you must be exceedingly careful
214 with your patterns. Laundering data using regular expression is the
215 only mechanism for untainting dirty data, unless you use the strategy
216 detailed below to fork a child of lesser privilege.
217
218 The example does not untaint $data if "use locale" is in effect,
219 because the characters matched by "\w" are determined by the locale.
220 Perl considers that locale definitions are untrustworthy because they
221 contain data from outside the program. If you are writing a locale-
222 aware program, and want to launder data with a regular expression
223 containing "\w", put "no locale" ahead of the expression in the same
224 block. See "SECURITY" in perllocale for further discussion and
225 examples.
226
227 Switches On the "#!" Line
228 When you make a script executable, in order to make it usable as a
229 command, the system will pass switches to perl from the script's #!
230 line. Perl checks that any command line switches given to a setuid (or
231 setgid) script actually match the ones set on the #! line. Some Unix
232 and Unix-like environments impose a one-switch limit on the #! line,
233 so you may need to use something like "-wU" instead of "-w -U" under
234 such systems. (This issue should arise only in Unix or Unix-like
235 environments that support #! and setuid or setgid scripts.)
236
237 Taint mode and @INC
238 When the taint mode ("-T") is in effect, the "." directory is removed
239 from @INC, and the environment variables "PERL5LIB" and "PERLLIB" are
240 ignored by Perl. You can still adjust @INC from outside the program by
241 using the "-I" command line option as explained in perlrun. The two
242 environment variables are ignored because they are obscured, and a user
243 running a program could be unaware that they are set, whereas the "-I"
244 option is clearly visible and therefore permitted.
245
246 Another way to modify @INC without modifying the program, is to use the
247 "lib" pragma, e.g.:
248
249 perl -Mlib=/foo program
250
251 The benefit of using "-Mlib=/foo" over "-I/foo", is that the former
252 will automagically remove any duplicated directories, while the latter
253 will not.
254
255 Note that if a tainted string is added to @INC, the following problem
256 will be reported:
257
258 Insecure dependency in require while running with -T switch
259
260 Cleaning Up Your Path
261 For "Insecure $ENV{PATH}" messages, you need to set $ENV{'PATH'} to a
262 known value, and each directory in the path must be absolute and non-
263 writable by others than its owner and group. You may be surprised to
264 get this message even if the pathname to your executable is fully
265 qualified. This is not generated because you didn't supply a full path
266 to the program; instead, it's generated because you never set your PATH
267 environment variable, or you didn't set it to something that was safe.
268 Because Perl can't guarantee that the executable in question isn't
269 itself going to turn around and execute some other program that is
270 dependent on your PATH, it makes sure you set the PATH.
271
272 The PATH isn't the only environment variable which can cause problems.
273 Because some shells may use the variables IFS, CDPATH, ENV, and
274 BASH_ENV, Perl checks that those are either empty or untainted when
275 starting subprocesses. You may wish to add something like this to your
276 setid and taint-checking scripts.
277
278 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
279
280 It's also possible to get into trouble with other operations that don't
281 care whether they use tainted values. Make judicious use of the file
282 tests in dealing with any user-supplied filenames. When possible, do
283 opens and such after properly dropping any special user (or group!)
284 privileges. Perl doesn't prevent you from opening tainted filenames
285 for reading, so be careful what you print out. The tainting mechanism
286 is intended to prevent stupid mistakes, not to remove the need for
287 thought.
288
289 Perl does not call the shell to expand wild cards when you pass
290 "system" and "exec" explicit parameter lists instead of strings with
291 possible shell wildcards in them. Unfortunately, the "open", "glob",
292 and backtick functions provide no such alternate calling convention, so
293 more subterfuge will be required.
294
295 Perl provides a reasonably safe way to open a file or pipe from a
296 setuid or setgid program: just create a child process with reduced
297 privilege who does the dirty work for you. First, fork a child using
298 the special "open" syntax that connects the parent and child by a pipe.
299 Now the child resets its ID set and any other per-process attributes,
300 like environment variables, umasks, current working directories, back
301 to the originals or known safe values. Then the child process, which
302 no longer has any special permissions, does the "open" or other system
303 call. Finally, the child passes the data it managed to access back to
304 the parent. Because the file or pipe was opened in the child while
305 running under less privilege than the parent, it's not apt to be
306 tricked into doing something it shouldn't.
307
308 Here's a way to do backticks reasonably safely. Notice how the "exec"
309 is not called with a string that the shell could expand. This is by
310 far the best way to call something that might be subjected to shell
311 escapes: just never call the shell at all.
312
313 use English;
314 die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
315 if ($pid) { # parent
316 while (<KID>) {
317 # do something
318 }
319 close KID;
320 } else {
321 my @temp = ($EUID, $EGID);
322 my $orig_uid = $UID;
323 my $orig_gid = $GID;
324 $EUID = $UID;
325 $EGID = $GID;
326 # Drop privileges
327 $UID = $orig_uid;
328 $GID = $orig_gid;
329 # Make sure privs are really gone
330 ($EUID, $EGID) = @temp;
331 die "Can't drop privileges"
332 unless $UID == $EUID && $GID eq $EGID;
333 $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH.
334 # Consider sanitizing the environment even more.
335 exec 'myprog', 'arg1', 'arg2'
336 or die "can't exec myprog: $!";
337 }
338
339 A similar strategy would work for wildcard expansion via "glob",
340 although you can use "readdir" instead.
341
342 Taint checking is most useful when although you trust yourself not to
343 have written a program to give away the farm, you don't necessarily
344 trust those who end up using it not to try to trick it into doing
345 something bad. This is the kind of security checking that's useful for
346 set-id programs and programs launched on someone else's behalf, like
347 CGI programs.
348
349 This is quite different, however, from not even trusting the writer of
350 the code not to try to do something evil. That's the kind of trust
351 needed when someone hands you a program you've never seen before and
352 says, "Here, run this." For that kind of safety, you might want to
353 check out the Safe module, included standard in the Perl distribution.
354 This module allows the programmer to set up special compartments in
355 which all system operations are trapped and namespace access is
356 carefully controlled. Safe should not be considered bullet-proof,
357 though: it will not prevent the foreign code to set up infinite loops,
358 allocate gigabytes of memory, or even abusing perl bugs to make the
359 host interpreter crash or behave in unpredictable ways. In any case
360 it's better avoided completely if you're really concerned about
361 security.
362
363 Security Bugs
364 Beyond the obvious problems that stem from giving special privileges to
365 systems as flexible as scripts, on many versions of Unix, set-id
366 scripts are inherently insecure right from the start. The problem is a
367 race condition in the kernel. Between the time the kernel opens the
368 file to see which interpreter to run and when the (now-set-id)
369 interpreter turns around and reopens the file to interpret it, the file
370 in question may have changed, especially if you have symbolic links on
371 your system.
372
373 Fortunately, sometimes this kernel "feature" can be disabled.
374 Unfortunately, there are two ways to disable it. The system can simply
375 outlaw scripts with any set-id bit set, which doesn't help much.
376 Alternately, it can simply ignore the set-id bits on scripts.
377
378 However, if the kernel set-id script feature isn't disabled, Perl will
379 complain loudly that your set-id script is insecure. You'll need to
380 either disable the kernel set-id script feature, or put a C wrapper
381 around the script. A C wrapper is just a compiled program that does
382 nothing except call your Perl program. Compiled programs are not
383 subject to the kernel bug that plagues set-id scripts. Here's a simple
384 wrapper, written in C:
385
386 #include <unistd.h>
387 #include <stdio.h>
388 #include <string.h>
389 #include <errno.h>
390
391 #define REAL_PATH "/path/to/script"
392
393 int main(int argc, char **argv)
394 {
395 execv(REAL_PATH, argv);
396 fprintf(stderr, "%s: %s: %s\n",
397 argv[0], REAL_PATH, strerror(errno));
398 return 127;
399 }
400
401 Compile this wrapper into a binary executable and then make it rather
402 than your script setuid or setgid.
403
404 In recent years, vendors have begun to supply systems free of this
405 inherent security bug. On such systems, when the kernel passes the
406 name of the set-id script to open to the interpreter, rather than using
407 a pathname subject to meddling, it instead passes /dev/fd/3. This is a
408 special file already opened on the script, so that there can be no race
409 condition for evil scripts to exploit. On these systems, Perl should
410 be compiled with "-DSETUID_SCRIPTS_ARE_SECURE_NOW". The Configure
411 program that builds Perl tries to figure this out for itself, so you
412 should never have to specify this yourself. Most modern releases of
413 SysVr4 and BSD 4.4 use this approach to avoid the kernel race
414 condition.
415
416 Protecting Your Programs
417 There are a number of ways to hide the source to your Perl programs,
418 with varying levels of "security".
419
420 First of all, however, you can't take away read permission, because the
421 source code has to be readable in order to be compiled and interpreted.
422 (That doesn't mean that a CGI script's source is readable by people on
423 the web, though.) So you have to leave the permissions at the socially
424 friendly 0755 level. This lets people on your local system only see
425 your source.
426
427 Some people mistakenly regard this as a security problem. If your
428 program does insecure things, and relies on people not knowing how to
429 exploit those insecurities, it is not secure. It is often possible for
430 someone to determine the insecure things and exploit them without
431 viewing the source. Security through obscurity, the name for hiding
432 your bugs instead of fixing them, is little security indeed.
433
434 You can try using encryption via source filters (Filter::* from CPAN,
435 or Filter::Util::Call and Filter::Simple since Perl 5.8). But crackers
436 might be able to decrypt it. You can try using the byte code compiler
437 and interpreter described below, but crackers might be able to de-
438 compile it. You can try using the native-code compiler described
439 below, but crackers might be able to disassemble it. These pose
440 varying degrees of difficulty to people wanting to get at your code,
441 but none can definitively conceal it (this is true of every language,
442 not just Perl).
443
444 If you're concerned about people profiting from your code, then the
445 bottom line is that nothing but a restrictive license will give you
446 legal security. License your software and pepper it with threatening
447 statements like "This is unpublished proprietary software of XYZ Corp.
448 Your access to it does not give you permission to use it blah blah
449 blah." You should see a lawyer to be sure your license's wording will
450 stand up in court.
451
452 Unicode
453 Unicode is a new and complex technology and one may easily overlook
454 certain security pitfalls. See perluniintro for an overview and
455 perlunicode for details, and "Security Implications of Unicode" in
456 perlunicode for security implications in particular.
457
458 Algorithmic Complexity Attacks
459 Certain internal algorithms used in the implementation of Perl can be
460 attacked by choosing the input carefully to consume large amounts of
461 either time or space or both. This can lead into the so-called Denial
462 of Service (DoS) attacks.
463
464 · Hash Algorithm - Hash algorithms like the one used in Perl are well
465 known to be vulnerable to collision attacks on their hash function.
466 Such attacks involve constructing a set of keys which collide into
467 the same bucket producing inefficient behavior. Such attacks often
468 depend on discovering the seed of the hash function used to map the
469 keys to buckets. That seed is then used to brute-force a key set
470 which can be used to mount a denial of service attack. In Perl
471 5.8.1 changes were introduced to harden Perl to such attacks, and
472 then later in Perl 5.18.0 these features were enhanced and
473 additional protections added.
474
475 At the time of this writing, Perl 5.18.0 is considered to be well-
476 hardened against algorithmic complexity attacks on its hash
477 implementation. This is largely owed to the following measures
478 mitigate attacks:
479
480 Hash Seed Randomization
481 In order to make it impossible to know what seed to generate an
482 attack key set for, this seed is randomly initialized at
483 process start. This may be overridden by using the
484 PERL_HASH_SEED environment variable, see "PERL_HASH_SEED" in
485 perlrun. This environment variable controls how items are
486 actually stored, not how they are presented via "keys",
487 "values" and "each".
488
489 Hash Traversal Randomization
490 Independent of which seed is used in the hash function, "keys",
491 "values", and "each" return items in a per-hash randomized
492 order. Modifying a hash by insertion will change the iteration
493 order of that hash. This behavior can be overridden by using
494 "hash_traversal_mask()" from Hash::Util or by using the
495 PERL_PERTURB_KEYS environment variable, see "PERL_PERTURB_KEYS"
496 in perlrun. Note that this feature controls the "visible"
497 order of the keys, and not the actual order they are stored in.
498
499 Bucket Order Perturbance
500 When items collide into a given hash bucket the order they are
501 stored in the chain is no longer predictable in Perl 5.18.
502 This has the intention to make it harder to observe a
503 collision. This behavior can be overridden by using the
504 PERL_PERTURB_KEYS environment variable, see "PERL_PERTURB_KEYS"
505 in perlrun.
506
507 New Default Hash Function
508 The default hash function has been modified with the intention
509 of making it harder to infer the hash seed.
510
511 Alternative Hash Functions
512 The source code includes multiple hash algorithms to choose
513 from. While we believe that the default perl hash is robust to
514 attack, we have included the hash function Siphash as a fall-
515 back option. At the time of release of Perl 5.18.0 Siphash is
516 believed to be of cryptographic strength. This is not the
517 default as it is much slower than the default hash.
518
519 Without compiling a special Perl, there is no way to get the exact
520 same behavior of any versions prior to Perl 5.18.0. The closest
521 one can get is by setting PERL_PERTURB_KEYS to 0 and setting the
522 PERL_HASH_SEED to a known value. We do not advise those settings
523 for production use due to the above security considerations.
524
525 Perl has never guaranteed any ordering of the hash keys, and the
526 ordering has already changed several times during the lifetime of
527 Perl 5. Also, the ordering of hash keys has always been, and
528 continues to be, affected by the insertion order and the history of
529 changes made to the hash over its lifetime.
530
531 Also note that while the order of the hash elements might be
532 randomized, this "pseudo-ordering" should not be used for
533 applications like shuffling a list randomly (use
534 "List::Util::shuffle()" for that, see List::Util, a standard core
535 module since Perl 5.8.0; or the CPAN module
536 "Algorithm::Numerical::Shuffle"), or for generating permutations
537 (use e.g. the CPAN modules "Algorithm::Permute" or
538 "Algorithm::FastPermute"), or for any cryptographic applications.
539
540 Tied hashes may have their own ordering and algorithmic complexity
541 attacks.
542
543 · Regular expressions - Perl's regular expression engine is so called
544 NFA (Non-deterministic Finite Automaton), which among other things
545 means that it can rather easily consume large amounts of both time
546 and space if the regular expression may match in several ways.
547 Careful crafting of the regular expressions can help but quite
548 often there really isn't much one can do (the book "Mastering
549 Regular Expressions" is required reading, see perlfaq2). Running
550 out of space manifests itself by Perl running out of memory.
551
552 · Sorting - the quicksort algorithm used in Perls before 5.8.0 to
553 implement the sort() function is very easy to trick into
554 misbehaving so that it consumes a lot of time. Starting from Perl
555 5.8.0 a different sorting algorithm, mergesort, is used by default.
556 Mergesort cannot misbehave on any input.
557
558 See
559 <https://www.usenix.org/legacy/events/sec03/tech/full_papers/crosby/crosby.pdf>
560 for more information, and any computer science textbook on algorithmic
561 complexity.
562
564 perlrun for its description of cleaning up environment variables.
565
566
567
568perl v5.26.3 2018-03-23 PERLSEC(1)