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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "PERLSEC 1" | |
132 | .TH PERLSEC 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | perlsec \- Perl security | |
135 | .SH "DESCRIPTION" | |
136 | .IX Header "DESCRIPTION" | |
137 | Perl is designed to make it easy to program securely even when running | |
138 | with extra privileges, like setuid or setgid programs. Unlike most | |
139 | command line shells, which are based on multiple substitution passes on | |
140 | each line of the script, Perl uses a more conventional evaluation scheme | |
141 | with fewer hidden snags. Additionally, because the language has more | |
142 | builtin functionality, it can rely less upon external (and possibly | |
143 | untrustworthy) programs to accomplish its purposes. | |
144 | .PP | |
145 | Perl automatically enables a set of special security checks, called \fItaint | |
146 | mode\fR, when it detects its program running with differing real and effective | |
147 | user or group IDs. The setuid bit in Unix permissions is mode 04000, the | |
148 | setgid bit mode 02000; either or both may be set. You can also enable taint | |
149 | mode explicitly by using the \fB\-T\fR command line flag. This flag is | |
150 | \&\fIstrongly\fR suggested for server programs and any program run on behalf of | |
151 | someone else, such as a \s-1CGI\s0 script. Once taint mode is on, it's on for | |
152 | the remainder of your script. | |
153 | .PP | |
154 | While in this mode, Perl takes special precautions called \fItaint | |
155 | checks\fR to prevent both obvious and subtle traps. Some of these checks | |
156 | are reasonably simple, such as verifying that path directories aren't | |
157 | writable by others; careful programmers have always used checks like | |
158 | these. Other checks, however, are best supported by the language itself, | |
159 | and it is these checks especially that contribute to making a set-id Perl | |
160 | program more secure than the corresponding C program. | |
161 | .PP | |
162 | You may not use data derived from outside your program to affect | |
163 | something else outside your program\*(--at least, not by accident. All | |
164 | command line arguments, environment variables, locale information (see | |
165 | perllocale), results of certain system calls (\fIreaddir()\fR, | |
166 | \&\fIreadlink()\fR, the variable of \fIshmread()\fR, the messages returned by | |
167 | \&\fImsgrcv()\fR, the password, gcos and shell fields returned by the | |
168 | \&\fIgetpwxxx()\fR calls), and all file input are marked as \*(L"tainted\*(R". | |
169 | Tainted data may not be used directly or indirectly in any command | |
170 | that invokes a sub\-shell, nor in any command that modifies files, | |
171 | directories, or processes, \fBwith the following exceptions\fR: | |
172 | .IP "\(bu" 4 | |
173 | If you pass more than one argument to either \f(CW\*(C`system\*(C'\fR or \f(CW\*(C`exec\*(C'\fR, | |
174 | the arguments are checked for taintedness \fBbut\fR the operation will still | |
175 | be attempted, emitting an optional warning. This will be fatal in a | |
176 | future version of perl so do not rely on it to bypass the tainting | |
177 | mechanism. | |
178 | .IP "\(bu" 4 | |
179 | Arguments to \f(CW\*(C`print\*(C'\fR and \f(CW\*(C`syswrite\*(C'\fR are \fBnot\fR checked for taintedness. | |
180 | .IP "\(bu" 4 | |
181 | Symbolic methods | |
182 | .Sp | |
183 | .Vb 1 | |
184 | \& $obj->$method(@args); | |
185 | .Ve | |
186 | .Sp | |
187 | and symbolic sub references | |
188 | .Sp | |
189 | .Vb 2 | |
190 | \& &{$foo}(@args); | |
191 | \& $foo->(@args); | |
192 | .Ve | |
193 | .Sp | |
194 | are not checked for taintedness. This requires extra carefulness | |
195 | unless you want external data to affect your control flow. Unless | |
196 | you carefully limit what these symbolic values are, people are able | |
197 | to call functions \fBoutside\fR your Perl code, such as POSIX::system, | |
198 | in which case they are able to run arbitrary external code. | |
199 | .PP | |
200 | The value of an expression containing tainted data will itself be | |
201 | tainted, even if it is logically impossible for the tainted data to | |
202 | affect the value. | |
203 | .PP | |
204 | Because taintedness is associated with each scalar value, some | |
205 | elements of an array can be tainted and others not. | |
206 | .PP | |
207 | For example: | |
208 | .PP | |
209 | .Vb 8 | |
210 | \& $arg = shift; # $arg is tainted | |
211 | \& $hid = $arg, 'bar'; # $hid is also tainted | |
212 | \& $line = <>; # Tainted | |
213 | \& $line = <STDIN>; # Also tainted | |
214 | \& open FOO, "/home/me/bar" or die $!; | |
215 | \& $line = <FOO>; # Still tainted | |
216 | \& $path = $ENV{'PATH'}; # Tainted, but see below | |
217 | \& $data = 'abc'; # Not tainted | |
218 | .Ve | |
219 | .PP | |
220 | .Vb 5 | |
221 | \& system "echo $arg"; # Insecure | |
222 | \& system "/bin/echo", $arg; # Allowed but considered insecure | |
223 | \& # (Perl doesn't know about /bin/echo) | |
224 | \& system "echo $hid"; # Insecure | |
225 | \& system "echo $data"; # Insecure until PATH set | |
226 | .Ve | |
227 | .PP | |
228 | .Vb 1 | |
229 | \& $path = $ENV{'PATH'}; # $path now tainted | |
230 | .Ve | |
231 | .PP | |
232 | .Vb 2 | |
233 | \& $ENV{'PATH'} = '/bin:/usr/bin'; | |
234 | \& delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; | |
235 | .Ve | |
236 | .PP | |
237 | .Vb 2 | |
238 | \& $path = $ENV{'PATH'}; # $path now NOT tainted | |
239 | \& system "echo $data"; # Is secure now! | |
240 | .Ve | |
241 | .PP | |
242 | .Vb 2 | |
243 | \& open(FOO, "< $arg"); # OK - read-only file | |
244 | \& open(FOO, "> $arg"); # Not OK - trying to write | |
245 | .Ve | |
246 | .PP | |
247 | .Vb 3 | |
248 | \& open(FOO,"echo $arg|"); # Not OK | |
249 | \& open(FOO,"-|") | |
250 | \& or exec 'echo', $arg; # Allowed but not really OK | |
251 | .Ve | |
252 | .PP | |
253 | .Vb 1 | |
254 | \& $shout = `echo $arg`; # Insecure, $shout now tainted | |
255 | .Ve | |
256 | .PP | |
257 | .Vb 2 | |
258 | \& unlink $data, $arg; # Insecure | |
259 | \& umask $arg; # Insecure | |
260 | .Ve | |
261 | .PP | |
262 | .Vb 3 | |
263 | \& exec "echo $arg"; # Insecure | |
264 | \& exec "echo", $arg; # Allowed but considered insecure | |
265 | \& exec "sh", '-c', $arg; # Considered secure, alas! | |
266 | .Ve | |
267 | .PP | |
268 | .Vb 2 | |
269 | \& @files = <*.c>; # insecure (uses readdir() or similar) | |
270 | \& @files = glob('*.c'); # insecure (uses readdir() or similar) | |
271 | .Ve | |
272 | .PP | |
273 | .Vb 4 | |
274 | \& # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would | |
275 | \& # have used an external program to do the filename expansion; but in | |
276 | \& # either case the result is tainted since the list of filenames comes | |
277 | \& # from outside of the program. | |
278 | .Ve | |
279 | .PP | |
280 | .Vb 2 | |
281 | \& $bad = ($arg, 23); # $bad will be tainted | |
282 | \& $arg, `true`; # Insecure (although it isn't really) | |
283 | .Ve | |
284 | .PP | |
285 | If you try to do something insecure, you will get a fatal error saying | |
286 | something like \*(L"Insecure dependency\*(R" or \*(L"Insecure \f(CW$ENV\fR{\s-1PATH\s0}\*(R". Note that you | |
287 | can still write an insecure \fBsystem\fR or \fBexec\fR, but only by explicitly | |
288 | doing something like the \*(L"considered secure\*(R" example above. This will not | |
289 | be possible in a future version of Perl. | |
290 | .Sh "Laundering and Detecting Tainted Data" | |
291 | .IX Subsection "Laundering and Detecting Tainted Data" | |
292 | To test whether a variable contains tainted data, and whose use would | |
293 | thus trigger an \*(L"Insecure dependency\*(R" message, you can use the | |
294 | \&\fItainted()\fR function of the Scalar::Util module, available in your | |
295 | nearby \s-1CPAN\s0 mirror, and included in Perl starting from the release 5.8.0. | |
296 | Or you may be able to use the following \fI\fIis_tainted()\fI\fR function. | |
297 | .PP | |
298 | .Vb 3 | |
299 | \& sub is_tainted { | |
300 | \& return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 }; | |
301 | \& } | |
302 | .Ve | |
303 | .PP | |
304 | This function makes use of the fact that the presence of tainted data | |
305 | anywhere within an expression renders the entire expression tainted. It | |
306 | would be inefficient for every operator to test every argument for | |
307 | taintedness. Instead, the slightly more efficient and conservative | |
308 | approach is used that if any tainted value has been accessed within the | |
309 | same expression, the whole expression is considered tainted. | |
310 | .PP | |
311 | But testing for taintedness gets you only so far. Sometimes you have just | |
312 | to clear your data's taintedness. The only way to bypass the tainting | |
313 | mechanism is by referencing subpatterns from a regular expression match. | |
314 | Perl presumes that if you reference a substring using \f(CW$1\fR, \f(CW$2\fR, etc., that | |
315 | you knew what you were doing when you wrote the pattern. That means using | |
316 | a bit of thought\*(--don't just blindly untaint anything, or you defeat the | |
317 | entire mechanism. It's better to verify that the variable has only good | |
318 | characters (for certain values of \*(L"good\*(R") rather than checking whether it | |
319 | has any bad characters. That's because it's far too easy to miss bad | |
320 | characters that you never thought of. | |
321 | .PP | |
322 | Here's a test to make sure that the data contains nothing but \*(L"word\*(R" | |
323 | characters (alphabetics, numerics, and underscores), a hyphen, an at sign, | |
324 | or a dot. | |
325 | .PP | |
326 | .Vb 5 | |
327 | \& if ($data =~ /^([-\e@\ew.]+)$/) { | |
328 | \& $data = $1; # $data now untainted | |
329 | \& } else { | |
330 | \& die "Bad data in $data"; # log this somewhere | |
331 | \& } | |
332 | .Ve | |
333 | .PP | |
334 | This is fairly secure because \f(CW\*(C`/\ew+/\*(C'\fR doesn't normally match shell | |
335 | metacharacters, nor are dot, dash, or at going to mean something special | |
336 | to the shell. Use of \f(CW\*(C`/.+/\*(C'\fR would have been insecure in theory because | |
337 | it lets everything through, but Perl doesn't check for that. The lesson | |
338 | is that when untainting, you must be exceedingly careful with your patterns. | |
339 | Laundering data using regular expression is the \fIonly\fR mechanism for | |
340 | untainting dirty data, unless you use the strategy detailed below to fork | |
341 | a child of lesser privilege. | |
342 | .PP | |
343 | The example does not untaint \f(CW$data\fR if \f(CW\*(C`use locale\*(C'\fR is in effect, | |
344 | because the characters matched by \f(CW\*(C`\ew\*(C'\fR are determined by the locale. | |
345 | Perl considers that locale definitions are untrustworthy because they | |
346 | contain data from outside the program. If you are writing a | |
347 | locale-aware program, and want to launder data with a regular expression | |
348 | containing \f(CW\*(C`\ew\*(C'\fR, put \f(CW\*(C`no locale\*(C'\fR ahead of the expression in the same | |
349 | block. See \*(L"\s-1SECURITY\s0\*(R" in perllocale for further discussion and examples. | |
350 | .ie n .Sh "Switches On the ""#!"" Line" | |
351 | .el .Sh "Switches On the ``#!'' Line" | |
352 | .IX Subsection "Switches On the #! Line" | |
353 | When you make a script executable, in order to make it usable as a | |
354 | command, the system will pass switches to perl from the script's #! | |
355 | line. Perl checks that any command line switches given to a setuid | |
356 | (or setgid) script actually match the ones set on the #! line. Some | |
357 | Unix and Unix-like environments impose a one-switch limit on the #! | |
358 | line, so you may need to use something like \f(CW\*(C`\-wU\*(C'\fR instead of \f(CW\*(C`\-w \-U\*(C'\fR | |
359 | under such systems. (This issue should arise only in Unix or | |
360 | Unix-like environments that support #! and setuid or setgid scripts.) | |
361 | .Sh "Cleaning Up Your Path" | |
362 | .IX Subsection "Cleaning Up Your Path" | |
363 | For "Insecure \f(CW$ENV{PATH}\fR" messages, you need to set \f(CW$ENV{'PATH'}\fR to a | |
364 | known value, and each directory in the path must be non-writable by others | |
365 | than its owner and group. You may be surprised to get this message even | |
366 | if the pathname to your executable is fully qualified. This is \fInot\fR | |
367 | generated because you didn't supply a full path to the program; instead, | |
368 | it's generated because you never set your \s-1PATH\s0 environment variable, or | |
369 | you didn't set it to something that was safe. Because Perl can't | |
370 | guarantee that the executable in question isn't itself going to turn | |
371 | around and execute some other program that is dependent on your \s-1PATH\s0, it | |
372 | makes sure you set the \s-1PATH\s0. | |
373 | .PP | |
374 | The \s-1PATH\s0 isn't the only environment variable which can cause problems. | |
375 | Because some shells may use the variables \s-1IFS\s0, \s-1CDPATH\s0, \s-1ENV\s0, and | |
376 | \&\s-1BASH_ENV\s0, Perl checks that those are either empty or untainted when | |
377 | starting subprocesses. You may wish to add something like this to your | |
378 | setid and taint-checking scripts. | |
379 | .PP | |
380 | .Vb 1 | |
381 | \& delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer | |
382 | .Ve | |
383 | .PP | |
384 | It's also possible to get into trouble with other operations that don't | |
385 | care whether they use tainted values. Make judicious use of the file | |
386 | tests in dealing with any user-supplied filenames. When possible, do | |
387 | opens and such \fBafter\fR properly dropping any special user (or group!) | |
388 | privileges. Perl doesn't prevent you from opening tainted filenames for reading, | |
389 | so be careful what you print out. The tainting mechanism is intended to | |
390 | prevent stupid mistakes, not to remove the need for thought. | |
391 | .PP | |
392 | Perl does not call the shell to expand wild cards when you pass \fBsystem\fR | |
393 | and \fBexec\fR explicit parameter lists instead of strings with possible shell | |
394 | wildcards in them. Unfortunately, the \fBopen\fR, \fBglob\fR, and | |
395 | backtick functions provide no such alternate calling convention, so more | |
396 | subterfuge will be required. | |
397 | .PP | |
398 | Perl provides a reasonably safe way to open a file or pipe from a setuid | |
399 | or setgid program: just create a child process with reduced privilege who | |
400 | does the dirty work for you. First, fork a child using the special | |
401 | \&\fBopen\fR syntax that connects the parent and child by a pipe. Now the | |
402 | child resets its \s-1ID\s0 set and any other per-process attributes, like | |
403 | environment variables, umasks, current working directories, back to the | |
404 | originals or known safe values. Then the child process, which no longer | |
405 | has any special permissions, does the \fBopen\fR or other system call. | |
406 | Finally, the child passes the data it managed to access back to the | |
407 | parent. Because the file or pipe was opened in the child while running | |
408 | under less privilege than the parent, it's not apt to be tricked into | |
409 | doing something it shouldn't. | |
410 | .PP | |
411 | Here's a way to do backticks reasonably safely. Notice how the \fBexec\fR is | |
412 | not called with a string that the shell could expand. This is by far the | |
413 | best way to call something that might be subjected to shell escapes: just | |
414 | never call the shell at all. | |
415 | .PP | |
416 | .Vb 25 | |
417 | \& use English '-no_match_vars'; | |
418 | \& die "Can't fork: $!" unless defined($pid = open(KID, "-|")); | |
419 | \& if ($pid) { # parent | |
420 | \& while (<KID>) { | |
421 | \& # do something | |
422 | \& } | |
423 | \& close KID; | |
424 | \& } else { | |
425 | \& my @temp = ($EUID, $EGID); | |
426 | \& my $orig_uid = $UID; | |
427 | \& my $orig_gid = $GID; | |
428 | \& $EUID = $UID; | |
429 | \& $EGID = $GID; | |
430 | \& # Drop privileges | |
431 | \& $UID = $orig_uid; | |
432 | \& $GID = $orig_gid; | |
433 | \& # Make sure privs are really gone | |
434 | \& ($EUID, $EGID) = @temp; | |
435 | \& die "Can't drop privileges" | |
436 | \& unless $UID == $EUID && $GID eq $EGID; | |
437 | \& $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH. | |
438 | \& # Consider sanitizing the environment even more. | |
439 | \& exec 'myprog', 'arg1', 'arg2' | |
440 | \& or die "can't exec myprog: $!"; | |
441 | \& } | |
442 | .Ve | |
443 | .PP | |
444 | A similar strategy would work for wildcard expansion via \f(CW\*(C`glob\*(C'\fR, although | |
445 | you can use \f(CW\*(C`readdir\*(C'\fR instead. | |
446 | .PP | |
447 | Taint checking is most useful when although you trust yourself not to have | |
448 | written a program to give away the farm, you don't necessarily trust those | |
449 | who end up using it not to try to trick it into doing something bad. This | |
450 | is the kind of security checking that's useful for set-id programs and | |
451 | programs launched on someone else's behalf, like \s-1CGI\s0 programs. | |
452 | .PP | |
453 | This is quite different, however, from not even trusting the writer of the | |
454 | code not to try to do something evil. That's the kind of trust needed | |
455 | when someone hands you a program you've never seen before and says, \*(L"Here, | |
456 | run this.\*(R" For that kind of safety, check out the Safe module, | |
457 | included standard in the Perl distribution. This module allows the | |
458 | programmer to set up special compartments in which all system operations | |
459 | are trapped and namespace access is carefully controlled. | |
460 | .Sh "Security Bugs" | |
461 | .IX Subsection "Security Bugs" | |
462 | Beyond the obvious problems that stem from giving special privileges to | |
463 | systems as flexible as scripts, on many versions of Unix, set-id scripts | |
464 | are inherently insecure right from the start. The problem is a race | |
465 | condition in the kernel. Between the time the kernel opens the file to | |
466 | see which interpreter to run and when the (now\-set\-id) interpreter turns | |
467 | around and reopens the file to interpret it, the file in question may have | |
468 | changed, especially if you have symbolic links on your system. | |
469 | .PP | |
470 | Fortunately, sometimes this kernel \*(L"feature\*(R" can be disabled. | |
471 | Unfortunately, there are two ways to disable it. The system can simply | |
472 | outlaw scripts with any set-id bit set, which doesn't help much. | |
473 | Alternately, it can simply ignore the set-id bits on scripts. If the | |
474 | latter is true, Perl can emulate the setuid and setgid mechanism when it | |
475 | notices the otherwise useless setuid/gid bits on Perl scripts. It does | |
476 | this via a special executable called \fBsuidperl\fR that is automatically | |
477 | invoked for you if it's needed. | |
478 | .PP | |
479 | However, if the kernel set-id script feature isn't disabled, Perl will | |
480 | complain loudly that your set-id script is insecure. You'll need to | |
481 | either disable the kernel set-id script feature, or put a C wrapper around | |
482 | the script. A C wrapper is just a compiled program that does nothing | |
483 | except call your Perl program. Compiled programs are not subject to the | |
484 | kernel bug that plagues set-id scripts. Here's a simple wrapper, written | |
485 | in C: | |
486 | .PP | |
487 | .Vb 6 | |
488 | \& #define REAL_PATH "/path/to/script" | |
489 | \& main(ac, av) | |
490 | \& char **av; | |
491 | \& { | |
492 | \& execv(REAL_PATH, av); | |
493 | \& } | |
494 | .Ve | |
495 | .PP | |
496 | Compile this wrapper into a binary executable and then make \fIit\fR rather | |
497 | than your script setuid or setgid. | |
498 | .PP | |
499 | In recent years, vendors have begun to supply systems free of this | |
500 | inherent security bug. On such systems, when the kernel passes the name | |
501 | of the set-id script to open to the interpreter, rather than using a | |
502 | pathname subject to meddling, it instead passes \fI/dev/fd/3\fR. This is a | |
503 | special file already opened on the script, so that there can be no race | |
504 | condition for evil scripts to exploit. On these systems, Perl should be | |
505 | compiled with \f(CW\*(C`\-DSETUID_SCRIPTS_ARE_SECURE_NOW\*(C'\fR. The \fBConfigure\fR | |
506 | program that builds Perl tries to figure this out for itself, so you | |
507 | should never have to specify this yourself. Most modern releases of | |
508 | SysVr4 and \s-1BSD\s0 4.4 use this approach to avoid the kernel race condition. | |
509 | .PP | |
510 | Prior to release 5.6.1 of Perl, bugs in the code of \fBsuidperl\fR could | |
511 | introduce a security hole. | |
512 | .Sh "Protecting Your Programs" | |
513 | .IX Subsection "Protecting Your Programs" | |
514 | There are a number of ways to hide the source to your Perl programs, | |
515 | with varying levels of \*(L"security\*(R". | |
516 | .PP | |
517 | First of all, however, you \fIcan't\fR take away read permission, because | |
518 | the source code has to be readable in order to be compiled and | |
519 | interpreted. (That doesn't mean that a \s-1CGI\s0 script's source is | |
520 | readable by people on the web, though.) So you have to leave the | |
521 | permissions at the socially friendly 0755 level. This lets | |
522 | people on your local system only see your source. | |
523 | .PP | |
524 | Some people mistakenly regard this as a security problem. If your program does | |
525 | insecure things, and relies on people not knowing how to exploit those | |
526 | insecurities, it is not secure. It is often possible for someone to | |
527 | determine the insecure things and exploit them without viewing the | |
528 | source. Security through obscurity, the name for hiding your bugs | |
529 | instead of fixing them, is little security indeed. | |
530 | .PP | |
531 | You can try using encryption via source filters (Filter::* from \s-1CPAN\s0, | |
532 | or Filter::Util::Call and Filter::Simple since Perl 5.8). | |
533 | But crackers might be able to decrypt it. You can try using the byte | |
534 | code compiler and interpreter described below, but crackers might be | |
535 | able to de-compile it. You can try using the native-code compiler | |
536 | described below, but crackers might be able to disassemble it. These | |
537 | pose varying degrees of difficulty to people wanting to get at your | |
538 | code, but none can definitively conceal it (this is true of every | |
539 | language, not just Perl). | |
540 | .PP | |
541 | If you're concerned about people profiting from your code, then the | |
542 | bottom line is that nothing but a restrictive licence will give you | |
543 | legal security. License your software and pepper it with threatening | |
544 | statements like \*(L"This is unpublished proprietary software of \s-1XYZ\s0 Corp. | |
545 | Your access to it does not give you permission to use it blah blah | |
546 | blah.\*(R" You should see a lawyer to be sure your licence's wording will | |
547 | stand up in court. | |
548 | .Sh "Unicode" | |
549 | .IX Subsection "Unicode" | |
550 | Unicode is a new and complex technology and one may easily overlook | |
551 | certain security pitfalls. See perluniintro for an overview and | |
552 | perlunicode for details, and \*(L"Security Implications of Unicode\*(R" in perlunicode for security implications in particular. | |
553 | .SH "SEE ALSO" | |
554 | .IX Header "SEE ALSO" | |
555 | perlrun for its description of cleaning up environment variables. |