1PERLSEC(1)             Perl Programmers Reference Guide             PERLSEC(1)
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NAME

6       perlsec - Perl security
7

DESCRIPTION

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

SECURITY VULNERABILITY CONTACT INFORMATION

18       If you believe you have found a security vulnerability in Perl, please
19       email perl5-security-report@perl.org with details.  This points to a
20       closed subscription, unarchived mailing list.  Please only use this
21       address for security issues in the Perl core, not for modules
22       independently distributed on CPAN.
23

SECURITY MECHANISMS AND CONCERNS

25   Taint mode
26       Perl automatically enables a set of special security checks, called
27       taint mode, when it detects its program running with differing real and
28       effective user or group IDs.  The setuid bit in Unix permissions is
29       mode 04000, the setgid bit mode 02000; either or both may be set.  You
30       can also enable taint mode explicitly by using the -T command line
31       flag. This flag is strongly suggested for server programs and any
32       program run on behalf of someone else, such as a CGI script. Once taint
33       mode is on, it's on for the remainder of your script.
34
35       While in this mode, Perl takes special precautions called taint checks
36       to prevent both obvious and subtle traps.  Some of these checks are
37       reasonably simple, such as verifying that path directories aren't
38       writable by others; careful programmers have always used checks like
39       these.  Other checks, however, are best supported by the language
40       itself, and it is these checks especially that contribute to making a
41       set-id Perl program more secure than the corresponding C program.
42
43       You may not use data derived from outside your program to affect
44       something else outside your program--at least, not by accident.  All
45       command line arguments, environment variables, locale information (see
46       perllocale), results of certain system calls ("readdir()",
47       "readlink()", the variable of "shmread()", the messages returned by
48       "msgrcv()", the password, gcos and shell fields returned by the
49       "getpwxxx()" calls), and all file input are marked as "tainted".
50       Tainted data may not be used directly or indirectly in any command that
51       invokes a sub-shell, nor in any command that modifies files,
52       directories, or processes, with the following exceptions:
53
54       ·   Arguments to "print" and "syswrite" are not checked for
55           taintedness.
56
57       ·   Symbolic methods
58
59               $obj->$method(@args);
60
61           and symbolic sub references
62
63               &{$foo}(@args);
64               $foo->(@args);
65
66           are not checked for taintedness.  This requires extra carefulness
67           unless you want external data to affect your control flow.  Unless
68           you carefully limit what these symbolic values are, people are able
69           to call functions outside your Perl code, such as POSIX::system, in
70           which case they are able to run arbitrary external code.
71
72       ·   Hash keys are never tainted.
73
74       For efficiency reasons, Perl takes a conservative view of whether data
75       is tainted.  If an expression contains tainted data, any subexpression
76       may be considered tainted, even if the value of the subexpression is
77       not itself affected by the tainted data.
78
79       Because taintedness is associated with each scalar value, some elements
80       of an array or hash can be tainted and others not.  The keys of a hash
81       are never tainted.
82
83       For example:
84
85           $arg = shift;               # $arg is tainted
86           $hid = $arg, 'bar';         # $hid is also tainted
87           $line = <>;                 # Tainted
88           $line = <STDIN>;            # Also tainted
89           open FOO, "/home/me/bar" or die $!;
90           $line = <FOO>;              # Still tainted
91           $path = $ENV{'PATH'};       # Tainted, but see below
92           $data = 'abc';              # Not tainted
93
94           system "echo $arg";         # Insecure
95           system "/bin/echo", $arg;   # Considered insecure
96                                       # (Perl doesn't know about /bin/echo)
97           system "echo $hid";         # Insecure
98           system "echo $data";        # Insecure until PATH set
99
100           $path = $ENV{'PATH'};       # $path now tainted
101
102           $ENV{'PATH'} = '/bin:/usr/bin';
103           delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
104
105           $path = $ENV{'PATH'};       # $path now NOT tainted
106           system "echo $data";        # Is secure now!
107
108           open(FOO, "< $arg");        # OK - read-only file
109           open(FOO, "> $arg");        # Not OK - trying to write
110
111           open(FOO,"echo $arg|");     # Not OK
112           open(FOO,"-|")
113               or exec 'echo', $arg;   # Also not OK
114
115           $shout = `echo $arg`;       # Insecure, $shout now tainted
116
117           unlink $data, $arg;         # Insecure
118           umask $arg;                 # Insecure
119
120           exec "echo $arg";           # Insecure
121           exec "echo", $arg;          # Insecure
122           exec "sh", '-c', $arg;      # Very insecure!
123
124           @files = <*.c>;             # insecure (uses readdir() or similar)
125           @files = glob('*.c');       # insecure (uses readdir() or similar)
126
127           # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would
128           # have used an external program to do the filename expansion; but in
129           # either case the result is tainted since the list of filenames comes
130           # from outside of the program.
131
132           $bad = ($arg, 23);          # $bad will be tainted
133           $arg, `true`;               # Insecure (although it isn't really)
134
135       If you try to do something insecure, you will get a fatal error saying
136       something like "Insecure dependency" or "Insecure $ENV{PATH}".
137
138       The exception to the principle of "one tainted value taints the whole
139       expression" is with the ternary conditional operator "?:".  Since code
140       with a ternary conditional
141
142           $result = $tainted_value ? "Untainted" : "Also untainted";
143
144       is effectively
145
146           if ( $tainted_value ) {
147               $result = "Untainted";
148           } else {
149               $result = "Also untainted";
150           }
151
152       it doesn't make sense for $result to be tainted.
153
154   Laundering and Detecting Tainted Data
155       To test whether a variable contains tainted data, and whose use would
156       thus trigger an "Insecure dependency" message, you can use the
157       "tainted()" function of the Scalar::Util module, available in your
158       nearby CPAN mirror, and included in Perl starting from the release
159       5.8.0.  Or you may be able to use the following "is_tainted()"
160       function.
161
162           sub is_tainted {
163               local $@;   # Don't pollute caller's value.
164               return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
165           }
166
167       This function makes use of the fact that the presence of tainted data
168       anywhere within an expression renders the entire expression tainted.
169       It would be inefficient for every operator to test every argument for
170       taintedness.  Instead, the slightly more efficient and conservative
171       approach is used that if any tainted value has been accessed within the
172       same expression, the whole expression is considered tainted.
173
174       But testing for taintedness gets you only so far.  Sometimes you have
175       just to clear your data's taintedness.  Values may be untainted by
176       using them as keys in a hash; otherwise the only way to bypass the
177       tainting mechanism is by referencing subpatterns from a regular
178       expression match.  Perl presumes that if you reference a substring
179       using $1, $2, etc., that you knew what you were doing when you wrote
180       the pattern.  That means using a bit of thought--don't just blindly
181       untaint anything, or you defeat the entire mechanism.  It's better to
182       verify that the variable has only good characters (for certain values
183       of "good") rather than checking whether it has any bad characters.
184       That's because it's far too easy to miss bad characters that you never
185       thought of.
186
187       Here's a test to make sure that the data contains nothing but "word"
188       characters (alphabetics, numerics, and underscores), a hyphen, an at
189       sign, or a dot.
190
191           if ($data =~ /^([-\@\w.]+)$/) {
192               $data = $1;                     # $data now untainted
193           } else {
194               die "Bad data in '$data'";      # log this somewhere
195           }
196
197       This is fairly secure because "/\w+/" doesn't normally match shell
198       metacharacters, nor are dot, dash, or at going to mean something
199       special to the shell.  Use of "/.+/" would have been insecure in theory
200       because it lets everything through, but Perl doesn't check for that.
201       The lesson is that when untainting, you must be exceedingly careful
202       with your patterns.  Laundering data using regular expression is the
203       only mechanism for untainting dirty data, unless you use the strategy
204       detailed below to fork a child of lesser privilege.
205
206       The example does not untaint $data if "use locale" is in effect,
207       because the characters matched by "\w" are determined by the locale.
208       Perl considers that locale definitions are untrustworthy because they
209       contain data from outside the program.  If you are writing a locale-
210       aware program, and want to launder data with a regular expression
211       containing "\w", put "no locale" ahead of the expression in the same
212       block.  See "SECURITY" in perllocale for further discussion and
213       examples.
214
215   Switches On the "#!" Line
216       When you make a script executable, in order to make it usable as a
217       command, the system will pass switches to perl from the script's #!
218       line.  Perl checks that any command line switches given to a setuid (or
219       setgid) script actually match the ones set on the #! line.  Some Unix
220       and Unix-like environments impose a one-switch limit on the #!  line,
221       so you may need to use something like "-wU" instead of "-w -U" under
222       such systems.  (This issue should arise only in Unix or Unix-like
223       environments that support #! and setuid or setgid scripts.)
224
225   Taint mode and @INC
226       When the taint mode ("-T") is in effect, the "." directory is removed
227       from @INC, and the environment variables "PERL5LIB" and "PERLLIB" are
228       ignored by Perl. You can still adjust @INC from outside the program by
229       using the "-I" command line option as explained in perlrun. The two
230       environment variables are ignored because they are obscured, and a user
231       running a program could be unaware that they are set, whereas the "-I"
232       option is clearly visible and therefore permitted.
233
234       Another way to modify @INC without modifying the program, is to use the
235       "lib" pragma, e.g.:
236
237         perl -Mlib=/foo program
238
239       The benefit of using "-Mlib=/foo" over "-I/foo", is that the former
240       will automagically remove any duplicated directories, while the later
241       will not.
242
243       Note that if a tainted string is added to @INC, the following problem
244       will be reported:
245
246         Insecure dependency in require while running with -T switch
247
248   Cleaning Up Your Path
249       For "Insecure $ENV{PATH}" messages, you need to set $ENV{'PATH'} to a
250       known value, and each directory in the path must be absolute and non-
251       writable by others than its owner and group.  You may be surprised to
252       get this message even if the pathname to your executable is fully
253       qualified.  This is not generated because you didn't supply a full path
254       to the program; instead, it's generated because you never set your PATH
255       environment variable, or you didn't set it to something that was safe.
256       Because Perl can't guarantee that the executable in question isn't
257       itself going to turn around and execute some other program that is
258       dependent on your PATH, it makes sure you set the PATH.
259
260       The PATH isn't the only environment variable which can cause problems.
261       Because some shells may use the variables IFS, CDPATH, ENV, and
262       BASH_ENV, Perl checks that those are either empty or untainted when
263       starting subprocesses. You may wish to add something like this to your
264       setid and taint-checking scripts.
265
266           delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};   # Make %ENV safer
267
268       It's also possible to get into trouble with other operations that don't
269       care whether they use tainted values.  Make judicious use of the file
270       tests in dealing with any user-supplied filenames.  When possible, do
271       opens and such after properly dropping any special user (or group!)
272       privileges. Perl doesn't prevent you from opening tainted filenames for
273       reading, so be careful what you print out.  The tainting mechanism is
274       intended to prevent stupid mistakes, not to remove the need for
275       thought.
276
277       Perl does not call the shell to expand wild cards when you pass
278       "system" and "exec" explicit parameter lists instead of strings with
279       possible shell wildcards in them.  Unfortunately, the "open", "glob",
280       and backtick functions provide no such alternate calling convention, so
281       more subterfuge will be required.
282
283       Perl provides a reasonably safe way to open a file or pipe from a
284       setuid or setgid program: just create a child process with reduced
285       privilege who does the dirty work for you.  First, fork a child using
286       the special "open" syntax that connects the parent and child by a pipe.
287       Now the child resets its ID set and any other per-process attributes,
288       like environment variables, umasks, current working directories, back
289       to the originals or known safe values.  Then the child process, which
290       no longer has any special permissions, does the "open" or other system
291       call.  Finally, the child passes the data it managed to access back to
292       the parent.  Because the file or pipe was opened in the child while
293       running under less privilege than the parent, it's not apt to be
294       tricked into doing something it shouldn't.
295
296       Here's a way to do backticks reasonably safely.  Notice how the "exec"
297       is not called with a string that the shell could expand.  This is by
298       far the best way to call something that might be subjected to shell
299       escapes: just never call the shell at all.
300
301               use English '-no_match_vars';
302               die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
303               if ($pid) {           # parent
304                   while (<KID>) {
305                       # do something
306                   }
307                   close KID;
308               } else {
309                   my @temp     = ($EUID, $EGID);
310                   my $orig_uid = $UID;
311                   my $orig_gid = $GID;
312                   $EUID = $UID;
313                   $EGID = $GID;
314                   # Drop privileges
315                   $UID  = $orig_uid;
316                   $GID  = $orig_gid;
317                   # Make sure privs are really gone
318                   ($EUID, $EGID) = @temp;
319                   die "Can't drop privileges"
320                       unless $UID == $EUID  && $GID eq $EGID;
321                   $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH.
322                   # Consider sanitizing the environment even more.
323                   exec 'myprog', 'arg1', 'arg2'
324                       or die "can't exec myprog: $!";
325               }
326
327       A similar strategy would work for wildcard expansion via "glob",
328       although you can use "readdir" instead.
329
330       Taint checking is most useful when although you trust yourself not to
331       have written a program to give away the farm, you don't necessarily
332       trust those who end up using it not to try to trick it into doing
333       something bad.  This is the kind of security checking that's useful for
334       set-id programs and programs launched on someone else's behalf, like
335       CGI programs.
336
337       This is quite different, however, from not even trusting the writer of
338       the code not to try to do something evil.  That's the kind of trust
339       needed when someone hands you a program you've never seen before and
340       says, "Here, run this."  For that kind of safety, you might want to
341       check out the Safe module, included standard in the Perl distribution.
342       This module allows the programmer to set up special compartments in
343       which all system operations are trapped and namespace access is
344       carefully controlled.  Safe should not be considered bullet-proof,
345       though: it will not prevent the foreign code to set up infinite loops,
346       allocate gigabytes of memory, or even abusing perl bugs to make the
347       host interpreter crash or behave in unpredictable ways. In any case
348       it's better avoided completely if you're really concerned about
349       security.
350
351   Security Bugs
352       Beyond the obvious problems that stem from giving special privileges to
353       systems as flexible as scripts, on many versions of Unix, set-id
354       scripts are inherently insecure right from the start.  The problem is a
355       race condition in the kernel.  Between the time the kernel opens the
356       file to see which interpreter to run and when the (now-set-id)
357       interpreter turns around and reopens the file to interpret it, the file
358       in question may have changed, especially if you have symbolic links on
359       your system.
360
361       Fortunately, sometimes this kernel "feature" can be disabled.
362       Unfortunately, there are two ways to disable it.  The system can simply
363       outlaw scripts with any set-id bit set, which doesn't help much.
364       Alternately, it can simply ignore the set-id bits on scripts.
365
366       However, if the kernel set-id script feature isn't disabled, Perl will
367       complain loudly that your set-id script is insecure.  You'll need to
368       either disable the kernel set-id script feature, or put a C wrapper
369       around the script.  A C wrapper is just a compiled program that does
370       nothing except call your Perl program.   Compiled programs are not
371       subject to the kernel bug that plagues set-id scripts.  Here's a simple
372       wrapper, written in C:
373
374           #define REAL_PATH "/path/to/script"
375           main(ac, av)
376               char **av;
377           {
378               execv(REAL_PATH, av);
379           }
380
381       Compile this wrapper into a binary executable and then make it rather
382       than your script setuid or setgid.
383
384       In recent years, vendors have begun to supply systems free of this
385       inherent security bug.  On such systems, when the kernel passes the
386       name of the set-id script to open to the interpreter, rather than using
387       a pathname subject to meddling, it instead passes /dev/fd/3.  This is a
388       special file already opened on the script, so that there can be no race
389       condition for evil scripts to exploit.  On these systems, Perl should
390       be compiled with "-DSETUID_SCRIPTS_ARE_SECURE_NOW".  The Configure
391       program that builds Perl tries to figure this out for itself, so you
392       should never have to specify this yourself.  Most modern releases of
393       SysVr4 and BSD 4.4 use this approach to avoid the kernel race
394       condition.
395
396   Protecting Your Programs
397       There are a number of ways to hide the source to your Perl programs,
398       with varying levels of "security".
399
400       First of all, however, you can't take away read permission, because the
401       source code has to be readable in order to be compiled and interpreted.
402       (That doesn't mean that a CGI script's source is readable by people on
403       the web, though.)  So you have to leave the permissions at the socially
404       friendly 0755 level.  This lets people on your local system only see
405       your source.
406
407       Some people mistakenly regard this as a security problem.  If your
408       program does insecure things, and relies on people not knowing how to
409       exploit those insecurities, it is not secure.  It is often possible for
410       someone to determine the insecure things and exploit them without
411       viewing the source.  Security through obscurity, the name for hiding
412       your bugs instead of fixing them, is little security indeed.
413
414       You can try using encryption via source filters (Filter::* from CPAN,
415       or Filter::Util::Call and Filter::Simple since Perl 5.8).  But crackers
416       might be able to decrypt it.  You can try using the byte code compiler
417       and interpreter described below, but crackers might be able to de-
418       compile it.  You can try using the native-code compiler described
419       below, but crackers might be able to disassemble it.  These pose
420       varying degrees of difficulty to people wanting to get at your code,
421       but none can definitively conceal it (this is true of every language,
422       not just Perl).
423
424       If you're concerned about people profiting from your code, then the
425       bottom line is that nothing but a restrictive license will give you
426       legal security.  License your software and pepper it with threatening
427       statements like "This is unpublished proprietary software of XYZ Corp.
428       Your access to it does not give you permission to use it blah blah
429       blah."  You should see a lawyer to be sure your license's wording will
430       stand up in court.
431
432   Unicode
433       Unicode is a new and complex technology and one may easily overlook
434       certain security pitfalls.  See perluniintro for an overview and
435       perlunicode for details, and "Security Implications of Unicode" in
436       perlunicode for security implications in particular.
437
438   Algorithmic Complexity Attacks
439       Certain internal algorithms used in the implementation of Perl can be
440       attacked by choosing the input carefully to consume large amounts of
441       either time or space or both.  This can lead into the so-called Denial
442       of Service (DoS) attacks.
443
444       ·   Hash Function - the algorithm used to "order" hash elements has
445           been changed several times during the development of Perl, mainly
446           to be reasonably fast.  In Perl 5.8.1 also the security aspect was
447           taken into account.
448
449           In Perls before 5.8.1 one could rather easily generate data that as
450           hash keys would cause Perl to consume large amounts of time because
451           internal structure of hashes would badly degenerate.  In Perl 5.8.1
452           the hash function is randomly perturbed by a pseudorandom seed
453           which makes generating such naughty hash keys harder.  See
454           "PERL_HASH_SEED" in perlrun for more information.
455
456           In Perl 5.8.1 the random perturbation was done by default, but as
457           of 5.8.2 it is only used on individual hashes if the internals
458           detect the insertion of pathological data. If one wants for some
459           reason emulate the old behaviour (and expose oneself to DoS
460           attacks) one can set the environment variable PERL_HASH_SEED to
461           zero to disable the protection (or any other integer to force a
462           known perturbation, rather than random).  One possible reason for
463           wanting to emulate the old behaviour is that in the new behaviour
464           consecutive runs of Perl will order hash keys differently, which
465           may confuse some applications (like Data::Dumper: the outputs of
466           two different runs are no longer identical).
467
468           Perl has never guaranteed any ordering of the hash keys, and the
469           ordering has already changed several times during the lifetime of
470           Perl 5.  Also, the ordering of hash keys has always been, and
471           continues to be, affected by the insertion order.
472
473           Also note that while the order of the hash elements might be
474           randomised, this "pseudoordering" should not be used for
475           applications like shuffling a list randomly (use
476           List::Util::shuffle() for that, see List::Util, a standard core
477           module since Perl 5.8.0; or the CPAN module
478           Algorithm::Numerical::Shuffle), or for generating permutations (use
479           e.g. the CPAN modules Algorithm::Permute or
480           Algorithm::FastPermute), or for any cryptographic applications.
481
482       ·   Regular expressions - Perl's regular expression engine is so called
483           NFA (Non-deterministic Finite Automaton), which among other things
484           means that it can rather easily consume large amounts of both time
485           and space if the regular expression may match in several ways.
486           Careful crafting of the regular expressions can help but quite
487           often there really isn't much one can do (the book "Mastering
488           Regular Expressions" is required reading, see perlfaq2).  Running
489           out of space manifests itself by Perl running out of memory.
490
491       ·   Sorting - the quicksort algorithm used in Perls before 5.8.0 to
492           implement the sort() function is very easy to trick into
493           misbehaving so that it consumes a lot of time.  Starting from Perl
494           5.8.0 a different sorting algorithm, mergesort, is used by default.
495           Mergesort cannot misbehave on any input.
496
497       See <http://www.cs.rice.edu/~scrosby/hash/> for more information, and
498       any computer science textbook on algorithmic complexity.
499

SEE ALSO

501       perlrun for its description of cleaning up environment variables.
502
503
504
505perl v5.16.3                      2013-03-04                        PERLSEC(1)
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