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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlsec - Perl security | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
425e5e39 | 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 | |
54310121 | 9 | command line shells, which are based on multiple substitution passes on |
425e5e39 | 10 | each line of the script, Perl uses a more conventional evaluation scheme |
11 | with fewer hidden snags. Additionally, because the language has more | |
54310121 | 12 | builtin functionality, it can rely less upon external (and possibly |
425e5e39 | 13 | untrustworthy) programs to accomplish its purposes. |
a0d0e21e | 14 | |
89f530a6 DG |
15 | =head1 SECURITY VULNERABILITY CONTACT INFORMATION |
16 | ||
87c118b9 DM |
17 | If you believe you have found a security vulnerability in Perl, please |
18 | email the details to perl5-security-report@perl.org. This creates a new | |
19 | Request Tracker ticket in a special queue which isn't initially publicly | |
20 | accessible. The email will also be copied to a closed subscription | |
21 | unarchived mailing list which includes all the core committers, who will | |
22 | be able to help assess the impact of issues, figure out a resolution, and | |
23 | help co-ordinate the release of patches to mitigate or fix the problem | |
24 | across all platforms on which Perl is supported. Please only use this | |
25 | address for security issues in the Perl core, not for modules | |
26 | independently distributed on CPAN. | |
27 | ||
28 | When sending an initial request to the security email address, please | |
29 | don't Cc any other parties, because if they reply to all, the reply will | |
30 | generate yet another new ticket. Once you have received an initial reply | |
31 | with a C<[perl #NNNNNN]> ticket number in the headline, it's okay to Cc | |
32 | subsequent replies to third parties: all emails to the | |
33 | perl5-security-report address with the ticket number in the subject line | |
34 | will be added to the ticket; without it, a new ticket will be created. | |
89f530a6 DG |
35 | |
36 | =head1 SECURITY MECHANISMS AND CONCERNS | |
37 | ||
38 | =head2 Taint mode | |
39 | ||
425e5e39 | 40 | Perl automatically enables a set of special security checks, called I<taint |
41 | mode>, when it detects its program running with differing real and effective | |
42 | user or group IDs. The setuid bit in Unix permissions is mode 04000, the | |
43 | setgid bit mode 02000; either or both may be set. You can also enable taint | |
91e64913 | 44 | mode explicitly by using the B<-T> command line flag. This flag is |
425e5e39 | 45 | I<strongly> suggested for server programs and any program run on behalf of |
91e64913 | 46 | someone else, such as a CGI script. Once taint mode is on, it's on for |
fb73857a | 47 | the remainder of your script. |
a0d0e21e | 48 | |
1e422769 | 49 | While in this mode, Perl takes special precautions called I<taint |
50 | checks> to prevent both obvious and subtle traps. Some of these checks | |
51 | are 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 itself, | |
fb73857a | 54 | and it is these checks especially that contribute to making a set-id Perl |
425e5e39 | 55 | program more secure than the corresponding C program. |
56 | ||
fb73857a | 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 | |
23634c10 AL |
60 | L<perllocale>), results of certain system calls (C<readdir()>, |
61 | C<readlink()>, the variable of C<shmread()>, the messages returned by | |
62 | C<msgrcv()>, the password, gcos and shell fields returned by the | |
63 | C<getpwxxx()> calls), and all file input are marked as "tainted". | |
41d6edb2 JH |
64 | Tainted data may not be used directly or indirectly in any command |
65 | that invokes a sub-shell, nor in any command that modifies files, | |
b7ee89ce AP |
66 | directories, or processes, B<with the following exceptions>: |
67 | ||
68 | =over 4 | |
69 | ||
70 | =item * | |
71 | ||
b7ee89ce AP |
72 | Arguments to C<print> and C<syswrite> are B<not> checked for taintedness. |
73 | ||
7f6513c1 JH |
74 | =item * |
75 | ||
76 | Symbolic methods | |
77 | ||
78 | $obj->$method(@args); | |
79 | ||
80 | and symbolic sub references | |
81 | ||
82 | &{$foo}(@args); | |
83 | $foo->(@args); | |
84 | ||
85 | are not checked for taintedness. This requires extra carefulness | |
86 | unless you want external data to affect your control flow. Unless | |
87 | you carefully limit what these symbolic values are, people are able | |
88 | to call functions B<outside> your Perl code, such as POSIX::system, | |
89 | in which case they are able to run arbitrary external code. | |
90 | ||
8ea1447c RD |
91 | =item * |
92 | ||
93 | Hash keys are B<never> tainted. | |
94 | ||
b7ee89ce AP |
95 | =back |
96 | ||
595bde10 MG |
97 | For efficiency reasons, Perl takes a conservative view of |
98 | whether data is tainted. If an expression contains tainted data, | |
99 | any subexpression may be considered tainted, even if the value | |
100 | of the subexpression is not itself affected by the tainted data. | |
ee556d55 | 101 | |
d929ce6f | 102 | Because taintedness is associated with each scalar value, some |
595bde10 | 103 | elements of an array or hash can be tainted and others not. |
8ea1447c | 104 | The keys of a hash are B<never> tainted. |
a0d0e21e | 105 | |
a0d0e21e LW |
106 | For example: |
107 | ||
425e5e39 | 108 | $arg = shift; # $arg is tainted |
048b63be | 109 | $hid = $arg . 'bar'; # $hid is also tainted |
425e5e39 | 110 | $line = <>; # Tainted |
8ebc5c01 | 111 | $line = <STDIN>; # Also tainted |
112 | open FOO, "/home/me/bar" or die $!; | |
113 | $line = <FOO>; # Still tainted | |
a0d0e21e | 114 | $path = $ENV{'PATH'}; # Tainted, but see below |
425e5e39 | 115 | $data = 'abc'; # Not tainted |
a0d0e21e | 116 | |
425e5e39 | 117 | system "echo $arg"; # Insecure |
7de90c4d | 118 | system "/bin/echo", $arg; # Considered insecure |
bbd7eb8a | 119 | # (Perl doesn't know about /bin/echo) |
425e5e39 | 120 | system "echo $hid"; # Insecure |
121 | system "echo $data"; # Insecure until PATH set | |
a0d0e21e | 122 | |
425e5e39 | 123 | $path = $ENV{'PATH'}; # $path now tainted |
a0d0e21e | 124 | |
54310121 | 125 | $ENV{'PATH'} = '/bin:/usr/bin'; |
c90c0ff4 | 126 | delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; |
a0d0e21e | 127 | |
425e5e39 | 128 | $path = $ENV{'PATH'}; # $path now NOT tainted |
129 | system "echo $data"; # Is secure now! | |
a0d0e21e | 130 | |
425e5e39 | 131 | open(FOO, "< $arg"); # OK - read-only file |
132 | open(FOO, "> $arg"); # Not OK - trying to write | |
a0d0e21e | 133 | |
bbd7eb8a | 134 | open(FOO,"echo $arg|"); # Not OK |
425e5e39 | 135 | open(FOO,"-|") |
7de90c4d | 136 | or exec 'echo', $arg; # Also not OK |
a0d0e21e | 137 | |
425e5e39 | 138 | $shout = `echo $arg`; # Insecure, $shout now tainted |
a0d0e21e | 139 | |
425e5e39 | 140 | unlink $data, $arg; # Insecure |
141 | umask $arg; # Insecure | |
a0d0e21e | 142 | |
bbd7eb8a | 143 | exec "echo $arg"; # Insecure |
7de90c4d RD |
144 | exec "echo", $arg; # Insecure |
145 | exec "sh", '-c', $arg; # Very insecure! | |
a0d0e21e | 146 | |
3a4b19e4 GS |
147 | @files = <*.c>; # insecure (uses readdir() or similar) |
148 | @files = glob('*.c'); # insecure (uses readdir() or similar) | |
7bac28a0 | 149 | |
dde0c558 BF |
150 | # In either case, the results of glob are tainted, since the list of |
151 | # filenames comes from outside of the program. | |
3f7d42d8 | 152 | |
ee556d55 MG |
153 | $bad = ($arg, 23); # $bad will be tainted |
154 | $arg, `true`; # Insecure (although it isn't really) | |
155 | ||
a0d0e21e | 156 | If you try to do something insecure, you will get a fatal error saying |
7de90c4d | 157 | something like "Insecure dependency" or "Insecure $ENV{PATH}". |
425e5e39 | 158 | |
23634c10 AL |
159 | The exception to the principle of "one tainted value taints the whole |
160 | expression" is with the ternary conditional operator C<?:>. Since code | |
161 | with a ternary conditional | |
162 | ||
163 | $result = $tainted_value ? "Untainted" : "Also untainted"; | |
164 | ||
165 | is effectively | |
166 | ||
167 | if ( $tainted_value ) { | |
168 | $result = "Untainted"; | |
169 | } else { | |
170 | $result = "Also untainted"; | |
171 | } | |
172 | ||
173 | it doesn't make sense for C<$result> to be tainted. | |
174 | ||
425e5e39 | 175 | =head2 Laundering and Detecting Tainted Data |
176 | ||
3f7d42d8 JH |
177 | To test whether a variable contains tainted data, and whose use would |
178 | thus trigger an "Insecure dependency" message, you can use the | |
23634c10 | 179 | C<tainted()> function of the Scalar::Util module, available in your |
3f7d42d8 | 180 | nearby CPAN mirror, and included in Perl starting from the release 5.8.0. |
595bde10 | 181 | Or you may be able to use the following C<is_tainted()> function. |
425e5e39 | 182 | |
183 | sub is_tainted { | |
7687d286 | 184 | local $@; # Don't pollute caller's value. |
61890e45 | 185 | return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 }; |
425e5e39 | 186 | } |
187 | ||
188 | This function makes use of the fact that the presence of tainted data | |
189 | anywhere within an expression renders the entire expression tainted. It | |
190 | would be inefficient for every operator to test every argument for | |
191 | taintedness. Instead, the slightly more efficient and conservative | |
192 | approach is used that if any tainted value has been accessed within the | |
193 | same expression, the whole expression is considered tainted. | |
194 | ||
5f05dabc | 195 | But testing for taintedness gets you only so far. Sometimes you have just |
595bde10 MG |
196 | to clear your data's taintedness. Values may be untainted by using them |
197 | as keys in a hash; otherwise the only way to bypass the tainting | |
54310121 | 198 | mechanism is by referencing subpatterns from a regular expression match. |
18512f39 KW |
199 | Perl presumes that if you reference a substring using $1, $2, etc. in a |
200 | non-tainting pattern, that | |
201 | you knew what you were doing when you wrote that pattern. That means using | |
425e5e39 | 202 | a bit of thought--don't just blindly untaint anything, or you defeat the |
a034a98d DD |
203 | entire mechanism. It's better to verify that the variable has only good |
204 | characters (for certain values of "good") rather than checking whether it | |
205 | has any bad characters. That's because it's far too easy to miss bad | |
206 | characters that you never thought of. | |
425e5e39 | 207 | |
208 | Here's a test to make sure that the data contains nothing but "word" | |
209 | characters (alphabetics, numerics, and underscores), a hyphen, an at sign, | |
210 | or a dot. | |
211 | ||
54310121 | 212 | if ($data =~ /^([-\@\w.]+)$/) { |
425e5e39 | 213 | $data = $1; # $data now untainted |
214 | } else { | |
3a2263fe | 215 | die "Bad data in '$data'"; # log this somewhere |
425e5e39 | 216 | } |
217 | ||
5f05dabc | 218 | This is fairly secure because C</\w+/> doesn't normally match shell |
425e5e39 | 219 | metacharacters, nor are dot, dash, or at going to mean something special |
220 | to the shell. Use of C</.+/> would have been insecure in theory because | |
221 | it lets everything through, but Perl doesn't check for that. The lesson | |
222 | is that when untainting, you must be exceedingly careful with your patterns. | |
19799a22 | 223 | Laundering data using regular expression is the I<only> mechanism for |
425e5e39 | 224 | untainting dirty data, unless you use the strategy detailed below to fork |
225 | a child of lesser privilege. | |
226 | ||
23634c10 | 227 | The example does not untaint C<$data> if C<use locale> is in effect, |
a034a98d DD |
228 | because the characters matched by C<\w> are determined by the locale. |
229 | Perl considers that locale definitions are untrustworthy because they | |
230 | contain data from outside the program. If you are writing a | |
231 | locale-aware program, and want to launder data with a regular expression | |
232 | containing C<\w>, put C<no locale> ahead of the expression in the same | |
233 | block. See L<perllocale/SECURITY> for further discussion and examples. | |
234 | ||
3a52c276 CS |
235 | =head2 Switches On the "#!" Line |
236 | ||
237 | When you make a script executable, in order to make it usable as a | |
238 | command, the system will pass switches to perl from the script's #! | |
54310121 | 239 | line. Perl checks that any command line switches given to a setuid |
3a52c276 | 240 | (or setgid) script actually match the ones set on the #! line. Some |
54310121 | 241 | Unix and Unix-like environments impose a one-switch limit on the #! |
3a52c276 | 242 | line, so you may need to use something like C<-wU> instead of C<-w -U> |
54310121 | 243 | under such systems. (This issue should arise only in Unix or |
244 | Unix-like environments that support #! and setuid or setgid scripts.) | |
3a52c276 | 245 | |
588f7210 SB |
246 | =head2 Taint mode and @INC |
247 | ||
248 | When the taint mode (C<-T>) is in effect, the "." directory is removed | |
249 | from C<@INC>, and the environment variables C<PERL5LIB> and C<PERLLIB> | |
91e64913 | 250 | are ignored by Perl. You can still adjust C<@INC> from outside the |
588f7210 | 251 | program by using the C<-I> command line option as explained in |
91e64913 | 252 | L<perlrun>. The two environment variables are ignored because |
588f7210 SB |
253 | they are obscured, and a user running a program could be unaware that |
254 | they are set, whereas the C<-I> option is clearly visible and | |
255 | therefore permitted. | |
256 | ||
257 | Another way to modify C<@INC> without modifying the program, is to use | |
258 | the C<lib> pragma, e.g.: | |
259 | ||
260 | perl -Mlib=/foo program | |
261 | ||
262 | The benefit of using C<-Mlib=/foo> over C<-I/foo>, is that the former | |
6fd9f613 | 263 | will automagically remove any duplicated directories, while the latter |
588f7210 SB |
264 | will not. |
265 | ||
6a268663 RGS |
266 | Note that if a tainted string is added to C<@INC>, the following |
267 | problem will be reported: | |
268 | ||
269 | Insecure dependency in require while running with -T switch | |
270 | ||
425e5e39 | 271 | =head2 Cleaning Up Your Path |
272 | ||
df98f984 RGS |
273 | For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to |
274 | a known value, and each directory in the path must be absolute and | |
275 | non-writable by others than its owner and group. You may be surprised to | |
276 | get this message even if the pathname to your executable is fully | |
277 | qualified. This is I<not> generated because you didn't supply a full path | |
278 | to the program; instead, it's generated because you never set your PATH | |
279 | environment variable, or you didn't set it to something that was safe. | |
280 | Because Perl can't guarantee that the executable in question isn't itself | |
281 | going to turn around and execute some other program that is dependent on | |
282 | your PATH, it makes sure you set the PATH. | |
a0d0e21e | 283 | |
a3cb178b GS |
284 | The PATH isn't the only environment variable which can cause problems. |
285 | Because some shells may use the variables IFS, CDPATH, ENV, and | |
286 | BASH_ENV, Perl checks that those are either empty or untainted when | |
91e64913 | 287 | starting subprocesses. You may wish to add something like this to your |
a3cb178b GS |
288 | setid and taint-checking scripts. |
289 | ||
290 | delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer | |
291 | ||
a0d0e21e LW |
292 | It's also possible to get into trouble with other operations that don't |
293 | care whether they use tainted values. Make judicious use of the file | |
294 | tests in dealing with any user-supplied filenames. When possible, do | |
fb73857a | 295 | opens and such B<after> properly dropping any special user (or group!) |
91e64913 FC |
296 | privileges. Perl doesn't prevent you from |
297 | opening tainted filenames for reading, | |
a0d0e21e LW |
298 | so be careful what you print out. The tainting mechanism is intended to |
299 | prevent stupid mistakes, not to remove the need for thought. | |
300 | ||
23634c10 AL |
301 | Perl does not call the shell to expand wild cards when you pass C<system> |
302 | and C<exec> explicit parameter lists instead of strings with possible shell | |
303 | wildcards in them. Unfortunately, the C<open>, C<glob>, and | |
54310121 | 304 | backtick functions provide no such alternate calling convention, so more |
305 | subterfuge will be required. | |
425e5e39 | 306 | |
307 | Perl provides a reasonably safe way to open a file or pipe from a setuid | |
308 | or setgid program: just create a child process with reduced privilege who | |
309 | does the dirty work for you. First, fork a child using the special | |
23634c10 | 310 | C<open> syntax that connects the parent and child by a pipe. Now the |
425e5e39 | 311 | child resets its ID set and any other per-process attributes, like |
312 | environment variables, umasks, current working directories, back to the | |
313 | originals or known safe values. Then the child process, which no longer | |
23634c10 | 314 | has any special permissions, does the C<open> or other system call. |
425e5e39 | 315 | Finally, the child passes the data it managed to access back to the |
5f05dabc | 316 | parent. Because the file or pipe was opened in the child while running |
425e5e39 | 317 | under less privilege than the parent, it's not apt to be tricked into |
318 | doing something it shouldn't. | |
319 | ||
23634c10 | 320 | Here's a way to do backticks reasonably safely. Notice how the C<exec> is |
425e5e39 | 321 | not called with a string that the shell could expand. This is by far the |
322 | best way to call something that might be subjected to shell escapes: just | |
fb73857a | 323 | never call the shell at all. |
cb1a09d0 | 324 | |
6ca3c6c6 | 325 | use English; |
e093bcf0 GW |
326 | die "Can't fork: $!" unless defined($pid = open(KID, "-|")); |
327 | if ($pid) { # parent | |
328 | while (<KID>) { | |
329 | # do something | |
330 | } | |
331 | close KID; | |
332 | } else { | |
333 | my @temp = ($EUID, $EGID); | |
334 | my $orig_uid = $UID; | |
335 | my $orig_gid = $GID; | |
336 | $EUID = $UID; | |
337 | $EGID = $GID; | |
338 | # Drop privileges | |
339 | $UID = $orig_uid; | |
340 | $GID = $orig_gid; | |
341 | # Make sure privs are really gone | |
342 | ($EUID, $EGID) = @temp; | |
343 | die "Can't drop privileges" | |
344 | unless $UID == $EUID && $GID eq $EGID; | |
345 | $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH. | |
346 | # Consider sanitizing the environment even more. | |
347 | exec 'myprog', 'arg1', 'arg2' | |
348 | or die "can't exec myprog: $!"; | |
349 | } | |
425e5e39 | 350 | |
fb73857a | 351 | A similar strategy would work for wildcard expansion via C<glob>, although |
352 | you can use C<readdir> instead. | |
425e5e39 | 353 | |
354 | Taint checking is most useful when although you trust yourself not to have | |
355 | written a program to give away the farm, you don't necessarily trust those | |
356 | who end up using it not to try to trick it into doing something bad. This | |
fb73857a | 357 | is the kind of security checking that's useful for set-id programs and |
425e5e39 | 358 | programs launched on someone else's behalf, like CGI programs. |
359 | ||
360 | This is quite different, however, from not even trusting the writer of the | |
361 | code not to try to do something evil. That's the kind of trust needed | |
362 | when someone hands you a program you've never seen before and says, "Here, | |
18d7fc85 RGS |
363 | run this." For that kind of safety, you might want to check out the Safe |
364 | module, included standard in the Perl distribution. This module allows the | |
425e5e39 | 365 | programmer to set up special compartments in which all system operations |
18d7fc85 RGS |
366 | are trapped and namespace access is carefully controlled. Safe should |
367 | not be considered bullet-proof, though: it will not prevent the foreign | |
368 | code to set up infinite loops, allocate gigabytes of memory, or even | |
369 | abusing perl bugs to make the host interpreter crash or behave in | |
91e64913 | 370 | unpredictable ways. In any case it's better avoided completely if you're |
18d7fc85 | 371 | really concerned about security. |
425e5e39 | 372 | |
373 | =head2 Security Bugs | |
374 | ||
375 | Beyond the obvious problems that stem from giving special privileges to | |
fb73857a | 376 | systems as flexible as scripts, on many versions of Unix, set-id scripts |
425e5e39 | 377 | are inherently insecure right from the start. The problem is a race |
378 | condition in the kernel. Between the time the kernel opens the file to | |
fb73857a | 379 | see which interpreter to run and when the (now-set-id) interpreter turns |
425e5e39 | 380 | around and reopens the file to interpret it, the file in question may have |
381 | changed, especially if you have symbolic links on your system. | |
382 | ||
383 | Fortunately, sometimes this kernel "feature" can be disabled. | |
384 | Unfortunately, there are two ways to disable it. The system can simply | |
fb73857a | 385 | outlaw scripts with any set-id bit set, which doesn't help much. |
cc69b689 | 386 | Alternately, it can simply ignore the set-id bits on scripts. |
425e5e39 | 387 | |
fb73857a | 388 | However, if the kernel set-id script feature isn't disabled, Perl will |
389 | complain loudly that your set-id script is insecure. You'll need to | |
390 | either disable the kernel set-id script feature, or put a C wrapper around | |
425e5e39 | 391 | the script. A C wrapper is just a compiled program that does nothing |
392 | except call your Perl program. Compiled programs are not subject to the | |
fb73857a | 393 | kernel bug that plagues set-id scripts. Here's a simple wrapper, written |
425e5e39 | 394 | in C: |
395 | ||
245c138e LM |
396 | #include <unistd.h> |
397 | #include <stdio.h> | |
398 | #include <string.h> | |
399 | #include <errno.h> | |
400 | ||
425e5e39 | 401 | #define REAL_PATH "/path/to/script" |
245c138e LM |
402 | |
403 | int main(int argc, char **argv) | |
425e5e39 | 404 | { |
245c138e LM |
405 | execv(REAL_PATH, argv); |
406 | fprintf(stderr, "%s: %s: %s\n", | |
407 | argv[0], REAL_PATH, strerror(errno)); | |
408 | return 127; | |
54310121 | 409 | } |
cb1a09d0 | 410 | |
54310121 | 411 | Compile this wrapper into a binary executable and then make I<it> rather |
412 | than your script setuid or setgid. | |
425e5e39 | 413 | |
425e5e39 | 414 | In recent years, vendors have begun to supply systems free of this |
415 | inherent security bug. On such systems, when the kernel passes the name | |
fb73857a | 416 | of the set-id script to open to the interpreter, rather than using a |
425e5e39 | 417 | pathname subject to meddling, it instead passes I</dev/fd/3>. This is a |
418 | special file already opened on the script, so that there can be no race | |
419 | condition for evil scripts to exploit. On these systems, Perl should be | |
23634c10 | 420 | compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F<Configure> |
425e5e39 | 421 | program that builds Perl tries to figure this out for itself, so you |
422 | should never have to specify this yourself. Most modern releases of | |
423 | SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. | |
424 | ||
68dc0745 | 425 | =head2 Protecting Your Programs |
426 | ||
427 | There are a number of ways to hide the source to your Perl programs, | |
428 | with varying levels of "security". | |
429 | ||
430 | First of all, however, you I<can't> take away read permission, because | |
431 | the source code has to be readable in order to be compiled and | |
432 | interpreted. (That doesn't mean that a CGI script's source is | |
433 | readable by people on the web, though.) So you have to leave the | |
5a964f20 TC |
434 | permissions at the socially friendly 0755 level. This lets |
435 | people on your local system only see your source. | |
68dc0745 | 436 | |
5a964f20 | 437 | Some people mistakenly regard this as a security problem. If your program does |
68dc0745 | 438 | insecure things, and relies on people not knowing how to exploit those |
439 | insecurities, it is not secure. It is often possible for someone to | |
440 | determine the insecure things and exploit them without viewing the | |
441 | source. Security through obscurity, the name for hiding your bugs | |
442 | instead of fixing them, is little security indeed. | |
443 | ||
83df6a1d JH |
444 | You can try using encryption via source filters (Filter::* from CPAN, |
445 | or Filter::Util::Call and Filter::Simple since Perl 5.8). | |
446 | But crackers might be able to decrypt it. You can try using the byte | |
447 | code compiler and interpreter described below, but crackers might be | |
448 | able to de-compile it. You can try using the native-code compiler | |
68dc0745 | 449 | described below, but crackers might be able to disassemble it. These |
450 | pose varying degrees of difficulty to people wanting to get at your | |
451 | code, but none can definitively conceal it (this is true of every | |
452 | language, not just Perl). | |
453 | ||
454 | If you're concerned about people profiting from your code, then the | |
3462340b | 455 | bottom line is that nothing but a restrictive license will give you |
68dc0745 | 456 | legal security. License your software and pepper it with threatening |
457 | statements like "This is unpublished proprietary software of XYZ Corp. | |
458 | Your access to it does not give you permission to use it blah blah | |
3462340b | 459 | blah." You should see a lawyer to be sure your license's wording will |
68dc0745 | 460 | stand up in court. |
5a964f20 | 461 | |
0d7c09bb JH |
462 | =head2 Unicode |
463 | ||
464 | Unicode is a new and complex technology and one may easily overlook | |
465 | certain security pitfalls. See L<perluniintro> for an overview and | |
466 | L<perlunicode> for details, and L<perlunicode/"Security Implications | |
467 | of Unicode"> for security implications in particular. | |
468 | ||
504f80c1 JH |
469 | =head2 Algorithmic Complexity Attacks |
470 | ||
471 | Certain internal algorithms used in the implementation of Perl can | |
472 | be attacked by choosing the input carefully to consume large amounts | |
473 | of either time or space or both. This can lead into the so-called | |
474 | I<Denial of Service> (DoS) attacks. | |
475 | ||
476 | =over 4 | |
477 | ||
478 | =item * | |
479 | ||
6a5b4183 YO |
480 | Hash Algorithm - Hash algorithms like the one used in Perl are well |
481 | known to be vulnerable to collision attacks on their hash function. | |
482 | Such attacks involve constructing a set of keys which collide into | |
91e64913 | 483 | the same bucket producing inefficient behavior. Such attacks often |
6a5b4183 | 484 | depend on discovering the seed of the hash function used to map the |
91e64913 FC |
485 | keys to buckets. That seed is then used to brute-force a key set which |
486 | can be used to mount a denial of service attack. In Perl 5.8.1 changes | |
6a5b4183 YO |
487 | were introduced to harden Perl to such attacks, and then later in |
488 | Perl 5.18.0 these features were enhanced and additional protections | |
489 | added. | |
490 | ||
4d74c8eb S |
491 | At the time of this writing, Perl 5.18.0 is considered to be |
492 | well-hardened against algorithmic complexity attacks on its hash | |
91e64913 | 493 | implementation. This is largely owed to the following measures |
4d74c8eb | 494 | mitigate attacks: |
6a5b4183 YO |
495 | |
496 | =over 4 | |
497 | ||
498 | =item Hash Seed Randomization | |
499 | ||
500 | In order to make it impossible to know what seed to generate an attack | |
91e64913 | 501 | key set for, this seed is randomly initialized at process start. This |
4d74c8eb | 502 | may be overridden by using the PERL_HASH_SEED environment variable, see |
91e64913 | 503 | L<perlrun/PERL_HASH_SEED>. This environment variable controls how |
4d74c8eb S |
504 | items are actually stored, not how they are presented via |
505 | C<keys>, C<values> and C<each>. | |
6a5b4183 YO |
506 | |
507 | =item Hash Traversal Randomization | |
508 | ||
4d74c8eb | 509 | Independent of which seed is used in the hash function, C<keys>, |
6a5b4183 YO |
510 | C<values>, and C<each> return items in a per-hash randomized order. |
511 | Modifying a hash by insertion will change the iteration order of that hash. | |
4d74c8eb | 512 | This behavior can be overridden by using C<hash_traversal_mask()> from |
6a5b4183 | 513 | L<Hash::Util> or by using the PERL_PERTURB_KEYS environment variable, |
91e64913 | 514 | see L<perlrun/PERL_PERTURB_KEYS>. Note that this feature controls the |
6a5b4183 YO |
515 | "visible" order of the keys, and not the actual order they are stored in. |
516 | ||
517 | =item Bucket Order Perturbance | |
518 | ||
4d74c8eb | 519 | When items collide into a given hash bucket the order they are stored in |
91e64913 FC |
520 | the chain is no longer predictable in Perl 5.18. This |
521 | has the intention to make it harder to observe a | |
c6c886ef | 522 | collision. This behavior can be overridden by using |
6a5b4183 YO |
523 | the PERL_PERTURB_KEYS environment variable, see L<perlrun/PERL_PERTURB_KEYS>. |
524 | ||
525 | =item New Default Hash Function | |
526 | ||
527 | The default hash function has been modified with the intention of making | |
528 | it harder to infer the hash seed. | |
529 | ||
530 | =item Alternative Hash Functions | |
531 | ||
532 | The source code includes multiple hash algorithms to choose from. While we | |
4d74c8eb | 533 | believe that the default perl hash is robust to attack, we have included the |
91e64913 | 534 | hash function Siphash as a fall-back option. At the time of release of |
6a5b4183 YO |
535 | Perl 5.18.0 Siphash is believed to be of cryptographic strength. This is |
536 | not the default as it is much slower than the default hash. | |
537 | ||
538 | =back | |
539 | ||
4d74c8eb | 540 | Without compiling a special Perl, there is no way to get the exact same |
91e64913 | 541 | behavior of any versions prior to Perl 5.18.0. The closest one can get |
6a5b4183 | 542 | is by setting PERL_PERTURB_KEYS to 0 and setting the PERL_HASH_SEED |
91e64913 | 543 | to a known value. We do not advise those settings for production use |
4d74c8eb | 544 | due to the above security considerations. |
6a5b4183 YO |
545 | |
546 | B<Perl has never guaranteed any ordering of the hash keys>, and | |
547 | the ordering has already changed several times during the lifetime of | |
548 | Perl 5. Also, the ordering of hash keys has always been, and continues | |
549 | to be, affected by the insertion order and the history of changes made | |
550 | to the hash over its lifetime. | |
7b3f7037 JH |
551 | |
552 | Also note that while the order of the hash elements might be | |
4d74c8eb S |
553 | randomized, this "pseudo-ordering" should B<not> be used for |
554 | applications like shuffling a list randomly (use C<List::Util::shuffle()> | |
7b3f7037 | 555 | for that, see L<List::Util>, a standard core module since Perl 5.8.0; |
4d74c8eb S |
556 | or the CPAN module C<Algorithm::Numerical::Shuffle>), or for generating |
557 | permutations (use e.g. the CPAN modules C<Algorithm::Permute> or | |
558 | C<Algorithm::FastPermute>), or for any cryptographic applications. | |
7b3f7037 | 559 | |
883f220b TC |
560 | Tied hashes may have their own ordering and algorithmic complexity |
561 | attacks. | |
562 | ||
504f80c1 JH |
563 | =item * |
564 | ||
5a4e8ea7 P |
565 | Regular expressions - Perl's regular expression engine is so called NFA |
566 | (Non-deterministic Finite Automaton), which among other things means that | |
567 | it can rather easily consume large amounts of both time and space if the | |
504f80c1 JH |
568 | regular expression may match in several ways. Careful crafting of the |
569 | regular expressions can help but quite often there really isn't much | |
570 | one can do (the book "Mastering Regular Expressions" is required | |
571 | reading, see L<perlfaq2>). Running out of space manifests itself by | |
572 | Perl running out of memory. | |
573 | ||
574 | =item * | |
575 | ||
576 | Sorting - the quicksort algorithm used in Perls before 5.8.0 to | |
577 | implement the sort() function is very easy to trick into misbehaving | |
3462340b JL |
578 | so that it consumes a lot of time. Starting from Perl 5.8.0 a different |
579 | sorting algorithm, mergesort, is used by default. Mergesort cannot | |
580 | misbehave on any input. | |
504f80c1 JH |
581 | |
582 | =back | |
583 | ||
b25b06cf | 584 | See L<https://www.usenix.org/legacy/events/sec03/tech/full_papers/crosby/crosby.pdf> for more information, |
3462340b | 585 | and any computer science textbook on algorithmic complexity. |
504f80c1 | 586 | |
5a964f20 TC |
587 | =head1 SEE ALSO |
588 | ||
589 | L<perlrun> for its description of cleaning up environment variables. |