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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 | ||
b135fd4a JL |
17 | If you believe you have found a security vulnerability in the Perl |
18 | interpreter or modules maintained in the core Perl codebase, | |
19 | email the details to | |
20 | L<perl-security@perl.org|mailto:perl-security@perl.org>. | |
21 | This address is a closed membership mailing list monitored by the Perl | |
22 | security team. | |
23 | ||
24 | See L<perlsecpolicy> for additional information. | |
89f530a6 DG |
25 | |
26 | =head1 SECURITY MECHANISMS AND CONCERNS | |
27 | ||
28 | =head2 Taint mode | |
29 | ||
425e5e39 | 30 | Perl automatically enables a set of special security checks, called I<taint |
31 | mode>, when it detects its program running with differing real and effective | |
32 | user or group IDs. The setuid bit in Unix permissions is mode 04000, the | |
33 | setgid bit mode 02000; either or both may be set. You can also enable taint | |
91e64913 | 34 | mode explicitly by using the B<-T> command line flag. This flag is |
425e5e39 | 35 | I<strongly> suggested for server programs and any program run on behalf of |
91e64913 | 36 | someone else, such as a CGI script. Once taint mode is on, it's on for |
fb73857a | 37 | the remainder of your script. |
a0d0e21e | 38 | |
1e422769 | 39 | While in this mode, Perl takes special precautions called I<taint |
40 | checks> to prevent both obvious and subtle traps. Some of these checks | |
41 | are reasonably simple, such as verifying that path directories aren't | |
42 | writable by others; careful programmers have always used checks like | |
43 | these. Other checks, however, are best supported by the language itself, | |
fb73857a | 44 | and it is these checks especially that contribute to making a set-id Perl |
425e5e39 | 45 | program more secure than the corresponding C program. |
46 | ||
fb73857a | 47 | You may not use data derived from outside your program to affect |
48 | something else outside your program--at least, not by accident. All | |
49 | command line arguments, environment variables, locale information (see | |
23634c10 AL |
50 | L<perllocale>), results of certain system calls (C<readdir()>, |
51 | C<readlink()>, the variable of C<shmread()>, the messages returned by | |
52 | C<msgrcv()>, the password, gcos and shell fields returned by the | |
53 | C<getpwxxx()> calls), and all file input are marked as "tainted". | |
41d6edb2 JH |
54 | Tainted data may not be used directly or indirectly in any command |
55 | that invokes a sub-shell, nor in any command that modifies files, | |
b7ee89ce AP |
56 | directories, or processes, B<with the following exceptions>: |
57 | ||
58 | =over 4 | |
59 | ||
60 | =item * | |
61 | ||
b7ee89ce AP |
62 | Arguments to C<print> and C<syswrite> are B<not> checked for taintedness. |
63 | ||
7f6513c1 JH |
64 | =item * |
65 | ||
66 | Symbolic methods | |
67 | ||
68 | $obj->$method(@args); | |
69 | ||
70 | and symbolic sub references | |
71 | ||
72 | &{$foo}(@args); | |
73 | $foo->(@args); | |
74 | ||
75 | are not checked for taintedness. This requires extra carefulness | |
76 | unless you want external data to affect your control flow. Unless | |
77 | you carefully limit what these symbolic values are, people are able | |
78 | to call functions B<outside> your Perl code, such as POSIX::system, | |
79 | in which case they are able to run arbitrary external code. | |
80 | ||
8ea1447c RD |
81 | =item * |
82 | ||
83 | Hash keys are B<never> tainted. | |
84 | ||
b7ee89ce AP |
85 | =back |
86 | ||
595bde10 MG |
87 | For efficiency reasons, Perl takes a conservative view of |
88 | whether data is tainted. If an expression contains tainted data, | |
89 | any subexpression may be considered tainted, even if the value | |
90 | of the subexpression is not itself affected by the tainted data. | |
ee556d55 | 91 | |
d929ce6f | 92 | Because taintedness is associated with each scalar value, some |
595bde10 | 93 | elements of an array or hash can be tainted and others not. |
8ea1447c | 94 | The keys of a hash are B<never> tainted. |
a0d0e21e | 95 | |
a0d0e21e LW |
96 | For example: |
97 | ||
425e5e39 | 98 | $arg = shift; # $arg is tainted |
048b63be | 99 | $hid = $arg . 'bar'; # $hid is also tainted |
425e5e39 | 100 | $line = <>; # Tainted |
8ebc5c01 | 101 | $line = <STDIN>; # Also tainted |
102 | open FOO, "/home/me/bar" or die $!; | |
103 | $line = <FOO>; # Still tainted | |
a0d0e21e | 104 | $path = $ENV{'PATH'}; # Tainted, but see below |
425e5e39 | 105 | $data = 'abc'; # Not tainted |
a0d0e21e | 106 | |
425e5e39 | 107 | system "echo $arg"; # Insecure |
7de90c4d | 108 | system "/bin/echo", $arg; # Considered insecure |
bbd7eb8a | 109 | # (Perl doesn't know about /bin/echo) |
425e5e39 | 110 | system "echo $hid"; # Insecure |
111 | system "echo $data"; # Insecure until PATH set | |
a0d0e21e | 112 | |
425e5e39 | 113 | $path = $ENV{'PATH'}; # $path now tainted |
a0d0e21e | 114 | |
54310121 | 115 | $ENV{'PATH'} = '/bin:/usr/bin'; |
c90c0ff4 | 116 | delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; |
a0d0e21e | 117 | |
425e5e39 | 118 | $path = $ENV{'PATH'}; # $path now NOT tainted |
119 | system "echo $data"; # Is secure now! | |
a0d0e21e | 120 | |
425e5e39 | 121 | open(FOO, "< $arg"); # OK - read-only file |
122 | open(FOO, "> $arg"); # Not OK - trying to write | |
a0d0e21e | 123 | |
bbd7eb8a | 124 | open(FOO,"echo $arg|"); # Not OK |
425e5e39 | 125 | open(FOO,"-|") |
7de90c4d | 126 | or exec 'echo', $arg; # Also not OK |
a0d0e21e | 127 | |
425e5e39 | 128 | $shout = `echo $arg`; # Insecure, $shout now tainted |
a0d0e21e | 129 | |
425e5e39 | 130 | unlink $data, $arg; # Insecure |
131 | umask $arg; # Insecure | |
a0d0e21e | 132 | |
bbd7eb8a | 133 | exec "echo $arg"; # Insecure |
7de90c4d RD |
134 | exec "echo", $arg; # Insecure |
135 | exec "sh", '-c', $arg; # Very insecure! | |
a0d0e21e | 136 | |
3a4b19e4 GS |
137 | @files = <*.c>; # insecure (uses readdir() or similar) |
138 | @files = glob('*.c'); # insecure (uses readdir() or similar) | |
7bac28a0 | 139 | |
dde0c558 BF |
140 | # In either case, the results of glob are tainted, since the list of |
141 | # filenames comes from outside of the program. | |
3f7d42d8 | 142 | |
ee556d55 MG |
143 | $bad = ($arg, 23); # $bad will be tainted |
144 | $arg, `true`; # Insecure (although it isn't really) | |
145 | ||
a0d0e21e | 146 | If you try to do something insecure, you will get a fatal error saying |
7de90c4d | 147 | something like "Insecure dependency" or "Insecure $ENV{PATH}". |
425e5e39 | 148 | |
23634c10 AL |
149 | The exception to the principle of "one tainted value taints the whole |
150 | expression" is with the ternary conditional operator C<?:>. Since code | |
151 | with a ternary conditional | |
152 | ||
153 | $result = $tainted_value ? "Untainted" : "Also untainted"; | |
154 | ||
155 | is effectively | |
156 | ||
157 | if ( $tainted_value ) { | |
158 | $result = "Untainted"; | |
159 | } else { | |
160 | $result = "Also untainted"; | |
161 | } | |
162 | ||
163 | it doesn't make sense for C<$result> to be tainted. | |
164 | ||
425e5e39 | 165 | =head2 Laundering and Detecting Tainted Data |
166 | ||
3f7d42d8 JH |
167 | To test whether a variable contains tainted data, and whose use would |
168 | thus trigger an "Insecure dependency" message, you can use the | |
23634c10 | 169 | C<tainted()> function of the Scalar::Util module, available in your |
3f7d42d8 | 170 | nearby CPAN mirror, and included in Perl starting from the release 5.8.0. |
595bde10 | 171 | Or you may be able to use the following C<is_tainted()> function. |
425e5e39 | 172 | |
173 | sub is_tainted { | |
7687d286 | 174 | local $@; # Don't pollute caller's value. |
61890e45 | 175 | return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 }; |
425e5e39 | 176 | } |
177 | ||
178 | This function makes use of the fact that the presence of tainted data | |
179 | anywhere within an expression renders the entire expression tainted. It | |
180 | would be inefficient for every operator to test every argument for | |
181 | taintedness. Instead, the slightly more efficient and conservative | |
182 | approach is used that if any tainted value has been accessed within the | |
183 | same expression, the whole expression is considered tainted. | |
184 | ||
5f05dabc | 185 | But testing for taintedness gets you only so far. Sometimes you have just |
595bde10 MG |
186 | to clear your data's taintedness. Values may be untainted by using them |
187 | as keys in a hash; otherwise the only way to bypass the tainting | |
54310121 | 188 | mechanism is by referencing subpatterns from a regular expression match. |
18512f39 KW |
189 | Perl presumes that if you reference a substring using $1, $2, etc. in a |
190 | non-tainting pattern, that | |
191 | you knew what you were doing when you wrote that pattern. That means using | |
425e5e39 | 192 | a bit of thought--don't just blindly untaint anything, or you defeat the |
a034a98d DD |
193 | entire mechanism. It's better to verify that the variable has only good |
194 | characters (for certain values of "good") rather than checking whether it | |
195 | has any bad characters. That's because it's far too easy to miss bad | |
196 | characters that you never thought of. | |
425e5e39 | 197 | |
198 | Here's a test to make sure that the data contains nothing but "word" | |
199 | characters (alphabetics, numerics, and underscores), a hyphen, an at sign, | |
200 | or a dot. | |
201 | ||
54310121 | 202 | if ($data =~ /^([-\@\w.]+)$/) { |
425e5e39 | 203 | $data = $1; # $data now untainted |
204 | } else { | |
3a2263fe | 205 | die "Bad data in '$data'"; # log this somewhere |
425e5e39 | 206 | } |
207 | ||
5f05dabc | 208 | This is fairly secure because C</\w+/> doesn't normally match shell |
425e5e39 | 209 | metacharacters, nor are dot, dash, or at going to mean something special |
210 | to the shell. Use of C</.+/> would have been insecure in theory because | |
211 | it lets everything through, but Perl doesn't check for that. The lesson | |
212 | is that when untainting, you must be exceedingly careful with your patterns. | |
19799a22 | 213 | Laundering data using regular expression is the I<only> mechanism for |
425e5e39 | 214 | untainting dirty data, unless you use the strategy detailed below to fork |
215 | a child of lesser privilege. | |
216 | ||
23634c10 | 217 | The example does not untaint C<$data> if C<use locale> is in effect, |
a034a98d DD |
218 | because the characters matched by C<\w> are determined by the locale. |
219 | Perl considers that locale definitions are untrustworthy because they | |
220 | contain data from outside the program. If you are writing a | |
221 | locale-aware program, and want to launder data with a regular expression | |
222 | containing C<\w>, put C<no locale> ahead of the expression in the same | |
223 | block. See L<perllocale/SECURITY> for further discussion and examples. | |
224 | ||
3a52c276 CS |
225 | =head2 Switches On the "#!" Line |
226 | ||
227 | When you make a script executable, in order to make it usable as a | |
228 | command, the system will pass switches to perl from the script's #! | |
54310121 | 229 | line. Perl checks that any command line switches given to a setuid |
3a52c276 | 230 | (or setgid) script actually match the ones set on the #! line. Some |
54310121 | 231 | Unix and Unix-like environments impose a one-switch limit on the #! |
3a52c276 | 232 | line, so you may need to use something like C<-wU> instead of C<-w -U> |
54310121 | 233 | under such systems. (This issue should arise only in Unix or |
234 | Unix-like environments that support #! and setuid or setgid scripts.) | |
3a52c276 | 235 | |
588f7210 SB |
236 | =head2 Taint mode and @INC |
237 | ||
f7335192 DC |
238 | When the taint mode (C<-T>) is in effect, the environment variables |
239 | C<PERL5LIB> and C<PERLLIB> | |
91e64913 | 240 | are ignored by Perl. You can still adjust C<@INC> from outside the |
588f7210 | 241 | program by using the C<-I> command line option as explained in |
028611fa DB |
242 | L<perlrun|perlrun/-Idirectory>. The two environment variables are |
243 | ignored because they are obscured, and a user running a program could | |
244 | be unaware that they are set, whereas the C<-I> option is clearly | |
245 | visible and therefore permitted. | |
588f7210 SB |
246 | |
247 | Another way to modify C<@INC> without modifying the program, is to use | |
248 | the C<lib> pragma, e.g.: | |
249 | ||
250 | perl -Mlib=/foo program | |
251 | ||
252 | The benefit of using C<-Mlib=/foo> over C<-I/foo>, is that the former | |
6fd9f613 | 253 | will automagically remove any duplicated directories, while the latter |
588f7210 SB |
254 | will not. |
255 | ||
6a268663 RGS |
256 | Note that if a tainted string is added to C<@INC>, the following |
257 | problem will be reported: | |
258 | ||
259 | Insecure dependency in require while running with -T switch | |
260 | ||
f7335192 | 261 | On versions of Perl before 5.26, activating taint mode will also remove |
a1c1fa25 DC |
262 | the current directory (".") from the default value of C<@INC>. Since |
263 | version 5.26, the current directory isn't included in C<@INC> by | |
264 | default. | |
f7335192 | 265 | |
425e5e39 | 266 | =head2 Cleaning Up Your Path |
267 | ||
df98f984 RGS |
268 | For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to |
269 | a known value, and each directory in the path must be absolute and | |
270 | non-writable by others than its owner and group. You may be surprised to | |
271 | get this message even if the pathname to your executable is fully | |
272 | qualified. This is I<not> generated because you didn't supply a full path | |
273 | to the program; instead, it's generated because you never set your PATH | |
274 | environment variable, or you didn't set it to something that was safe. | |
275 | Because Perl can't guarantee that the executable in question isn't itself | |
276 | going to turn around and execute some other program that is dependent on | |
277 | your PATH, it makes sure you set the PATH. | |
a0d0e21e | 278 | |
a3cb178b GS |
279 | The PATH isn't the only environment variable which can cause problems. |
280 | Because some shells may use the variables IFS, CDPATH, ENV, and | |
281 | BASH_ENV, Perl checks that those are either empty or untainted when | |
91e64913 | 282 | starting subprocesses. You may wish to add something like this to your |
a3cb178b GS |
283 | setid and taint-checking scripts. |
284 | ||
285 | delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer | |
286 | ||
a0d0e21e LW |
287 | It's also possible to get into trouble with other operations that don't |
288 | care whether they use tainted values. Make judicious use of the file | |
289 | tests in dealing with any user-supplied filenames. When possible, do | |
fb73857a | 290 | opens and such B<after> properly dropping any special user (or group!) |
91e64913 FC |
291 | privileges. Perl doesn't prevent you from |
292 | opening tainted filenames for reading, | |
a0d0e21e LW |
293 | so be careful what you print out. The tainting mechanism is intended to |
294 | prevent stupid mistakes, not to remove the need for thought. | |
295 | ||
23634c10 AL |
296 | Perl does not call the shell to expand wild cards when you pass C<system> |
297 | and C<exec> explicit parameter lists instead of strings with possible shell | |
298 | wildcards in them. Unfortunately, the C<open>, C<glob>, and | |
54310121 | 299 | backtick functions provide no such alternate calling convention, so more |
300 | subterfuge will be required. | |
425e5e39 | 301 | |
302 | Perl provides a reasonably safe way to open a file or pipe from a setuid | |
303 | or setgid program: just create a child process with reduced privilege who | |
304 | does the dirty work for you. First, fork a child using the special | |
23634c10 | 305 | C<open> syntax that connects the parent and child by a pipe. Now the |
425e5e39 | 306 | child resets its ID set and any other per-process attributes, like |
307 | environment variables, umasks, current working directories, back to the | |
308 | originals or known safe values. Then the child process, which no longer | |
23634c10 | 309 | has any special permissions, does the C<open> or other system call. |
425e5e39 | 310 | Finally, the child passes the data it managed to access back to the |
5f05dabc | 311 | parent. Because the file or pipe was opened in the child while running |
425e5e39 | 312 | under less privilege than the parent, it's not apt to be tricked into |
313 | doing something it shouldn't. | |
314 | ||
23634c10 | 315 | Here's a way to do backticks reasonably safely. Notice how the C<exec> is |
425e5e39 | 316 | not called with a string that the shell could expand. This is by far the |
317 | best way to call something that might be subjected to shell escapes: just | |
fb73857a | 318 | never call the shell at all. |
cb1a09d0 | 319 | |
6ca3c6c6 | 320 | use English; |
e093bcf0 GW |
321 | die "Can't fork: $!" unless defined($pid = open(KID, "-|")); |
322 | if ($pid) { # parent | |
323 | while (<KID>) { | |
324 | # do something | |
325 | } | |
326 | close KID; | |
327 | } else { | |
328 | my @temp = ($EUID, $EGID); | |
329 | my $orig_uid = $UID; | |
330 | my $orig_gid = $GID; | |
331 | $EUID = $UID; | |
332 | $EGID = $GID; | |
333 | # Drop privileges | |
334 | $UID = $orig_uid; | |
335 | $GID = $orig_gid; | |
336 | # Make sure privs are really gone | |
337 | ($EUID, $EGID) = @temp; | |
338 | die "Can't drop privileges" | |
339 | unless $UID == $EUID && $GID eq $EGID; | |
340 | $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH. | |
341 | # Consider sanitizing the environment even more. | |
342 | exec 'myprog', 'arg1', 'arg2' | |
343 | or die "can't exec myprog: $!"; | |
344 | } | |
425e5e39 | 345 | |
fb73857a | 346 | A similar strategy would work for wildcard expansion via C<glob>, although |
347 | you can use C<readdir> instead. | |
425e5e39 | 348 | |
349 | Taint checking is most useful when although you trust yourself not to have | |
350 | written a program to give away the farm, you don't necessarily trust those | |
351 | who end up using it not to try to trick it into doing something bad. This | |
fb73857a | 352 | is the kind of security checking that's useful for set-id programs and |
425e5e39 | 353 | programs launched on someone else's behalf, like CGI programs. |
354 | ||
355 | This is quite different, however, from not even trusting the writer of the | |
356 | code not to try to do something evil. That's the kind of trust needed | |
357 | when someone hands you a program you've never seen before and says, "Here, | |
18d7fc85 RGS |
358 | run this." For that kind of safety, you might want to check out the Safe |
359 | module, included standard in the Perl distribution. This module allows the | |
425e5e39 | 360 | programmer to set up special compartments in which all system operations |
18d7fc85 RGS |
361 | are trapped and namespace access is carefully controlled. Safe should |
362 | not be considered bullet-proof, though: it will not prevent the foreign | |
363 | code to set up infinite loops, allocate gigabytes of memory, or even | |
364 | abusing perl bugs to make the host interpreter crash or behave in | |
91e64913 | 365 | unpredictable ways. In any case it's better avoided completely if you're |
18d7fc85 | 366 | really concerned about security. |
425e5e39 | 367 | |
b5145c7d | 368 | =head2 Shebang Race Condition |
425e5e39 | 369 | |
370 | Beyond the obvious problems that stem from giving special privileges to | |
fb73857a | 371 | systems as flexible as scripts, on many versions of Unix, set-id scripts |
425e5e39 | 372 | are inherently insecure right from the start. The problem is a race |
373 | condition in the kernel. Between the time the kernel opens the file to | |
fb73857a | 374 | see which interpreter to run and when the (now-set-id) interpreter turns |
425e5e39 | 375 | around and reopens the file to interpret it, the file in question may have |
376 | changed, especially if you have symbolic links on your system. | |
377 | ||
dabde021 | 378 | Some Unixes, especially more recent ones, are free of this |
b5145c7d Z |
379 | inherent security bug. On such systems, when the kernel passes the name |
380 | of the set-id script to open to the interpreter, rather than using a | |
381 | pathname subject to meddling, it instead passes I</dev/fd/3>. This is a | |
382 | special file already opened on the script, so that there can be no race | |
383 | condition for evil scripts to exploit. On these systems, Perl should be | |
384 | compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F<Configure> | |
385 | program that builds Perl tries to figure this out for itself, so you | |
386 | should never have to specify this yourself. Most modern releases of | |
387 | SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. | |
425e5e39 | 388 | |
b5145c7d Z |
389 | If you don't have the safe version of set-id scripts, all is not lost. |
390 | Sometimes this kernel "feature" can be disabled, so that the kernel | |
391 | either doesn't run set-id scripts with the set-id or doesn't run them | |
392 | at all. Either way avoids the exploitability of the race condition, | |
393 | but doesn't help in actually running scripts set-id. | |
394 | ||
395 | If the kernel set-id script feature isn't disabled, then any set-id | |
396 | script provides an exploitable vulnerability. Perl can't avoid being | |
397 | exploitable, but will point out vulnerable scripts where it can. If Perl | |
398 | detects that it is being applied to a set-id script then it will complain | |
399 | loudly that your set-id script is insecure, and won't run it. When Perl | |
400 | complains, you need to remove the set-id bit from the script to eliminate | |
401 | the vulnerability. Refusing to run the script doesn't in itself close | |
402 | the vulnerability; it is just Perl's way of encouraging you to do this. | |
403 | ||
404 | To actually run a script set-id, if you don't have the safe version of | |
405 | set-id scripts, you'll need to put a C wrapper around | |
425e5e39 | 406 | the script. A C wrapper is just a compiled program that does nothing |
407 | except call your Perl program. Compiled programs are not subject to the | |
fb73857a | 408 | kernel bug that plagues set-id scripts. Here's a simple wrapper, written |
425e5e39 | 409 | in C: |
410 | ||
245c138e LM |
411 | #include <unistd.h> |
412 | #include <stdio.h> | |
413 | #include <string.h> | |
414 | #include <errno.h> | |
415 | ||
425e5e39 | 416 | #define REAL_PATH "/path/to/script" |
245c138e LM |
417 | |
418 | int main(int argc, char **argv) | |
425e5e39 | 419 | { |
245c138e LM |
420 | execv(REAL_PATH, argv); |
421 | fprintf(stderr, "%s: %s: %s\n", | |
422 | argv[0], REAL_PATH, strerror(errno)); | |
423 | return 127; | |
54310121 | 424 | } |
cb1a09d0 | 425 | |
54310121 | 426 | Compile this wrapper into a binary executable and then make I<it> rather |
b5145c7d | 427 | than your script setuid or setgid. Note that this wrapper isn't doing |
dabde021 | 428 | anything to sanitise the execution environment other than ensuring |
b5145c7d Z |
429 | that a safe path to the script is used. It only avoids the shebang |
430 | race condition. It relies on Perl's own features, and on the script | |
431 | itself being careful, to make it safe enough to run the script set-id. | |
425e5e39 | 432 | |
68dc0745 | 433 | =head2 Protecting Your Programs |
434 | ||
435 | There are a number of ways to hide the source to your Perl programs, | |
436 | with varying levels of "security". | |
437 | ||
438 | First of all, however, you I<can't> take away read permission, because | |
439 | the source code has to be readable in order to be compiled and | |
440 | interpreted. (That doesn't mean that a CGI script's source is | |
441 | readable by people on the web, though.) So you have to leave the | |
5a964f20 TC |
442 | permissions at the socially friendly 0755 level. This lets |
443 | people on your local system only see your source. | |
68dc0745 | 444 | |
5a964f20 | 445 | Some people mistakenly regard this as a security problem. If your program does |
68dc0745 | 446 | insecure things, and relies on people not knowing how to exploit those |
447 | insecurities, it is not secure. It is often possible for someone to | |
448 | determine the insecure things and exploit them without viewing the | |
449 | source. Security through obscurity, the name for hiding your bugs | |
450 | instead of fixing them, is little security indeed. | |
451 | ||
83df6a1d JH |
452 | You can try using encryption via source filters (Filter::* from CPAN, |
453 | or Filter::Util::Call and Filter::Simple since Perl 5.8). | |
454 | But crackers might be able to decrypt it. You can try using the byte | |
455 | code compiler and interpreter described below, but crackers might be | |
456 | able to de-compile it. You can try using the native-code compiler | |
68dc0745 | 457 | described below, but crackers might be able to disassemble it. These |
458 | pose varying degrees of difficulty to people wanting to get at your | |
459 | code, but none can definitively conceal it (this is true of every | |
460 | language, not just Perl). | |
461 | ||
462 | If you're concerned about people profiting from your code, then the | |
3462340b | 463 | bottom line is that nothing but a restrictive license will give you |
68dc0745 | 464 | legal security. License your software and pepper it with threatening |
465 | statements like "This is unpublished proprietary software of XYZ Corp. | |
466 | Your access to it does not give you permission to use it blah blah | |
3462340b | 467 | blah." You should see a lawyer to be sure your license's wording will |
68dc0745 | 468 | stand up in court. |
5a964f20 | 469 | |
0d7c09bb JH |
470 | =head2 Unicode |
471 | ||
472 | Unicode is a new and complex technology and one may easily overlook | |
473 | certain security pitfalls. See L<perluniintro> for an overview and | |
474 | L<perlunicode> for details, and L<perlunicode/"Security Implications | |
475 | of Unicode"> for security implications in particular. | |
476 | ||
504f80c1 JH |
477 | =head2 Algorithmic Complexity Attacks |
478 | ||
479 | Certain internal algorithms used in the implementation of Perl can | |
480 | be attacked by choosing the input carefully to consume large amounts | |
481 | of either time or space or both. This can lead into the so-called | |
482 | I<Denial of Service> (DoS) attacks. | |
483 | ||
484 | =over 4 | |
485 | ||
486 | =item * | |
487 | ||
6a5b4183 YO |
488 | Hash Algorithm - Hash algorithms like the one used in Perl are well |
489 | known to be vulnerable to collision attacks on their hash function. | |
490 | Such attacks involve constructing a set of keys which collide into | |
91e64913 | 491 | the same bucket producing inefficient behavior. Such attacks often |
6a5b4183 | 492 | depend on discovering the seed of the hash function used to map the |
91e64913 FC |
493 | keys to buckets. That seed is then used to brute-force a key set which |
494 | can be used to mount a denial of service attack. In Perl 5.8.1 changes | |
6a5b4183 YO |
495 | were introduced to harden Perl to such attacks, and then later in |
496 | Perl 5.18.0 these features were enhanced and additional protections | |
497 | added. | |
498 | ||
4d74c8eb S |
499 | At the time of this writing, Perl 5.18.0 is considered to be |
500 | well-hardened against algorithmic complexity attacks on its hash | |
91e64913 | 501 | implementation. This is largely owed to the following measures |
4d74c8eb | 502 | mitigate attacks: |
6a5b4183 YO |
503 | |
504 | =over 4 | |
505 | ||
506 | =item Hash Seed Randomization | |
507 | ||
508 | In order to make it impossible to know what seed to generate an attack | |
91e64913 | 509 | key set for, this seed is randomly initialized at process start. This |
4d74c8eb | 510 | may be overridden by using the PERL_HASH_SEED environment variable, see |
91e64913 | 511 | L<perlrun/PERL_HASH_SEED>. This environment variable controls how |
4d74c8eb S |
512 | items are actually stored, not how they are presented via |
513 | C<keys>, C<values> and C<each>. | |
6a5b4183 YO |
514 | |
515 | =item Hash Traversal Randomization | |
516 | ||
4d74c8eb | 517 | Independent of which seed is used in the hash function, C<keys>, |
6a5b4183 YO |
518 | C<values>, and C<each> return items in a per-hash randomized order. |
519 | Modifying a hash by insertion will change the iteration order of that hash. | |
4d74c8eb | 520 | This behavior can be overridden by using C<hash_traversal_mask()> from |
6a5b4183 | 521 | L<Hash::Util> or by using the PERL_PERTURB_KEYS environment variable, |
91e64913 | 522 | see L<perlrun/PERL_PERTURB_KEYS>. Note that this feature controls the |
6a5b4183 YO |
523 | "visible" order of the keys, and not the actual order they are stored in. |
524 | ||
525 | =item Bucket Order Perturbance | |
526 | ||
4d74c8eb | 527 | When items collide into a given hash bucket the order they are stored in |
91e64913 FC |
528 | the chain is no longer predictable in Perl 5.18. This |
529 | has the intention to make it harder to observe a | |
c6c886ef | 530 | collision. This behavior can be overridden by using |
6a5b4183 YO |
531 | the PERL_PERTURB_KEYS environment variable, see L<perlrun/PERL_PERTURB_KEYS>. |
532 | ||
533 | =item New Default Hash Function | |
534 | ||
535 | The default hash function has been modified with the intention of making | |
536 | it harder to infer the hash seed. | |
537 | ||
538 | =item Alternative Hash Functions | |
539 | ||
540 | The source code includes multiple hash algorithms to choose from. While we | |
4d74c8eb | 541 | believe that the default perl hash is robust to attack, we have included the |
91e64913 | 542 | hash function Siphash as a fall-back option. At the time of release of |
6a5b4183 YO |
543 | Perl 5.18.0 Siphash is believed to be of cryptographic strength. This is |
544 | not the default as it is much slower than the default hash. | |
545 | ||
546 | =back | |
547 | ||
4d74c8eb | 548 | Without compiling a special Perl, there is no way to get the exact same |
91e64913 | 549 | behavior of any versions prior to Perl 5.18.0. The closest one can get |
6a5b4183 | 550 | is by setting PERL_PERTURB_KEYS to 0 and setting the PERL_HASH_SEED |
91e64913 | 551 | to a known value. We do not advise those settings for production use |
4d74c8eb | 552 | due to the above security considerations. |
6a5b4183 YO |
553 | |
554 | B<Perl has never guaranteed any ordering of the hash keys>, and | |
555 | the ordering has already changed several times during the lifetime of | |
556 | Perl 5. Also, the ordering of hash keys has always been, and continues | |
557 | to be, affected by the insertion order and the history of changes made | |
558 | to the hash over its lifetime. | |
7b3f7037 JH |
559 | |
560 | Also note that while the order of the hash elements might be | |
4d74c8eb S |
561 | randomized, this "pseudo-ordering" should B<not> be used for |
562 | applications like shuffling a list randomly (use C<List::Util::shuffle()> | |
7b3f7037 | 563 | for that, see L<List::Util>, a standard core module since Perl 5.8.0; |
4d74c8eb S |
564 | or the CPAN module C<Algorithm::Numerical::Shuffle>), or for generating |
565 | permutations (use e.g. the CPAN modules C<Algorithm::Permute> or | |
566 | C<Algorithm::FastPermute>), or for any cryptographic applications. | |
7b3f7037 | 567 | |
883f220b TC |
568 | Tied hashes may have their own ordering and algorithmic complexity |
569 | attacks. | |
570 | ||
504f80c1 JH |
571 | =item * |
572 | ||
5a4e8ea7 P |
573 | Regular expressions - Perl's regular expression engine is so called NFA |
574 | (Non-deterministic Finite Automaton), which among other things means that | |
575 | it can rather easily consume large amounts of both time and space if the | |
504f80c1 JH |
576 | regular expression may match in several ways. Careful crafting of the |
577 | regular expressions can help but quite often there really isn't much | |
578 | one can do (the book "Mastering Regular Expressions" is required | |
579 | reading, see L<perlfaq2>). Running out of space manifests itself by | |
580 | Perl running out of memory. | |
581 | ||
582 | =item * | |
583 | ||
584 | Sorting - the quicksort algorithm used in Perls before 5.8.0 to | |
e2091bb6 | 585 | implement the sort() function was very easy to trick into misbehaving |
3462340b JL |
586 | so that it consumes a lot of time. Starting from Perl 5.8.0 a different |
587 | sorting algorithm, mergesort, is used by default. Mergesort cannot | |
588 | misbehave on any input. | |
504f80c1 JH |
589 | |
590 | =back | |
591 | ||
b25b06cf | 592 | See L<https://www.usenix.org/legacy/events/sec03/tech/full_papers/crosby/crosby.pdf> for more information, |
3462340b | 593 | and any computer science textbook on algorithmic complexity. |
504f80c1 | 594 | |
b5145c7d Z |
595 | =head2 Using Sudo |
596 | ||
597 | The popular tool C<sudo> provides a controlled way for users to be able | |
598 | to run programs as other users. It sanitises the execution environment | |
599 | to some extent, and will avoid the L<shebang race condition|/"Shebang | |
600 | Race Condition">. If you don't have the safe version of set-id scripts, | |
601 | then C<sudo> may be a more convenient way of executing a script as | |
602 | another user than writing a C wrapper would be. | |
603 | ||
604 | However, C<sudo> sets the real user or group ID to that of the target | |
605 | identity, not just the effective ID as set-id bits do. As a result, Perl | |
606 | can't detect that it is running under C<sudo>, and so won't automatically | |
607 | take its own security precautions such as turning on taint mode. Where | |
608 | C<sudo> configuration dictates exactly which command can be run, the | |
609 | approved command may include a C<-T> option to perl to enable taint mode. | |
610 | ||
f1460a66 | 611 | In general, it is necessary to evaluate the suitability of a script to |
b5145c7d Z |
612 | run under C<sudo> specifically with that kind of execution environment |
613 | in mind. It is neither necessary nor sufficient for the same script to | |
614 | be suitable to run in a traditional set-id arrangement, though many of | |
615 | the issues overlap. | |
616 | ||
5a964f20 TC |
617 | =head1 SEE ALSO |
618 | ||
028611fa DB |
619 | L<perlrun/ENVIRONMENT> for its description of cleaning up environment |
620 | variables. |