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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 | ||
17 | If you believe you have found a security vulnerability in Perl, please email | |
18 | perl5-security-report@perl.org with details. This points to a closed | |
19 | subscription, unarchived mailing list. Please only use this address for | |
20 | security issues in the Perl core, not for modules independently distributed on | |
21 | CPAN. | |
22 | ||
23 | =head1 SECURITY MECHANISMS AND CONCERNS | |
24 | ||
25 | =head2 Taint mode | |
26 | ||
425e5e39 | 27 | Perl automatically enables a set of special security checks, called I<taint |
28 | mode>, when it detects its program running with differing real and effective | |
29 | user or group IDs. The setuid bit in Unix permissions is mode 04000, the | |
30 | setgid bit mode 02000; either or both may be set. You can also enable taint | |
91e64913 | 31 | mode explicitly by using the B<-T> command line flag. This flag is |
425e5e39 | 32 | I<strongly> suggested for server programs and any program run on behalf of |
91e64913 | 33 | someone else, such as a CGI script. Once taint mode is on, it's on for |
fb73857a | 34 | the remainder of your script. |
a0d0e21e | 35 | |
1e422769 | 36 | While in this mode, Perl takes special precautions called I<taint |
37 | checks> to prevent both obvious and subtle traps. Some of these checks | |
38 | are reasonably simple, such as verifying that path directories aren't | |
39 | writable by others; careful programmers have always used checks like | |
40 | these. Other checks, however, are best supported by the language itself, | |
fb73857a | 41 | and it is these checks especially that contribute to making a set-id Perl |
425e5e39 | 42 | program more secure than the corresponding C program. |
43 | ||
fb73857a | 44 | You may not use data derived from outside your program to affect |
45 | something else outside your program--at least, not by accident. All | |
46 | command line arguments, environment variables, locale information (see | |
23634c10 AL |
47 | L<perllocale>), results of certain system calls (C<readdir()>, |
48 | C<readlink()>, the variable of C<shmread()>, the messages returned by | |
49 | C<msgrcv()>, the password, gcos and shell fields returned by the | |
50 | C<getpwxxx()> calls), and all file input are marked as "tainted". | |
41d6edb2 JH |
51 | Tainted data may not be used directly or indirectly in any command |
52 | that invokes a sub-shell, nor in any command that modifies files, | |
b7ee89ce AP |
53 | directories, or processes, B<with the following exceptions>: |
54 | ||
55 | =over 4 | |
56 | ||
57 | =item * | |
58 | ||
b7ee89ce AP |
59 | Arguments to C<print> and C<syswrite> are B<not> checked for taintedness. |
60 | ||
7f6513c1 JH |
61 | =item * |
62 | ||
63 | Symbolic methods | |
64 | ||
65 | $obj->$method(@args); | |
66 | ||
67 | and symbolic sub references | |
68 | ||
69 | &{$foo}(@args); | |
70 | $foo->(@args); | |
71 | ||
72 | are not checked for taintedness. This requires extra carefulness | |
73 | unless you want external data to affect your control flow. Unless | |
74 | you carefully limit what these symbolic values are, people are able | |
75 | to call functions B<outside> your Perl code, such as POSIX::system, | |
76 | in which case they are able to run arbitrary external code. | |
77 | ||
8ea1447c RD |
78 | =item * |
79 | ||
80 | Hash keys are B<never> tainted. | |
81 | ||
b7ee89ce AP |
82 | =back |
83 | ||
595bde10 MG |
84 | For efficiency reasons, Perl takes a conservative view of |
85 | whether data is tainted. If an expression contains tainted data, | |
86 | any subexpression may be considered tainted, even if the value | |
87 | of the subexpression is not itself affected by the tainted data. | |
ee556d55 | 88 | |
d929ce6f | 89 | Because taintedness is associated with each scalar value, some |
595bde10 | 90 | elements of an array or hash can be tainted and others not. |
8ea1447c | 91 | The keys of a hash are B<never> tainted. |
a0d0e21e | 92 | |
a0d0e21e LW |
93 | For example: |
94 | ||
425e5e39 | 95 | $arg = shift; # $arg is tainted |
048b63be | 96 | $hid = $arg . 'bar'; # $hid is also tainted |
425e5e39 | 97 | $line = <>; # Tainted |
8ebc5c01 | 98 | $line = <STDIN>; # Also tainted |
99 | open FOO, "/home/me/bar" or die $!; | |
100 | $line = <FOO>; # Still tainted | |
a0d0e21e | 101 | $path = $ENV{'PATH'}; # Tainted, but see below |
425e5e39 | 102 | $data = 'abc'; # Not tainted |
a0d0e21e | 103 | |
425e5e39 | 104 | system "echo $arg"; # Insecure |
7de90c4d | 105 | system "/bin/echo", $arg; # Considered insecure |
bbd7eb8a | 106 | # (Perl doesn't know about /bin/echo) |
425e5e39 | 107 | system "echo $hid"; # Insecure |
108 | system "echo $data"; # Insecure until PATH set | |
a0d0e21e | 109 | |
425e5e39 | 110 | $path = $ENV{'PATH'}; # $path now tainted |
a0d0e21e | 111 | |
54310121 | 112 | $ENV{'PATH'} = '/bin:/usr/bin'; |
c90c0ff4 | 113 | delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; |
a0d0e21e | 114 | |
425e5e39 | 115 | $path = $ENV{'PATH'}; # $path now NOT tainted |
116 | system "echo $data"; # Is secure now! | |
a0d0e21e | 117 | |
425e5e39 | 118 | open(FOO, "< $arg"); # OK - read-only file |
119 | open(FOO, "> $arg"); # Not OK - trying to write | |
a0d0e21e | 120 | |
bbd7eb8a | 121 | open(FOO,"echo $arg|"); # Not OK |
425e5e39 | 122 | open(FOO,"-|") |
7de90c4d | 123 | or exec 'echo', $arg; # Also not OK |
a0d0e21e | 124 | |
425e5e39 | 125 | $shout = `echo $arg`; # Insecure, $shout now tainted |
a0d0e21e | 126 | |
425e5e39 | 127 | unlink $data, $arg; # Insecure |
128 | umask $arg; # Insecure | |
a0d0e21e | 129 | |
bbd7eb8a | 130 | exec "echo $arg"; # Insecure |
7de90c4d RD |
131 | exec "echo", $arg; # Insecure |
132 | exec "sh", '-c', $arg; # Very insecure! | |
a0d0e21e | 133 | |
3a4b19e4 GS |
134 | @files = <*.c>; # insecure (uses readdir() or similar) |
135 | @files = glob('*.c'); # insecure (uses readdir() or similar) | |
7bac28a0 | 136 | |
dde0c558 BF |
137 | # In either case, the results of glob are tainted, since the list of |
138 | # filenames comes from outside of the program. | |
3f7d42d8 | 139 | |
ee556d55 MG |
140 | $bad = ($arg, 23); # $bad will be tainted |
141 | $arg, `true`; # Insecure (although it isn't really) | |
142 | ||
a0d0e21e | 143 | If you try to do something insecure, you will get a fatal error saying |
7de90c4d | 144 | something like "Insecure dependency" or "Insecure $ENV{PATH}". |
425e5e39 | 145 | |
23634c10 AL |
146 | The exception to the principle of "one tainted value taints the whole |
147 | expression" is with the ternary conditional operator C<?:>. Since code | |
148 | with a ternary conditional | |
149 | ||
150 | $result = $tainted_value ? "Untainted" : "Also untainted"; | |
151 | ||
152 | is effectively | |
153 | ||
154 | if ( $tainted_value ) { | |
155 | $result = "Untainted"; | |
156 | } else { | |
157 | $result = "Also untainted"; | |
158 | } | |
159 | ||
160 | it doesn't make sense for C<$result> to be tainted. | |
161 | ||
425e5e39 | 162 | =head2 Laundering and Detecting Tainted Data |
163 | ||
3f7d42d8 JH |
164 | To test whether a variable contains tainted data, and whose use would |
165 | thus trigger an "Insecure dependency" message, you can use the | |
23634c10 | 166 | C<tainted()> function of the Scalar::Util module, available in your |
3f7d42d8 | 167 | nearby CPAN mirror, and included in Perl starting from the release 5.8.0. |
595bde10 | 168 | Or you may be able to use the following C<is_tainted()> function. |
425e5e39 | 169 | |
170 | sub is_tainted { | |
7687d286 | 171 | local $@; # Don't pollute caller's value. |
61890e45 | 172 | return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 }; |
425e5e39 | 173 | } |
174 | ||
175 | This function makes use of the fact that the presence of tainted data | |
176 | anywhere within an expression renders the entire expression tainted. It | |
177 | would be inefficient for every operator to test every argument for | |
178 | taintedness. Instead, the slightly more efficient and conservative | |
179 | approach is used that if any tainted value has been accessed within the | |
180 | same expression, the whole expression is considered tainted. | |
181 | ||
5f05dabc | 182 | But testing for taintedness gets you only so far. Sometimes you have just |
595bde10 MG |
183 | to clear your data's taintedness. Values may be untainted by using them |
184 | as keys in a hash; otherwise the only way to bypass the tainting | |
54310121 | 185 | mechanism is by referencing subpatterns from a regular expression match. |
18512f39 KW |
186 | Perl presumes that if you reference a substring using $1, $2, etc. in a |
187 | non-tainting pattern, that | |
188 | you knew what you were doing when you wrote that pattern. That means using | |
425e5e39 | 189 | a bit of thought--don't just blindly untaint anything, or you defeat the |
a034a98d DD |
190 | entire mechanism. It's better to verify that the variable has only good |
191 | characters (for certain values of "good") rather than checking whether it | |
192 | has any bad characters. That's because it's far too easy to miss bad | |
193 | characters that you never thought of. | |
425e5e39 | 194 | |
195 | Here's a test to make sure that the data contains nothing but "word" | |
196 | characters (alphabetics, numerics, and underscores), a hyphen, an at sign, | |
197 | or a dot. | |
198 | ||
54310121 | 199 | if ($data =~ /^([-\@\w.]+)$/) { |
425e5e39 | 200 | $data = $1; # $data now untainted |
201 | } else { | |
3a2263fe | 202 | die "Bad data in '$data'"; # log this somewhere |
425e5e39 | 203 | } |
204 | ||
5f05dabc | 205 | This is fairly secure because C</\w+/> doesn't normally match shell |
425e5e39 | 206 | metacharacters, nor are dot, dash, or at going to mean something special |
207 | to the shell. Use of C</.+/> would have been insecure in theory because | |
208 | it lets everything through, but Perl doesn't check for that. The lesson | |
209 | is that when untainting, you must be exceedingly careful with your patterns. | |
19799a22 | 210 | Laundering data using regular expression is the I<only> mechanism for |
425e5e39 | 211 | untainting dirty data, unless you use the strategy detailed below to fork |
212 | a child of lesser privilege. | |
213 | ||
23634c10 | 214 | The example does not untaint C<$data> if C<use locale> is in effect, |
a034a98d DD |
215 | because the characters matched by C<\w> are determined by the locale. |
216 | Perl considers that locale definitions are untrustworthy because they | |
217 | contain data from outside the program. If you are writing a | |
218 | locale-aware program, and want to launder data with a regular expression | |
219 | containing C<\w>, put C<no locale> ahead of the expression in the same | |
220 | block. See L<perllocale/SECURITY> for further discussion and examples. | |
221 | ||
3a52c276 CS |
222 | =head2 Switches On the "#!" Line |
223 | ||
224 | When you make a script executable, in order to make it usable as a | |
225 | command, the system will pass switches to perl from the script's #! | |
54310121 | 226 | line. Perl checks that any command line switches given to a setuid |
3a52c276 | 227 | (or setgid) script actually match the ones set on the #! line. Some |
54310121 | 228 | Unix and Unix-like environments impose a one-switch limit on the #! |
3a52c276 | 229 | line, so you may need to use something like C<-wU> instead of C<-w -U> |
54310121 | 230 | under such systems. (This issue should arise only in Unix or |
231 | Unix-like environments that support #! and setuid or setgid scripts.) | |
3a52c276 | 232 | |
588f7210 SB |
233 | =head2 Taint mode and @INC |
234 | ||
235 | When the taint mode (C<-T>) is in effect, the "." directory is removed | |
236 | from C<@INC>, and the environment variables C<PERL5LIB> and C<PERLLIB> | |
91e64913 | 237 | are ignored by Perl. You can still adjust C<@INC> from outside the |
588f7210 | 238 | program by using the C<-I> command line option as explained in |
91e64913 | 239 | L<perlrun>. The two environment variables are ignored because |
588f7210 SB |
240 | they are obscured, and a user running a program could be unaware that |
241 | they are set, whereas the C<-I> option is clearly visible and | |
242 | therefore permitted. | |
243 | ||
244 | Another way to modify C<@INC> without modifying the program, is to use | |
245 | the C<lib> pragma, e.g.: | |
246 | ||
247 | perl -Mlib=/foo program | |
248 | ||
249 | The benefit of using C<-Mlib=/foo> over C<-I/foo>, is that the former | |
6fd9f613 | 250 | will automagically remove any duplicated directories, while the latter |
588f7210 SB |
251 | will not. |
252 | ||
6a268663 RGS |
253 | Note that if a tainted string is added to C<@INC>, the following |
254 | problem will be reported: | |
255 | ||
256 | Insecure dependency in require while running with -T switch | |
257 | ||
425e5e39 | 258 | =head2 Cleaning Up Your Path |
259 | ||
df98f984 RGS |
260 | For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to |
261 | a known value, and each directory in the path must be absolute and | |
262 | non-writable by others than its owner and group. You may be surprised to | |
263 | get this message even if the pathname to your executable is fully | |
264 | qualified. This is I<not> generated because you didn't supply a full path | |
265 | to the program; instead, it's generated because you never set your PATH | |
266 | environment variable, or you didn't set it to something that was safe. | |
267 | Because Perl can't guarantee that the executable in question isn't itself | |
268 | going to turn around and execute some other program that is dependent on | |
269 | your PATH, it makes sure you set the PATH. | |
a0d0e21e | 270 | |
a3cb178b GS |
271 | The PATH isn't the only environment variable which can cause problems. |
272 | Because some shells may use the variables IFS, CDPATH, ENV, and | |
273 | BASH_ENV, Perl checks that those are either empty or untainted when | |
91e64913 | 274 | starting subprocesses. You may wish to add something like this to your |
a3cb178b GS |
275 | setid and taint-checking scripts. |
276 | ||
277 | delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer | |
278 | ||
a0d0e21e LW |
279 | It's also possible to get into trouble with other operations that don't |
280 | care whether they use tainted values. Make judicious use of the file | |
281 | tests in dealing with any user-supplied filenames. When possible, do | |
fb73857a | 282 | opens and such B<after> properly dropping any special user (or group!) |
91e64913 FC |
283 | privileges. Perl doesn't prevent you from |
284 | opening tainted filenames for reading, | |
a0d0e21e LW |
285 | so be careful what you print out. The tainting mechanism is intended to |
286 | prevent stupid mistakes, not to remove the need for thought. | |
287 | ||
23634c10 AL |
288 | Perl does not call the shell to expand wild cards when you pass C<system> |
289 | and C<exec> explicit parameter lists instead of strings with possible shell | |
290 | wildcards in them. Unfortunately, the C<open>, C<glob>, and | |
54310121 | 291 | backtick functions provide no such alternate calling convention, so more |
292 | subterfuge will be required. | |
425e5e39 | 293 | |
294 | Perl provides a reasonably safe way to open a file or pipe from a setuid | |
295 | or setgid program: just create a child process with reduced privilege who | |
296 | does the dirty work for you. First, fork a child using the special | |
23634c10 | 297 | C<open> syntax that connects the parent and child by a pipe. Now the |
425e5e39 | 298 | child resets its ID set and any other per-process attributes, like |
299 | environment variables, umasks, current working directories, back to the | |
300 | originals or known safe values. Then the child process, which no longer | |
23634c10 | 301 | has any special permissions, does the C<open> or other system call. |
425e5e39 | 302 | Finally, the child passes the data it managed to access back to the |
5f05dabc | 303 | parent. Because the file or pipe was opened in the child while running |
425e5e39 | 304 | under less privilege than the parent, it's not apt to be tricked into |
305 | doing something it shouldn't. | |
306 | ||
23634c10 | 307 | Here's a way to do backticks reasonably safely. Notice how the C<exec> is |
425e5e39 | 308 | not called with a string that the shell could expand. This is by far the |
309 | best way to call something that might be subjected to shell escapes: just | |
fb73857a | 310 | never call the shell at all. |
cb1a09d0 | 311 | |
6ca3c6c6 | 312 | use English; |
e093bcf0 GW |
313 | die "Can't fork: $!" unless defined($pid = open(KID, "-|")); |
314 | if ($pid) { # parent | |
315 | while (<KID>) { | |
316 | # do something | |
317 | } | |
318 | close KID; | |
319 | } else { | |
320 | my @temp = ($EUID, $EGID); | |
321 | my $orig_uid = $UID; | |
322 | my $orig_gid = $GID; | |
323 | $EUID = $UID; | |
324 | $EGID = $GID; | |
325 | # Drop privileges | |
326 | $UID = $orig_uid; | |
327 | $GID = $orig_gid; | |
328 | # Make sure privs are really gone | |
329 | ($EUID, $EGID) = @temp; | |
330 | die "Can't drop privileges" | |
331 | unless $UID == $EUID && $GID eq $EGID; | |
332 | $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH. | |
333 | # Consider sanitizing the environment even more. | |
334 | exec 'myprog', 'arg1', 'arg2' | |
335 | or die "can't exec myprog: $!"; | |
336 | } | |
425e5e39 | 337 | |
fb73857a | 338 | A similar strategy would work for wildcard expansion via C<glob>, although |
339 | you can use C<readdir> instead. | |
425e5e39 | 340 | |
341 | Taint checking is most useful when although you trust yourself not to have | |
342 | written a program to give away the farm, you don't necessarily trust those | |
343 | who end up using it not to try to trick it into doing something bad. This | |
fb73857a | 344 | is the kind of security checking that's useful for set-id programs and |
425e5e39 | 345 | programs launched on someone else's behalf, like CGI programs. |
346 | ||
347 | This is quite different, however, from not even trusting the writer of the | |
348 | code not to try to do something evil. That's the kind of trust needed | |
349 | when someone hands you a program you've never seen before and says, "Here, | |
18d7fc85 RGS |
350 | run this." For that kind of safety, you might want to check out the Safe |
351 | module, included standard in the Perl distribution. This module allows the | |
425e5e39 | 352 | programmer to set up special compartments in which all system operations |
18d7fc85 RGS |
353 | are trapped and namespace access is carefully controlled. Safe should |
354 | not be considered bullet-proof, though: it will not prevent the foreign | |
355 | code to set up infinite loops, allocate gigabytes of memory, or even | |
356 | abusing perl bugs to make the host interpreter crash or behave in | |
91e64913 | 357 | unpredictable ways. In any case it's better avoided completely if you're |
18d7fc85 | 358 | really concerned about security. |
425e5e39 | 359 | |
360 | =head2 Security Bugs | |
361 | ||
362 | Beyond the obvious problems that stem from giving special privileges to | |
fb73857a | 363 | systems as flexible as scripts, on many versions of Unix, set-id scripts |
425e5e39 | 364 | are inherently insecure right from the start. The problem is a race |
365 | condition in the kernel. Between the time the kernel opens the file to | |
fb73857a | 366 | see which interpreter to run and when the (now-set-id) interpreter turns |
425e5e39 | 367 | around and reopens the file to interpret it, the file in question may have |
368 | changed, especially if you have symbolic links on your system. | |
369 | ||
370 | Fortunately, sometimes this kernel "feature" can be disabled. | |
371 | Unfortunately, there are two ways to disable it. The system can simply | |
fb73857a | 372 | outlaw scripts with any set-id bit set, which doesn't help much. |
cc69b689 | 373 | Alternately, it can simply ignore the set-id bits on scripts. |
425e5e39 | 374 | |
fb73857a | 375 | However, if the kernel set-id script feature isn't disabled, Perl will |
376 | complain loudly that your set-id script is insecure. You'll need to | |
377 | either disable the kernel set-id script feature, or put a C wrapper around | |
425e5e39 | 378 | the script. A C wrapper is just a compiled program that does nothing |
379 | except call your Perl program. Compiled programs are not subject to the | |
fb73857a | 380 | kernel bug that plagues set-id scripts. Here's a simple wrapper, written |
425e5e39 | 381 | in C: |
382 | ||
383 | #define REAL_PATH "/path/to/script" | |
54310121 | 384 | main(ac, av) |
425e5e39 | 385 | char **av; |
386 | { | |
387 | execv(REAL_PATH, av); | |
54310121 | 388 | } |
cb1a09d0 | 389 | |
54310121 | 390 | Compile this wrapper into a binary executable and then make I<it> rather |
391 | than your script setuid or setgid. | |
425e5e39 | 392 | |
425e5e39 | 393 | In recent years, vendors have begun to supply systems free of this |
394 | inherent security bug. On such systems, when the kernel passes the name | |
fb73857a | 395 | of the set-id script to open to the interpreter, rather than using a |
425e5e39 | 396 | pathname subject to meddling, it instead passes I</dev/fd/3>. This is a |
397 | special file already opened on the script, so that there can be no race | |
398 | condition for evil scripts to exploit. On these systems, Perl should be | |
23634c10 | 399 | compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F<Configure> |
425e5e39 | 400 | program that builds Perl tries to figure this out for itself, so you |
401 | should never have to specify this yourself. Most modern releases of | |
402 | SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. | |
403 | ||
68dc0745 | 404 | =head2 Protecting Your Programs |
405 | ||
406 | There are a number of ways to hide the source to your Perl programs, | |
407 | with varying levels of "security". | |
408 | ||
409 | First of all, however, you I<can't> take away read permission, because | |
410 | the source code has to be readable in order to be compiled and | |
411 | interpreted. (That doesn't mean that a CGI script's source is | |
412 | readable by people on the web, though.) So you have to leave the | |
5a964f20 TC |
413 | permissions at the socially friendly 0755 level. This lets |
414 | people on your local system only see your source. | |
68dc0745 | 415 | |
5a964f20 | 416 | Some people mistakenly regard this as a security problem. If your program does |
68dc0745 | 417 | insecure things, and relies on people not knowing how to exploit those |
418 | insecurities, it is not secure. It is often possible for someone to | |
419 | determine the insecure things and exploit them without viewing the | |
420 | source. Security through obscurity, the name for hiding your bugs | |
421 | instead of fixing them, is little security indeed. | |
422 | ||
83df6a1d JH |
423 | You can try using encryption via source filters (Filter::* from CPAN, |
424 | or Filter::Util::Call and Filter::Simple since Perl 5.8). | |
425 | But crackers might be able to decrypt it. You can try using the byte | |
426 | code compiler and interpreter described below, but crackers might be | |
427 | able to de-compile it. You can try using the native-code compiler | |
68dc0745 | 428 | described below, but crackers might be able to disassemble it. These |
429 | pose varying degrees of difficulty to people wanting to get at your | |
430 | code, but none can definitively conceal it (this is true of every | |
431 | language, not just Perl). | |
432 | ||
433 | If you're concerned about people profiting from your code, then the | |
3462340b | 434 | bottom line is that nothing but a restrictive license will give you |
68dc0745 | 435 | legal security. License your software and pepper it with threatening |
436 | statements like "This is unpublished proprietary software of XYZ Corp. | |
437 | Your access to it does not give you permission to use it blah blah | |
3462340b | 438 | blah." You should see a lawyer to be sure your license's wording will |
68dc0745 | 439 | stand up in court. |
5a964f20 | 440 | |
0d7c09bb JH |
441 | =head2 Unicode |
442 | ||
443 | Unicode is a new and complex technology and one may easily overlook | |
444 | certain security pitfalls. See L<perluniintro> for an overview and | |
445 | L<perlunicode> for details, and L<perlunicode/"Security Implications | |
446 | of Unicode"> for security implications in particular. | |
447 | ||
504f80c1 JH |
448 | =head2 Algorithmic Complexity Attacks |
449 | ||
450 | Certain internal algorithms used in the implementation of Perl can | |
451 | be attacked by choosing the input carefully to consume large amounts | |
452 | of either time or space or both. This can lead into the so-called | |
453 | I<Denial of Service> (DoS) attacks. | |
454 | ||
455 | =over 4 | |
456 | ||
457 | =item * | |
458 | ||
6a5b4183 YO |
459 | Hash Algorithm - Hash algorithms like the one used in Perl are well |
460 | known to be vulnerable to collision attacks on their hash function. | |
461 | Such attacks involve constructing a set of keys which collide into | |
91e64913 | 462 | the same bucket producing inefficient behavior. Such attacks often |
6a5b4183 | 463 | depend on discovering the seed of the hash function used to map the |
91e64913 FC |
464 | keys to buckets. That seed is then used to brute-force a key set which |
465 | can be used to mount a denial of service attack. In Perl 5.8.1 changes | |
6a5b4183 YO |
466 | were introduced to harden Perl to such attacks, and then later in |
467 | Perl 5.18.0 these features were enhanced and additional protections | |
468 | added. | |
469 | ||
4d74c8eb S |
470 | At the time of this writing, Perl 5.18.0 is considered to be |
471 | well-hardened against algorithmic complexity attacks on its hash | |
91e64913 | 472 | implementation. This is largely owed to the following measures |
4d74c8eb | 473 | mitigate attacks: |
6a5b4183 YO |
474 | |
475 | =over 4 | |
476 | ||
477 | =item Hash Seed Randomization | |
478 | ||
479 | In order to make it impossible to know what seed to generate an attack | |
91e64913 | 480 | key set for, this seed is randomly initialized at process start. This |
4d74c8eb | 481 | may be overridden by using the PERL_HASH_SEED environment variable, see |
91e64913 | 482 | L<perlrun/PERL_HASH_SEED>. This environment variable controls how |
4d74c8eb S |
483 | items are actually stored, not how they are presented via |
484 | C<keys>, C<values> and C<each>. | |
6a5b4183 YO |
485 | |
486 | =item Hash Traversal Randomization | |
487 | ||
4d74c8eb | 488 | Independent of which seed is used in the hash function, C<keys>, |
6a5b4183 YO |
489 | C<values>, and C<each> return items in a per-hash randomized order. |
490 | Modifying a hash by insertion will change the iteration order of that hash. | |
4d74c8eb | 491 | This behavior can be overridden by using C<hash_traversal_mask()> from |
6a5b4183 | 492 | L<Hash::Util> or by using the PERL_PERTURB_KEYS environment variable, |
91e64913 | 493 | see L<perlrun/PERL_PERTURB_KEYS>. Note that this feature controls the |
6a5b4183 YO |
494 | "visible" order of the keys, and not the actual order they are stored in. |
495 | ||
496 | =item Bucket Order Perturbance | |
497 | ||
4d74c8eb | 498 | When items collide into a given hash bucket the order they are stored in |
91e64913 FC |
499 | the chain is no longer predictable in Perl 5.18. This |
500 | has the intention to make it harder to observe a | |
c6c886ef | 501 | collision. This behavior can be overridden by using |
6a5b4183 YO |
502 | the PERL_PERTURB_KEYS environment variable, see L<perlrun/PERL_PERTURB_KEYS>. |
503 | ||
504 | =item New Default Hash Function | |
505 | ||
506 | The default hash function has been modified with the intention of making | |
507 | it harder to infer the hash seed. | |
508 | ||
509 | =item Alternative Hash Functions | |
510 | ||
511 | The source code includes multiple hash algorithms to choose from. While we | |
4d74c8eb | 512 | believe that the default perl hash is robust to attack, we have included the |
91e64913 | 513 | hash function Siphash as a fall-back option. At the time of release of |
6a5b4183 YO |
514 | Perl 5.18.0 Siphash is believed to be of cryptographic strength. This is |
515 | not the default as it is much slower than the default hash. | |
516 | ||
517 | =back | |
518 | ||
4d74c8eb | 519 | Without compiling a special Perl, there is no way to get the exact same |
91e64913 | 520 | behavior of any versions prior to Perl 5.18.0. The closest one can get |
6a5b4183 | 521 | is by setting PERL_PERTURB_KEYS to 0 and setting the PERL_HASH_SEED |
91e64913 | 522 | to a known value. We do not advise those settings for production use |
4d74c8eb | 523 | due to the above security considerations. |
6a5b4183 YO |
524 | |
525 | B<Perl has never guaranteed any ordering of the hash keys>, and | |
526 | the ordering has already changed several times during the lifetime of | |
527 | Perl 5. Also, the ordering of hash keys has always been, and continues | |
528 | to be, affected by the insertion order and the history of changes made | |
529 | to the hash over its lifetime. | |
7b3f7037 JH |
530 | |
531 | Also note that while the order of the hash elements might be | |
4d74c8eb S |
532 | randomized, this "pseudo-ordering" should B<not> be used for |
533 | applications like shuffling a list randomly (use C<List::Util::shuffle()> | |
7b3f7037 | 534 | for that, see L<List::Util>, a standard core module since Perl 5.8.0; |
4d74c8eb S |
535 | or the CPAN module C<Algorithm::Numerical::Shuffle>), or for generating |
536 | permutations (use e.g. the CPAN modules C<Algorithm::Permute> or | |
537 | C<Algorithm::FastPermute>), or for any cryptographic applications. | |
7b3f7037 | 538 | |
883f220b TC |
539 | Tied hashes may have their own ordering and algorithmic complexity |
540 | attacks. | |
541 | ||
504f80c1 JH |
542 | =item * |
543 | ||
5a4e8ea7 P |
544 | Regular expressions - Perl's regular expression engine is so called NFA |
545 | (Non-deterministic Finite Automaton), which among other things means that | |
546 | it can rather easily consume large amounts of both time and space if the | |
504f80c1 JH |
547 | regular expression may match in several ways. Careful crafting of the |
548 | regular expressions can help but quite often there really isn't much | |
549 | one can do (the book "Mastering Regular Expressions" is required | |
550 | reading, see L<perlfaq2>). Running out of space manifests itself by | |
551 | Perl running out of memory. | |
552 | ||
553 | =item * | |
554 | ||
555 | Sorting - the quicksort algorithm used in Perls before 5.8.0 to | |
556 | implement the sort() function is very easy to trick into misbehaving | |
3462340b JL |
557 | so that it consumes a lot of time. Starting from Perl 5.8.0 a different |
558 | sorting algorithm, mergesort, is used by default. Mergesort cannot | |
559 | misbehave on any input. | |
504f80c1 JH |
560 | |
561 | =back | |
562 | ||
563 | See L<http://www.cs.rice.edu/~scrosby/hash/> for more information, | |
3462340b | 564 | and any computer science textbook on algorithmic complexity. |
504f80c1 | 565 | |
5a964f20 TC |
566 | =head1 SEE ALSO |
567 | ||
568 | L<perlrun> for its description of cleaning up environment variables. |