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Small addition to perlsec by Stas Bekman.
[perl5.git] / pod / perlsec.pod
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1=head1 NAME
2
3perlsec - Perl security
4
5=head1 DESCRIPTION
6
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7Perl is designed to make it easy to program securely even when running
8with extra privileges, like setuid or setgid programs. Unlike most
54310121 9command line shells, which are based on multiple substitution passes on
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10each line of the script, Perl uses a more conventional evaluation scheme
11with fewer hidden snags. Additionally, because the language has more
54310121 12builtin functionality, it can rely less upon external (and possibly
425e5e39 13untrustworthy) programs to accomplish its purposes.
a0d0e21e 14
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15Perl automatically enables a set of special security checks, called I<taint
16mode>, when it detects its program running with differing real and effective
17user or group IDs. The setuid bit in Unix permissions is mode 04000, the
18setgid bit mode 02000; either or both may be set. You can also enable taint
5f05dabc 19mode explicitly by using the B<-T> command line flag. This flag is
425e5e39 20I<strongly> suggested for server programs and any program run on behalf of
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21someone else, such as a CGI script. Once taint mode is on, it's on for
22the remainder of your script.
a0d0e21e 23
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24While in this mode, Perl takes special precautions called I<taint
25checks> to prevent both obvious and subtle traps. Some of these checks
26are reasonably simple, such as verifying that path directories aren't
27writable by others; careful programmers have always used checks like
28these. Other checks, however, are best supported by the language itself,
fb73857a 29and it is these checks especially that contribute to making a set-id Perl
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30program more secure than the corresponding C program.
31
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32You may not use data derived from outside your program to affect
33something else outside your program--at least, not by accident. All
34command line arguments, environment variables, locale information (see
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35L<perllocale>), results of certain system calls (C<readdir()>,
36C<readlink()>, the variable of C<shmread()>, the messages returned by
37C<msgrcv()>, the password, gcos and shell fields returned by the
38C<getpwxxx()> calls), and all file input are marked as "tainted".
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39Tainted data may not be used directly or indirectly in any command
40that invokes a sub-shell, nor in any command that modifies files,
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41directories, or processes, B<with the following exceptions>:
42
43=over 4
44
45=item *
46
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47Arguments to C<print> and C<syswrite> are B<not> checked for taintedness.
48
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49=item *
50
51Symbolic methods
52
53 $obj->$method(@args);
54
55and symbolic sub references
56
57 &{$foo}(@args);
58 $foo->(@args);
59
60are not checked for taintedness. This requires extra carefulness
61unless you want external data to affect your control flow. Unless
62you carefully limit what these symbolic values are, people are able
63to call functions B<outside> your Perl code, such as POSIX::system,
64in which case they are able to run arbitrary external code.
65
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66=back
67
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68For efficiency reasons, Perl takes a conservative view of
69whether data is tainted. If an expression contains tainted data,
70any subexpression may be considered tainted, even if the value
71of the subexpression is not itself affected by the tainted data.
ee556d55 72
d929ce6f 73Because taintedness is associated with each scalar value, some
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74elements of an array or hash can be tainted and others not.
75The keys of a hash are never tainted.
a0d0e21e 76
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77For example:
78
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79 $arg = shift; # $arg is tainted
80 $hid = $arg, 'bar'; # $hid is also tainted
81 $line = <>; # Tainted
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82 $line = <STDIN>; # Also tainted
83 open FOO, "/home/me/bar" or die $!;
84 $line = <FOO>; # Still tainted
a0d0e21e 85 $path = $ENV{'PATH'}; # Tainted, but see below
425e5e39 86 $data = 'abc'; # Not tainted
a0d0e21e 87
425e5e39 88 system "echo $arg"; # Insecure
7de90c4d 89 system "/bin/echo", $arg; # Considered insecure
bbd7eb8a 90 # (Perl doesn't know about /bin/echo)
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91 system "echo $hid"; # Insecure
92 system "echo $data"; # Insecure until PATH set
a0d0e21e 93
425e5e39 94 $path = $ENV{'PATH'}; # $path now tainted
a0d0e21e 95
54310121 96 $ENV{'PATH'} = '/bin:/usr/bin';
c90c0ff4 97 delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
a0d0e21e 98
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99 $path = $ENV{'PATH'}; # $path now NOT tainted
100 system "echo $data"; # Is secure now!
a0d0e21e 101
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102 open(FOO, "< $arg"); # OK - read-only file
103 open(FOO, "> $arg"); # Not OK - trying to write
a0d0e21e 104
bbd7eb8a 105 open(FOO,"echo $arg|"); # Not OK
425e5e39 106 open(FOO,"-|")
7de90c4d 107 or exec 'echo', $arg; # Also not OK
a0d0e21e 108
425e5e39 109 $shout = `echo $arg`; # Insecure, $shout now tainted
a0d0e21e 110
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111 unlink $data, $arg; # Insecure
112 umask $arg; # Insecure
a0d0e21e 113
bbd7eb8a 114 exec "echo $arg"; # Insecure
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115 exec "echo", $arg; # Insecure
116 exec "sh", '-c', $arg; # Very insecure!
a0d0e21e 117
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118 @files = <*.c>; # insecure (uses readdir() or similar)
119 @files = glob('*.c'); # insecure (uses readdir() or similar)
7bac28a0 120
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121 # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would
122 # have used an external program to do the filename expansion; but in
123 # either case the result is tainted since the list of filenames comes
124 # from outside of the program.
125
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126 $bad = ($arg, 23); # $bad will be tainted
127 $arg, `true`; # Insecure (although it isn't really)
128
a0d0e21e 129If you try to do something insecure, you will get a fatal error saying
7de90c4d 130something like "Insecure dependency" or "Insecure $ENV{PATH}".
425e5e39 131
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132The exception to the principle of "one tainted value taints the whole
133expression" is with the ternary conditional operator C<?:>. Since code
134with a ternary conditional
135
136 $result = $tainted_value ? "Untainted" : "Also untainted";
137
138is effectively
139
140 if ( $tainted_value ) {
141 $result = "Untainted";
142 } else {
143 $result = "Also untainted";
144 }
145
146it doesn't make sense for C<$result> to be tainted.
147
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148=head2 Laundering and Detecting Tainted Data
149
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150To test whether a variable contains tainted data, and whose use would
151thus trigger an "Insecure dependency" message, you can use the
23634c10 152C<tainted()> function of the Scalar::Util module, available in your
3f7d42d8 153nearby CPAN mirror, and included in Perl starting from the release 5.8.0.
595bde10 154Or you may be able to use the following C<is_tainted()> function.
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155
156 sub is_tainted {
61890e45 157 return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
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158 }
159
160This function makes use of the fact that the presence of tainted data
161anywhere within an expression renders the entire expression tainted. It
162would be inefficient for every operator to test every argument for
163taintedness. Instead, the slightly more efficient and conservative
164approach is used that if any tainted value has been accessed within the
165same expression, the whole expression is considered tainted.
166
5f05dabc 167But testing for taintedness gets you only so far. Sometimes you have just
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168to clear your data's taintedness. Values may be untainted by using them
169as keys in a hash; otherwise the only way to bypass the tainting
54310121 170mechanism is by referencing subpatterns from a regular expression match.
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171Perl presumes that if you reference a substring using $1, $2, etc., that
172you knew what you were doing when you wrote the pattern. That means using
173a bit of thought--don't just blindly untaint anything, or you defeat the
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174entire mechanism. It's better to verify that the variable has only good
175characters (for certain values of "good") rather than checking whether it
176has any bad characters. That's because it's far too easy to miss bad
177characters that you never thought of.
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178
179Here's a test to make sure that the data contains nothing but "word"
180characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
181or a dot.
182
54310121 183 if ($data =~ /^([-\@\w.]+)$/) {
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184 $data = $1; # $data now untainted
185 } else {
3a2263fe 186 die "Bad data in '$data'"; # log this somewhere
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187 }
188
5f05dabc 189This is fairly secure because C</\w+/> doesn't normally match shell
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190metacharacters, nor are dot, dash, or at going to mean something special
191to the shell. Use of C</.+/> would have been insecure in theory because
192it lets everything through, but Perl doesn't check for that. The lesson
193is that when untainting, you must be exceedingly careful with your patterns.
19799a22 194Laundering data using regular expression is the I<only> mechanism for
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195untainting dirty data, unless you use the strategy detailed below to fork
196a child of lesser privilege.
197
23634c10 198The example does not untaint C<$data> if C<use locale> is in effect,
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199because the characters matched by C<\w> are determined by the locale.
200Perl considers that locale definitions are untrustworthy because they
201contain data from outside the program. If you are writing a
202locale-aware program, and want to launder data with a regular expression
203containing C<\w>, put C<no locale> ahead of the expression in the same
204block. See L<perllocale/SECURITY> for further discussion and examples.
205
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206=head2 Switches On the "#!" Line
207
208When you make a script executable, in order to make it usable as a
209command, the system will pass switches to perl from the script's #!
54310121 210line. Perl checks that any command line switches given to a setuid
3a52c276 211(or setgid) script actually match the ones set on the #! line. Some
54310121 212Unix and Unix-like environments impose a one-switch limit on the #!
3a52c276 213line, so you may need to use something like C<-wU> instead of C<-w -U>
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214under such systems. (This issue should arise only in Unix or
215Unix-like environments that support #! and setuid or setgid scripts.)
3a52c276 216
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217=head2 Taint mode and @INC
218
219When the taint mode (C<-T>) is in effect, the "." directory is removed
220from C<@INC>, and the environment variables C<PERL5LIB> and C<PERLLIB>
221are ignored by Perl. You can still adjust C<@INC> from outside the
222program by using the C<-I> command line option as explained in
223L<perlrun>. The two environment variables are ignored because
224they are obscured, and a user running a program could be unaware that
225they are set, whereas the C<-I> option is clearly visible and
226therefore permitted.
227
228Another way to modify C<@INC> without modifying the program, is to use
229the C<lib> pragma, e.g.:
230
231 perl -Mlib=/foo program
232
233The benefit of using C<-Mlib=/foo> over C<-I/foo>, is that the former
234will automagically remove any duplicated directories, while the later
235will not.
236
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237Note that if a tainted string is added to C<@INC>, the following
238problem will be reported:
239
240 Insecure dependency in require while running with -T switch
241
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242=head2 Cleaning Up Your Path
243
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244For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to
245a known value, and each directory in the path must be absolute and
246non-writable by others than its owner and group. You may be surprised to
247get this message even if the pathname to your executable is fully
248qualified. This is I<not> generated because you didn't supply a full path
249to the program; instead, it's generated because you never set your PATH
250environment variable, or you didn't set it to something that was safe.
251Because Perl can't guarantee that the executable in question isn't itself
252going to turn around and execute some other program that is dependent on
253your PATH, it makes sure you set the PATH.
a0d0e21e 254
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255The PATH isn't the only environment variable which can cause problems.
256Because some shells may use the variables IFS, CDPATH, ENV, and
257BASH_ENV, Perl checks that those are either empty or untainted when
258starting subprocesses. You may wish to add something like this to your
259setid and taint-checking scripts.
260
261 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
262
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263It's also possible to get into trouble with other operations that don't
264care whether they use tainted values. Make judicious use of the file
265tests in dealing with any user-supplied filenames. When possible, do
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266opens and such B<after> properly dropping any special user (or group!)
267privileges. Perl doesn't prevent you from opening tainted filenames for reading,
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268so be careful what you print out. The tainting mechanism is intended to
269prevent stupid mistakes, not to remove the need for thought.
270
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271Perl does not call the shell to expand wild cards when you pass C<system>
272and C<exec> explicit parameter lists instead of strings with possible shell
273wildcards in them. Unfortunately, the C<open>, C<glob>, and
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274backtick functions provide no such alternate calling convention, so more
275subterfuge will be required.
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276
277Perl provides a reasonably safe way to open a file or pipe from a setuid
278or setgid program: just create a child process with reduced privilege who
279does the dirty work for you. First, fork a child using the special
23634c10 280C<open> syntax that connects the parent and child by a pipe. Now the
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281child resets its ID set and any other per-process attributes, like
282environment variables, umasks, current working directories, back to the
283originals or known safe values. Then the child process, which no longer
23634c10 284has any special permissions, does the C<open> or other system call.
425e5e39 285Finally, the child passes the data it managed to access back to the
5f05dabc 286parent. Because the file or pipe was opened in the child while running
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287under less privilege than the parent, it's not apt to be tricked into
288doing something it shouldn't.
289
23634c10 290Here's a way to do backticks reasonably safely. Notice how the C<exec> is
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291not called with a string that the shell could expand. This is by far the
292best way to call something that might be subjected to shell escapes: just
fb73857a 293never call the shell at all.
cb1a09d0 294
a1ce9542 295 use English '-no_match_vars';
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296 die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
297 if ($pid) { # parent
298 while (<KID>) {
299 # do something
300 }
301 close KID;
302 } else {
303 my @temp = ($EUID, $EGID);
304 my $orig_uid = $UID;
305 my $orig_gid = $GID;
306 $EUID = $UID;
307 $EGID = $GID;
308 # Drop privileges
309 $UID = $orig_uid;
310 $GID = $orig_gid;
311 # Make sure privs are really gone
312 ($EUID, $EGID) = @temp;
313 die "Can't drop privileges"
314 unless $UID == $EUID && $GID eq $EGID;
315 $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH.
316 # Consider sanitizing the environment even more.
317 exec 'myprog', 'arg1', 'arg2'
318 or die "can't exec myprog: $!";
319 }
425e5e39 320
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321A similar strategy would work for wildcard expansion via C<glob>, although
322you can use C<readdir> instead.
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323
324Taint checking is most useful when although you trust yourself not to have
325written a program to give away the farm, you don't necessarily trust those
326who end up using it not to try to trick it into doing something bad. This
fb73857a 327is the kind of security checking that's useful for set-id programs and
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328programs launched on someone else's behalf, like CGI programs.
329
330This is quite different, however, from not even trusting the writer of the
331code not to try to do something evil. That's the kind of trust needed
332when someone hands you a program you've never seen before and says, "Here,
333run this." For that kind of safety, check out the Safe module,
334included standard in the Perl distribution. This module allows the
335programmer to set up special compartments in which all system operations
336are trapped and namespace access is carefully controlled.
337
338=head2 Security Bugs
339
340Beyond the obvious problems that stem from giving special privileges to
fb73857a 341systems as flexible as scripts, on many versions of Unix, set-id scripts
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342are inherently insecure right from the start. The problem is a race
343condition in the kernel. Between the time the kernel opens the file to
fb73857a 344see which interpreter to run and when the (now-set-id) interpreter turns
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345around and reopens the file to interpret it, the file in question may have
346changed, especially if you have symbolic links on your system.
347
348Fortunately, sometimes this kernel "feature" can be disabled.
349Unfortunately, there are two ways to disable it. The system can simply
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350outlaw scripts with any set-id bit set, which doesn't help much.
351Alternately, it can simply ignore the set-id bits on scripts. If the
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352latter is true, Perl can emulate the setuid and setgid mechanism when it
353notices the otherwise useless setuid/gid bits on Perl scripts. It does
23634c10 354this via a special executable called F<suidperl> that is automatically
54310121 355invoked for you if it's needed.
425e5e39 356
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357However, if the kernel set-id script feature isn't disabled, Perl will
358complain loudly that your set-id script is insecure. You'll need to
359either disable the kernel set-id script feature, or put a C wrapper around
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360the script. A C wrapper is just a compiled program that does nothing
361except call your Perl program. Compiled programs are not subject to the
fb73857a 362kernel bug that plagues set-id scripts. Here's a simple wrapper, written
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363in C:
364
365 #define REAL_PATH "/path/to/script"
54310121 366 main(ac, av)
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367 char **av;
368 {
369 execv(REAL_PATH, av);
54310121 370 }
cb1a09d0 371
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372Compile this wrapper into a binary executable and then make I<it> rather
373than your script setuid or setgid.
425e5e39 374
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375In recent years, vendors have begun to supply systems free of this
376inherent security bug. On such systems, when the kernel passes the name
fb73857a 377of the set-id script to open to the interpreter, rather than using a
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378pathname subject to meddling, it instead passes I</dev/fd/3>. This is a
379special file already opened on the script, so that there can be no race
380condition for evil scripts to exploit. On these systems, Perl should be
23634c10 381compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F<Configure>
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382program that builds Perl tries to figure this out for itself, so you
383should never have to specify this yourself. Most modern releases of
384SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.
385
23634c10 386Prior to release 5.6.1 of Perl, bugs in the code of F<suidperl> could
0325b4c4 387introduce a security hole.
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388
389=head2 Protecting Your Programs
390
391There are a number of ways to hide the source to your Perl programs,
392with varying levels of "security".
393
394First of all, however, you I<can't> take away read permission, because
395the source code has to be readable in order to be compiled and
396interpreted. (That doesn't mean that a CGI script's source is
397readable by people on the web, though.) So you have to leave the
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398permissions at the socially friendly 0755 level. This lets
399people on your local system only see your source.
68dc0745 400
5a964f20 401Some people mistakenly regard this as a security problem. If your program does
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402insecure things, and relies on people not knowing how to exploit those
403insecurities, it is not secure. It is often possible for someone to
404determine the insecure things and exploit them without viewing the
405source. Security through obscurity, the name for hiding your bugs
406instead of fixing them, is little security indeed.
407
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408You can try using encryption via source filters (Filter::* from CPAN,
409or Filter::Util::Call and Filter::Simple since Perl 5.8).
410But crackers might be able to decrypt it. You can try using the byte
411code compiler and interpreter described below, but crackers might be
412able to de-compile it. You can try using the native-code compiler
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413described below, but crackers might be able to disassemble it. These
414pose varying degrees of difficulty to people wanting to get at your
415code, but none can definitively conceal it (this is true of every
416language, not just Perl).
417
418If you're concerned about people profiting from your code, then the
419bottom line is that nothing but a restrictive licence will give you
420legal security. License your software and pepper it with threatening
421statements like "This is unpublished proprietary software of XYZ Corp.
422Your access to it does not give you permission to use it blah blah
423blah." You should see a lawyer to be sure your licence's wording will
424stand up in court.
5a964f20 425
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426=head2 Unicode
427
428Unicode is a new and complex technology and one may easily overlook
429certain security pitfalls. See L<perluniintro> for an overview and
430L<perlunicode> for details, and L<perlunicode/"Security Implications
431of Unicode"> for security implications in particular.
432
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433=head2 Algorithmic Complexity Attacks
434
435Certain internal algorithms used in the implementation of Perl can
436be attacked by choosing the input carefully to consume large amounts
437of either time or space or both. This can lead into the so-called
438I<Denial of Service> (DoS) attacks.
439
440=over 4
441
442=item *
443
444Hash Function - the algorithm used to "order" hash elements has been
445changed several times during the development of Perl, mainly to be
446reasonably fast. In Perl 5.8.1 also the security aspect was taken
447into account.
448
449In Perls before 5.8.1 one could rather easily generate data that as
450hash keys would cause Perl to consume large amounts of time because
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451internal structure of hashes would badly degenerate. In Perl 5.8.1
452the hash function is randomly perturbed by a pseudorandom seed which
453makes generating such naughty hash keys harder.
454See L<perlrun/PERL_HASH_SEED> for more information.
455
456The random perturbation is done by default but if one wants for some
457reason emulate the old behaviour one can set the environment variable
458PERL_HASH_SEED to zero (or any other integer). One possible reason
459for wanting to emulate the old behaviour is that in the new behaviour
460consecutive runs of Perl will order hash keys differently, which may
461confuse some applications (like Data::Dumper: the outputs of two
462different runs are no more identical).
504f80c1 463
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464B<Perl has never guaranteed any ordering of the hash keys>, and the
465ordering has already changed several times during the lifetime of
466Perl 5. Also, the ordering of hash keys has always been, and
467continues to be, affected by the insertion order.
468
469Also note that while the order of the hash elements might be
470randomised, this "pseudoordering" should B<not> be used for
471applications like shuffling a list randomly (use List::Util::shuffle()
472for that, see L<List::Util>, a standard core module since Perl 5.8.0;
473or the CPAN module Algorithm::Numerical::Shuffle), or for generating
474permutations (use e.g. the CPAN modules Algorithm::Permute or
475Algorithm::FastPermute), or for any cryptographic applications.
476
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477=item *
478
479Regular expressions - Perl's regular expression engine is so called
480NFA (Non-Finite Automaton), which among other things means that it can
481rather easily consume large amounts of both time and space if the
482regular expression may match in several ways. Careful crafting of the
483regular expressions can help but quite often there really isn't much
484one can do (the book "Mastering Regular Expressions" is required
485reading, see L<perlfaq2>). Running out of space manifests itself by
486Perl running out of memory.
487
488=item *
489
490Sorting - the quicksort algorithm used in Perls before 5.8.0 to
491implement the sort() function is very easy to trick into misbehaving
492so that it consumes a lot of time. Nothing more is required than
493resorting a list already sorted. Starting from Perl 5.8.0 a different
494sorting algorithm, mergesort, is used. Mergesort is insensitive to
495its input data, so it cannot be similarly fooled.
496
497=back
498
499See L<http://www.cs.rice.edu/~scrosby/hash/> for more information,
500and any computer science text book on the algorithmic complexity.
501
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502=head1 SEE ALSO
503
504L<perlrun> for its description of cleaning up environment variables.