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1 | =head1 NAME |
2 | |
3 | perlsec - Perl security |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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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 |
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9 | command line shells, which are based on multiple substitution passes on |
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10 | each line of the script, Perl uses a more conventional evaluation scheme |
11 | with fewer hidden snags. Additionally, because the language has more |
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12 | builtin functionality, it can rely less upon external (and possibly |
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13 | untrustworthy) programs to accomplish its purposes. |
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14 | |
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15 | Perl automatically enables a set of special security checks, called I<taint |
16 | mode>, when it detects its program running with differing real and effective |
17 | user or group IDs. The setuid bit in Unix permissions is mode 04000, the |
18 | setgid bit mode 02000; either or both may be set. You can also enable taint |
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19 | mode explicitly by using the B<-T> command line flag. This flag is |
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20 | I<strongly> suggested for server programs and any program run on behalf of |
21 | someone else, such as a CGI script. |
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22 | |
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23 | While in this mode, Perl takes special precautions called I<taint |
24 | checks> to prevent both obvious and subtle traps. Some of these checks |
25 | are reasonably simple, such as verifying that path directories aren't |
26 | writable by others; careful programmers have always used checks like |
27 | these. Other checks, however, are best supported by the language itself, |
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28 | and it is these checks especially that contribute to making a setuid Perl |
29 | program more secure than the corresponding C program. |
30 | |
31 | You may not use data derived from outside your program to affect something |
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32 | else outside your program--at least, not by accident. All command line |
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33 | arguments, environment variables, locale information (see L<perllocale>), |
34 | and file input are marked as "tainted". Tainted data may not be used |
35 | directly or indirectly in any command that invokes a sub-shell, nor in any |
36 | command that modifies files, directories, or processes. Any variable set |
37 | within an expression that has previously referenced a tainted value itself |
38 | becomes tainted, even if it is logically impossible for the tainted value |
39 | to influence the variable. Because taintedness is associated with each |
40 | scalar value, some elements of an array can be tainted and others not. |
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41 | |
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42 | For example: |
43 | |
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44 | $arg = shift; # $arg is tainted |
45 | $hid = $arg, 'bar'; # $hid is also tainted |
46 | $line = <>; # Tainted |
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47 | $line = <STDIN>; # Also tainted |
48 | open FOO, "/home/me/bar" or die $!; |
49 | $line = <FOO>; # Still tainted |
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50 | $path = $ENV{'PATH'}; # Tainted, but see below |
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51 | $data = 'abc'; # Not tainted |
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52 | |
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53 | system "echo $arg"; # Insecure |
54 | system "/bin/echo", $arg; # Secure (doesn't use sh) |
55 | system "echo $hid"; # Insecure |
56 | system "echo $data"; # Insecure until PATH set |
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57 | |
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58 | $path = $ENV{'PATH'}; # $path now tainted |
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59 | |
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60 | $ENV{'PATH'} = '/bin:/usr/bin'; |
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61 | delete $ENV{'IFS'}; |
62 | delete $ENV{'CDPATH'}; |
63 | delete $ENV{'ENV'}; |
64 | $ENV{'TERM'} = 'dumb'; |
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65 | |
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66 | $path = $ENV{'PATH'}; # $path now NOT tainted |
67 | system "echo $data"; # Is secure now! |
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68 | |
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69 | open(FOO, "< $arg"); # OK - read-only file |
70 | open(FOO, "> $arg"); # Not OK - trying to write |
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71 | |
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72 | open(FOO,"echo $arg|"); # Not OK, but... |
73 | open(FOO,"-|") |
74 | or exec 'echo', $arg; # OK |
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75 | |
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76 | $shout = `echo $arg`; # Insecure, $shout now tainted |
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77 | |
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78 | unlink $data, $arg; # Insecure |
79 | umask $arg; # Insecure |
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80 | |
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81 | exec "echo $arg"; # Insecure |
82 | exec "echo", $arg; # Secure (doesn't use the shell) |
83 | exec "sh", '-c', $arg; # Considered secure, alas! |
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84 | |
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85 | @files = <*.c>; # Always insecure (uses csh) |
86 | @files = glob('*.c'); # Always insecure (uses csh) |
87 | |
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88 | If you try to do something insecure, you will get a fatal error saying |
89 | something like "Insecure dependency" or "Insecure PATH". Note that you |
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90 | can still write an insecure B<system> or B<exec>, but only by explicitly |
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91 | doing something like the last example above. |
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92 | |
93 | =head2 Laundering and Detecting Tainted Data |
94 | |
95 | To test whether a variable contains tainted data, and whose use would thus |
96 | trigger an "Insecure dependency" message, you can use the following |
97 | I<is_tainted()> function. |
98 | |
99 | sub is_tainted { |
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100 | return ! eval { |
101 | join('',@_), kill 0; |
102 | 1; |
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103 | }; |
104 | } |
105 | |
106 | This function makes use of the fact that the presence of tainted data |
107 | anywhere within an expression renders the entire expression tainted. It |
108 | would be inefficient for every operator to test every argument for |
109 | taintedness. Instead, the slightly more efficient and conservative |
110 | approach is used that if any tainted value has been accessed within the |
111 | same expression, the whole expression is considered tainted. |
112 | |
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113 | But testing for taintedness gets you only so far. Sometimes you have just |
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114 | to clear your data's taintedness. The only way to bypass the tainting |
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115 | mechanism is by referencing subpatterns from a regular expression match. |
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116 | Perl presumes that if you reference a substring using $1, $2, etc., that |
117 | you knew what you were doing when you wrote the pattern. That means using |
118 | a bit of thought--don't just blindly untaint anything, or you defeat the |
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119 | entire mechanism. It's better to verify that the variable has only good |
120 | characters (for certain values of "good") rather than checking whether it |
121 | has any bad characters. That's because it's far too easy to miss bad |
122 | characters that you never thought of. |
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123 | |
124 | Here's a test to make sure that the data contains nothing but "word" |
125 | characters (alphabetics, numerics, and underscores), a hyphen, an at sign, |
126 | or a dot. |
127 | |
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128 | if ($data =~ /^([-\@\w.]+)$/) { |
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129 | $data = $1; # $data now untainted |
130 | } else { |
131 | die "Bad data in $data"; # log this somewhere |
132 | } |
133 | |
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134 | This is fairly secure because C</\w+/> doesn't normally match shell |
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135 | metacharacters, nor are dot, dash, or at going to mean something special |
136 | to the shell. Use of C</.+/> would have been insecure in theory because |
137 | it lets everything through, but Perl doesn't check for that. The lesson |
138 | is that when untainting, you must be exceedingly careful with your patterns. |
139 | Laundering data using regular expression is the I<ONLY> mechanism for |
140 | untainting dirty data, unless you use the strategy detailed below to fork |
141 | a child of lesser privilege. |
142 | |
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143 | The example does not untaint $data if C<use locale> is in effect, |
144 | because the characters matched by C<\w> are determined by the locale. |
145 | Perl considers that locale definitions are untrustworthy because they |
146 | contain data from outside the program. If you are writing a |
147 | locale-aware program, and want to launder data with a regular expression |
148 | containing C<\w>, put C<no locale> ahead of the expression in the same |
149 | block. See L<perllocale/SECURITY> for further discussion and examples. |
150 | |
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151 | =head2 Switches On the "#!" Line |
152 | |
153 | When you make a script executable, in order to make it usable as a |
154 | command, the system will pass switches to perl from the script's #! |
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155 | line. Perl checks that any command line switches given to a setuid |
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156 | (or setgid) script actually match the ones set on the #! line. Some |
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157 | Unix and Unix-like environments impose a one-switch limit on the #! |
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158 | line, so you may need to use something like C<-wU> instead of C<-w -U> |
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159 | under such systems. (This issue should arise only in Unix or |
160 | Unix-like environments that support #! and setuid or setgid scripts.) |
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161 | |
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162 | =head2 Cleaning Up Your Path |
163 | |
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164 | For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a |
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165 | known value, and each directory in the path must be non-writable by others |
166 | than its owner and group. You may be surprised to get this message even |
167 | if the pathname to your executable is fully qualified. This is I<not> |
168 | generated because you didn't supply a full path to the program; instead, |
169 | it's generated because you never set your PATH environment variable, or |
170 | you didn't set it to something that was safe. Because Perl can't |
171 | guarantee that the executable in question isn't itself going to turn |
172 | around and execute some other program that is dependent on your PATH, it |
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173 | makes sure you set the PATH. |
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174 | |
175 | It's also possible to get into trouble with other operations that don't |
176 | care whether they use tainted values. Make judicious use of the file |
177 | tests in dealing with any user-supplied filenames. When possible, do |
178 | opens and such after setting C<$E<gt> = $E<lt>>. (Remember group IDs, |
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179 | too!) Perl doesn't prevent you from opening tainted filenames for reading, |
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180 | so be careful what you print out. The tainting mechanism is intended to |
181 | prevent stupid mistakes, not to remove the need for thought. |
182 | |
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183 | Perl does not call the shell to expand wild cards when you pass B<system> |
184 | and B<exec> explicit parameter lists instead of strings with possible shell |
185 | wildcards in them. Unfortunately, the B<open>, B<glob>, and |
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186 | backtick functions provide no such alternate calling convention, so more |
187 | subterfuge will be required. |
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188 | |
189 | Perl provides a reasonably safe way to open a file or pipe from a setuid |
190 | or setgid program: just create a child process with reduced privilege who |
191 | does the dirty work for you. First, fork a child using the special |
192 | B<open> syntax that connects the parent and child by a pipe. Now the |
193 | child resets its ID set and any other per-process attributes, like |
194 | environment variables, umasks, current working directories, back to the |
195 | originals or known safe values. Then the child process, which no longer |
196 | has any special permissions, does the B<open> or other system call. |
197 | Finally, the child passes the data it managed to access back to the |
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198 | parent. Because the file or pipe was opened in the child while running |
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199 | under less privilege than the parent, it's not apt to be tricked into |
200 | doing something it shouldn't. |
201 | |
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202 | Here's a way to do backticks reasonably safely. Notice how the B<exec> is |
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203 | not called with a string that the shell could expand. This is by far the |
204 | best way to call something that might be subjected to shell escapes: just |
205 | never call the shell at all. By the time we get to the B<exec>, tainting |
206 | is turned off, however, so be careful what you call and what you pass it. |
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207 | |
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208 | use English; |
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209 | die unless defined $pid = open(KID, "-|"); |
210 | if ($pid) { # parent |
211 | while (<KID>) { |
212 | # do something |
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213 | } |
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214 | close KID; |
215 | } else { |
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216 | $EUID = $UID; |
217 | $EGID = $GID; # XXX: initgroups() not called |
218 | $ENV{PATH} = "/bin:/usr/bin"; |
219 | exec 'myprog', 'arg1', 'arg2'; |
220 | die "can't exec myprog: $!"; |
221 | } |
222 | |
223 | A similar strategy would work for wildcard expansion via C<glob>. |
224 | |
225 | Taint checking is most useful when although you trust yourself not to have |
226 | written a program to give away the farm, you don't necessarily trust those |
227 | who end up using it not to try to trick it into doing something bad. This |
228 | is the kind of security checking that's useful for setuid programs and |
229 | programs launched on someone else's behalf, like CGI programs. |
230 | |
231 | This is quite different, however, from not even trusting the writer of the |
232 | code not to try to do something evil. That's the kind of trust needed |
233 | when someone hands you a program you've never seen before and says, "Here, |
234 | run this." For that kind of safety, check out the Safe module, |
235 | included standard in the Perl distribution. This module allows the |
236 | programmer to set up special compartments in which all system operations |
237 | are trapped and namespace access is carefully controlled. |
238 | |
239 | =head2 Security Bugs |
240 | |
241 | Beyond the obvious problems that stem from giving special privileges to |
242 | systems as flexible as scripts, on many versions of Unix, setuid scripts |
243 | are inherently insecure right from the start. The problem is a race |
244 | condition in the kernel. Between the time the kernel opens the file to |
245 | see which interpreter to run and when the (now-setuid) interpreter turns |
246 | around and reopens the file to interpret it, the file in question may have |
247 | changed, especially if you have symbolic links on your system. |
248 | |
249 | Fortunately, sometimes this kernel "feature" can be disabled. |
250 | Unfortunately, there are two ways to disable it. The system can simply |
251 | outlaw scripts with the setuid bit set, which doesn't help much. |
252 | Alternately, it can simply ignore the setuid bit on scripts. If the |
253 | latter is true, Perl can emulate the setuid and setgid mechanism when it |
254 | notices the otherwise useless setuid/gid bits on Perl scripts. It does |
255 | this via a special executable called B<suidperl> that is automatically |
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256 | invoked for you if it's needed. |
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257 | |
258 | However, if the kernel setuid script feature isn't disabled, Perl will |
259 | complain loudly that your setuid script is insecure. You'll need to |
260 | either disable the kernel setuid script feature, or put a C wrapper around |
261 | the script. A C wrapper is just a compiled program that does nothing |
262 | except call your Perl program. Compiled programs are not subject to the |
263 | kernel bug that plagues setuid scripts. Here's a simple wrapper, written |
264 | in C: |
265 | |
266 | #define REAL_PATH "/path/to/script" |
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267 | main(ac, av) |
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268 | char **av; |
269 | { |
270 | execv(REAL_PATH, av); |
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271 | } |
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272 | |
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273 | Compile this wrapper into a binary executable and then make I<it> rather |
274 | than your script setuid or setgid. |
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275 | |
276 | See the program B<wrapsuid> in the F<eg> directory of your Perl |
277 | distribution for a convenient way to do this automatically for all your |
278 | setuid Perl programs. It moves setuid scripts into files with the same |
279 | name plus a leading dot, and then compiles a wrapper like the one above |
280 | for each of them. |
281 | |
282 | In recent years, vendors have begun to supply systems free of this |
283 | inherent security bug. On such systems, when the kernel passes the name |
284 | of the setuid script to open to the interpreter, rather than using a |
285 | pathname subject to meddling, it instead passes I</dev/fd/3>. This is a |
286 | special file already opened on the script, so that there can be no race |
287 | condition for evil scripts to exploit. On these systems, Perl should be |
288 | compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure> |
289 | program that builds Perl tries to figure this out for itself, so you |
290 | should never have to specify this yourself. Most modern releases of |
291 | SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. |
292 | |
293 | Prior to release 5.003 of Perl, a bug in the code of B<suidperl> could |
294 | introduce a security hole in systems compiled with strict POSIX |
295 | compliance. |
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296 | |
297 | =head2 Protecting Your Programs |
298 | |
299 | There are a number of ways to hide the source to your Perl programs, |
300 | with varying levels of "security". |
301 | |
302 | First of all, however, you I<can't> take away read permission, because |
303 | the source code has to be readable in order to be compiled and |
304 | interpreted. (That doesn't mean that a CGI script's source is |
305 | readable by people on the web, though.) So you have to leave the |
306 | permissions at the socially friendly 0755 level. |
307 | |
308 | Some people regard this as a security problem. If your program does |
309 | insecure things, and relies on people not knowing how to exploit those |
310 | insecurities, it is not secure. It is often possible for someone to |
311 | determine the insecure things and exploit them without viewing the |
312 | source. Security through obscurity, the name for hiding your bugs |
313 | instead of fixing them, is little security indeed. |
314 | |
315 | You can try using encryption via source filters (Filter::* from CPAN). |
316 | But crackers might be able to decrypt it. You can try using the |
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317 | byte code compiler and interpreter described below, but crackers might |
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318 | be able to de-compile it. You can try using the native-code compiler |
319 | described below, but crackers might be able to disassemble it. These |
320 | pose varying degrees of difficulty to people wanting to get at your |
321 | code, but none can definitively conceal it (this is true of every |
322 | language, not just Perl). |
323 | |
324 | If you're concerned about people profiting from your code, then the |
325 | bottom line is that nothing but a restrictive licence will give you |
326 | legal security. License your software and pepper it with threatening |
327 | statements like "This is unpublished proprietary software of XYZ Corp. |
328 | Your access to it does not give you permission to use it blah blah |
329 | blah." You should see a lawyer to be sure your licence's wording will |
330 | stand up in court. |