Commit | Line | Data |
---|---|---|
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 | |
425e5e39 | 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 | |
5f05dabc | 19 | mode explicitly by using the B<-T> command line flag. This flag is |
425e5e39 | 20 | I<strongly> suggested for server programs and any program run on behalf of |
21 | someone else, such as a CGI script. | |
a0d0e21e | 22 | |
1e422769 | 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, | |
425e5e39 | 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 | |
54310121 | 32 | else outside your program--at least, not by accident. All command line |
a034a98d DD |
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. | |
a0d0e21e | 41 | |
a0d0e21e LW |
42 | For example: |
43 | ||
425e5e39 | 44 | $arg = shift; # $arg is tainted |
45 | $hid = $arg, 'bar'; # $hid is also tainted | |
46 | $line = <>; # Tainted | |
8ebc5c01 | 47 | $line = <STDIN>; # Also tainted |
48 | open FOO, "/home/me/bar" or die $!; | |
49 | $line = <FOO>; # Still tainted | |
a0d0e21e | 50 | $path = $ENV{'PATH'}; # Tainted, but see below |
425e5e39 | 51 | $data = 'abc'; # Not tainted |
a0d0e21e | 52 | |
425e5e39 | 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 | |
a0d0e21e | 57 | |
425e5e39 | 58 | $path = $ENV{'PATH'}; # $path now tainted |
a0d0e21e | 59 | |
54310121 | 60 | $ENV{'PATH'} = '/bin:/usr/bin'; |
c90c0ff4 | 61 | delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; |
a0d0e21e | 62 | |
425e5e39 | 63 | $path = $ENV{'PATH'}; # $path now NOT tainted |
64 | system "echo $data"; # Is secure now! | |
a0d0e21e | 65 | |
425e5e39 | 66 | open(FOO, "< $arg"); # OK - read-only file |
67 | open(FOO, "> $arg"); # Not OK - trying to write | |
a0d0e21e | 68 | |
425e5e39 | 69 | open(FOO,"echo $arg|"); # Not OK, but... |
70 | open(FOO,"-|") | |
71 | or exec 'echo', $arg; # OK | |
a0d0e21e | 72 | |
425e5e39 | 73 | $shout = `echo $arg`; # Insecure, $shout now tainted |
a0d0e21e | 74 | |
425e5e39 | 75 | unlink $data, $arg; # Insecure |
76 | umask $arg; # Insecure | |
a0d0e21e | 77 | |
425e5e39 | 78 | exec "echo $arg"; # Insecure |
79 | exec "echo", $arg; # Secure (doesn't use the shell) | |
80 | exec "sh", '-c', $arg; # Considered secure, alas! | |
a0d0e21e | 81 | |
7bac28a0 | 82 | @files = <*.c>; # Always insecure (uses csh) |
83 | @files = glob('*.c'); # Always insecure (uses csh) | |
84 | ||
a0d0e21e LW |
85 | If you try to do something insecure, you will get a fatal error saying |
86 | something like "Insecure dependency" or "Insecure PATH". Note that you | |
425e5e39 | 87 | can still write an insecure B<system> or B<exec>, but only by explicitly |
54310121 | 88 | doing something like the last example above. |
425e5e39 | 89 | |
90 | =head2 Laundering and Detecting Tainted Data | |
91 | ||
92 | To test whether a variable contains tainted data, and whose use would thus | |
93 | trigger an "Insecure dependency" message, you can use the following | |
94 | I<is_tainted()> function. | |
95 | ||
96 | sub is_tainted { | |
54310121 | 97 | return ! eval { |
98 | join('',@_), kill 0; | |
99 | 1; | |
425e5e39 | 100 | }; |
101 | } | |
102 | ||
103 | This function makes use of the fact that the presence of tainted data | |
104 | anywhere within an expression renders the entire expression tainted. It | |
105 | would be inefficient for every operator to test every argument for | |
106 | taintedness. Instead, the slightly more efficient and conservative | |
107 | approach is used that if any tainted value has been accessed within the | |
108 | same expression, the whole expression is considered tainted. | |
109 | ||
5f05dabc | 110 | But testing for taintedness gets you only so far. Sometimes you have just |
425e5e39 | 111 | to clear your data's taintedness. The only way to bypass the tainting |
54310121 | 112 | mechanism is by referencing subpatterns from a regular expression match. |
425e5e39 | 113 | Perl presumes that if you reference a substring using $1, $2, etc., that |
114 | you knew what you were doing when you wrote the pattern. That means using | |
115 | a bit of thought--don't just blindly untaint anything, or you defeat the | |
a034a98d DD |
116 | entire mechanism. It's better to verify that the variable has only good |
117 | characters (for certain values of "good") rather than checking whether it | |
118 | has any bad characters. That's because it's far too easy to miss bad | |
119 | characters that you never thought of. | |
425e5e39 | 120 | |
121 | Here's a test to make sure that the data contains nothing but "word" | |
122 | characters (alphabetics, numerics, and underscores), a hyphen, an at sign, | |
123 | or a dot. | |
124 | ||
54310121 | 125 | if ($data =~ /^([-\@\w.]+)$/) { |
425e5e39 | 126 | $data = $1; # $data now untainted |
127 | } else { | |
128 | die "Bad data in $data"; # log this somewhere | |
129 | } | |
130 | ||
5f05dabc | 131 | This is fairly secure because C</\w+/> doesn't normally match shell |
425e5e39 | 132 | metacharacters, nor are dot, dash, or at going to mean something special |
133 | to the shell. Use of C</.+/> would have been insecure in theory because | |
134 | it lets everything through, but Perl doesn't check for that. The lesson | |
135 | is that when untainting, you must be exceedingly careful with your patterns. | |
136 | Laundering data using regular expression is the I<ONLY> mechanism for | |
137 | untainting dirty data, unless you use the strategy detailed below to fork | |
138 | a child of lesser privilege. | |
139 | ||
a034a98d DD |
140 | The example does not untaint $data if C<use locale> is in effect, |
141 | because the characters matched by C<\w> are determined by the locale. | |
142 | Perl considers that locale definitions are untrustworthy because they | |
143 | contain data from outside the program. If you are writing a | |
144 | locale-aware program, and want to launder data with a regular expression | |
145 | containing C<\w>, put C<no locale> ahead of the expression in the same | |
146 | block. See L<perllocale/SECURITY> for further discussion and examples. | |
147 | ||
3a52c276 CS |
148 | =head2 Switches On the "#!" Line |
149 | ||
150 | When you make a script executable, in order to make it usable as a | |
151 | command, the system will pass switches to perl from the script's #! | |
54310121 | 152 | line. Perl checks that any command line switches given to a setuid |
3a52c276 | 153 | (or setgid) script actually match the ones set on the #! line. Some |
54310121 | 154 | Unix and Unix-like environments impose a one-switch limit on the #! |
3a52c276 | 155 | line, so you may need to use something like C<-wU> instead of C<-w -U> |
54310121 | 156 | under such systems. (This issue should arise only in Unix or |
157 | Unix-like environments that support #! and setuid or setgid scripts.) | |
3a52c276 | 158 | |
425e5e39 | 159 | =head2 Cleaning Up Your Path |
160 | ||
1fef88e7 | 161 | For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a |
1e422769 | 162 | known value, and each directory in the path must be non-writable by others |
163 | than its owner and group. You may be surprised to get this message even | |
164 | if the pathname to your executable is fully qualified. This is I<not> | |
165 | generated because you didn't supply a full path to the program; instead, | |
166 | it's generated because you never set your PATH environment variable, or | |
167 | you didn't set it to something that was safe. Because Perl can't | |
168 | guarantee that the executable in question isn't itself going to turn | |
169 | around and execute some other program that is dependent on your PATH, it | |
54310121 | 170 | makes sure you set the PATH. |
a0d0e21e LW |
171 | |
172 | It's also possible to get into trouble with other operations that don't | |
173 | care whether they use tainted values. Make judicious use of the file | |
174 | tests in dealing with any user-supplied filenames. When possible, do | |
175 | opens and such after setting C<$E<gt> = $E<lt>>. (Remember group IDs, | |
425e5e39 | 176 | too!) Perl doesn't prevent you from opening tainted filenames for reading, |
a0d0e21e LW |
177 | so be careful what you print out. The tainting mechanism is intended to |
178 | prevent stupid mistakes, not to remove the need for thought. | |
179 | ||
425e5e39 | 180 | Perl does not call the shell to expand wild cards when you pass B<system> |
181 | and B<exec> explicit parameter lists instead of strings with possible shell | |
182 | wildcards in them. Unfortunately, the B<open>, B<glob>, and | |
54310121 | 183 | backtick functions provide no such alternate calling convention, so more |
184 | subterfuge will be required. | |
425e5e39 | 185 | |
186 | Perl provides a reasonably safe way to open a file or pipe from a setuid | |
187 | or setgid program: just create a child process with reduced privilege who | |
188 | does the dirty work for you. First, fork a child using the special | |
189 | B<open> syntax that connects the parent and child by a pipe. Now the | |
190 | child resets its ID set and any other per-process attributes, like | |
191 | environment variables, umasks, current working directories, back to the | |
192 | originals or known safe values. Then the child process, which no longer | |
193 | has any special permissions, does the B<open> or other system call. | |
194 | Finally, the child passes the data it managed to access back to the | |
5f05dabc | 195 | parent. Because the file or pipe was opened in the child while running |
425e5e39 | 196 | under less privilege than the parent, it's not apt to be tricked into |
197 | doing something it shouldn't. | |
198 | ||
54310121 | 199 | Here's a way to do backticks reasonably safely. Notice how the B<exec> is |
425e5e39 | 200 | not called with a string that the shell could expand. This is by far the |
201 | best way to call something that might be subjected to shell escapes: just | |
202 | never call the shell at all. By the time we get to the B<exec>, tainting | |
203 | is turned off, however, so be careful what you call and what you pass it. | |
cb1a09d0 | 204 | |
54310121 | 205 | use English; |
cb1a09d0 AD |
206 | die unless defined $pid = open(KID, "-|"); |
207 | if ($pid) { # parent | |
208 | while (<KID>) { | |
209 | # do something | |
425e5e39 | 210 | } |
cb1a09d0 AD |
211 | close KID; |
212 | } else { | |
425e5e39 | 213 | $EUID = $UID; |
214 | $EGID = $GID; # XXX: initgroups() not called | |
215 | $ENV{PATH} = "/bin:/usr/bin"; | |
216 | exec 'myprog', 'arg1', 'arg2'; | |
217 | die "can't exec myprog: $!"; | |
218 | } | |
219 | ||
220 | A similar strategy would work for wildcard expansion via C<glob>. | |
221 | ||
222 | Taint checking is most useful when although you trust yourself not to have | |
223 | written a program to give away the farm, you don't necessarily trust those | |
224 | who end up using it not to try to trick it into doing something bad. This | |
225 | is the kind of security checking that's useful for setuid programs and | |
226 | programs launched on someone else's behalf, like CGI programs. | |
227 | ||
228 | This is quite different, however, from not even trusting the writer of the | |
229 | code not to try to do something evil. That's the kind of trust needed | |
230 | when someone hands you a program you've never seen before and says, "Here, | |
231 | run this." For that kind of safety, check out the Safe module, | |
232 | included standard in the Perl distribution. This module allows the | |
233 | programmer to set up special compartments in which all system operations | |
234 | are trapped and namespace access is carefully controlled. | |
235 | ||
236 | =head2 Security Bugs | |
237 | ||
238 | Beyond the obvious problems that stem from giving special privileges to | |
239 | systems as flexible as scripts, on many versions of Unix, setuid scripts | |
240 | are inherently insecure right from the start. The problem is a race | |
241 | condition in the kernel. Between the time the kernel opens the file to | |
242 | see which interpreter to run and when the (now-setuid) interpreter turns | |
243 | around and reopens the file to interpret it, the file in question may have | |
244 | changed, especially if you have symbolic links on your system. | |
245 | ||
246 | Fortunately, sometimes this kernel "feature" can be disabled. | |
247 | Unfortunately, there are two ways to disable it. The system can simply | |
248 | outlaw scripts with the setuid bit set, which doesn't help much. | |
249 | Alternately, it can simply ignore the setuid bit on scripts. If the | |
250 | latter is true, Perl can emulate the setuid and setgid mechanism when it | |
251 | notices the otherwise useless setuid/gid bits on Perl scripts. It does | |
252 | this via a special executable called B<suidperl> that is automatically | |
54310121 | 253 | invoked for you if it's needed. |
425e5e39 | 254 | |
255 | However, if the kernel setuid script feature isn't disabled, Perl will | |
256 | complain loudly that your setuid script is insecure. You'll need to | |
257 | either disable the kernel setuid script feature, or put a C wrapper around | |
258 | the script. A C wrapper is just a compiled program that does nothing | |
259 | except call your Perl program. Compiled programs are not subject to the | |
260 | kernel bug that plagues setuid scripts. Here's a simple wrapper, written | |
261 | in C: | |
262 | ||
263 | #define REAL_PATH "/path/to/script" | |
54310121 | 264 | main(ac, av) |
425e5e39 | 265 | char **av; |
266 | { | |
267 | execv(REAL_PATH, av); | |
54310121 | 268 | } |
cb1a09d0 | 269 | |
54310121 | 270 | Compile this wrapper into a binary executable and then make I<it> rather |
271 | than your script setuid or setgid. | |
425e5e39 | 272 | |
273 | See the program B<wrapsuid> in the F<eg> directory of your Perl | |
274 | distribution for a convenient way to do this automatically for all your | |
275 | setuid Perl programs. It moves setuid scripts into files with the same | |
276 | name plus a leading dot, and then compiles a wrapper like the one above | |
277 | for each of them. | |
278 | ||
279 | In recent years, vendors have begun to supply systems free of this | |
280 | inherent security bug. On such systems, when the kernel passes the name | |
281 | of the setuid script to open to the interpreter, rather than using a | |
282 | pathname subject to meddling, it instead passes I</dev/fd/3>. This is a | |
283 | special file already opened on the script, so that there can be no race | |
284 | condition for evil scripts to exploit. On these systems, Perl should be | |
285 | compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure> | |
286 | program that builds Perl tries to figure this out for itself, so you | |
287 | should never have to specify this yourself. Most modern releases of | |
288 | SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. | |
289 | ||
290 | Prior to release 5.003 of Perl, a bug in the code of B<suidperl> could | |
291 | introduce a security hole in systems compiled with strict POSIX | |
292 | compliance. | |
68dc0745 | 293 | |
294 | =head2 Protecting Your Programs | |
295 | ||
296 | There are a number of ways to hide the source to your Perl programs, | |
297 | with varying levels of "security". | |
298 | ||
299 | First of all, however, you I<can't> take away read permission, because | |
300 | the source code has to be readable in order to be compiled and | |
301 | interpreted. (That doesn't mean that a CGI script's source is | |
302 | readable by people on the web, though.) So you have to leave the | |
303 | permissions at the socially friendly 0755 level. | |
304 | ||
305 | Some people regard this as a security problem. If your program does | |
306 | insecure things, and relies on people not knowing how to exploit those | |
307 | insecurities, it is not secure. It is often possible for someone to | |
308 | determine the insecure things and exploit them without viewing the | |
309 | source. Security through obscurity, the name for hiding your bugs | |
310 | instead of fixing them, is little security indeed. | |
311 | ||
312 | You can try using encryption via source filters (Filter::* from CPAN). | |
313 | But crackers might be able to decrypt it. You can try using the | |
54310121 | 314 | byte code compiler and interpreter described below, but crackers might |
68dc0745 | 315 | be able to de-compile it. You can try using the native-code compiler |
316 | described below, but crackers might be able to disassemble it. These | |
317 | pose varying degrees of difficulty to people wanting to get at your | |
318 | code, but none can definitively conceal it (this is true of every | |
319 | language, not just Perl). | |
320 | ||
321 | If you're concerned about people profiting from your code, then the | |
322 | bottom line is that nothing but a restrictive licence will give you | |
323 | legal security. License your software and pepper it with threatening | |
324 | statements like "This is unpublished proprietary software of XYZ Corp. | |
325 | Your access to it does not give you permission to use it blah blah | |
326 | blah." You should see a lawyer to be sure your licence's wording will | |
327 | stand up in court. |