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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 |
9 | command-line shells, which are based on multiple substitution passes on |
10 | each line of the script, Perl uses a more conventional evaluation scheme |
11 | with fewer hidden snags. Additionally, because the language has more |
12 | built-in functionality, it can rely less upon external (and possibly |
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, |
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 |
32 | else outside your program--at least, not by accident. All command-line |
33 | arguments, environment variables, and file input are marked as "tainted". |
34 | Tainted data may not be used directly or indirectly in any command that |
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35 | invokes a sub-shell, nor in any command that modifies files, directories, |
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36 | or processes. Any variable set within an expression that has previously |
37 | referenced a tainted value itself becomes tainted, even if it is logically |
38 | impossible for the tainted value to influence the variable. Because |
39 | taintedness is associated with each scalar value, some elements of an |
40 | 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 | $path = $ENV{'PATH'}; # Tainted, but see below |
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48 | $data = 'abc'; # Not tainted |
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49 | |
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50 | system "echo $arg"; # Insecure |
51 | system "/bin/echo", $arg; # Secure (doesn't use sh) |
52 | system "echo $hid"; # Insecure |
53 | system "echo $data"; # Insecure until PATH set |
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54 | |
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55 | $path = $ENV{'PATH'}; # $path now tainted |
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56 | |
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57 | $ENV{'PATH'} = '/bin:/usr/bin'; |
58 | $ENV{'IFS'} = '' if $ENV{'IFS'} ne ''; |
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59 | |
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60 | $path = $ENV{'PATH'}; # $path now NOT tainted |
61 | system "echo $data"; # Is secure now! |
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62 | |
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63 | open(FOO, "< $arg"); # OK - read-only file |
64 | open(FOO, "> $arg"); # Not OK - trying to write |
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65 | |
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66 | open(FOO,"echo $arg|"); # Not OK, but... |
67 | open(FOO,"-|") |
68 | or exec 'echo', $arg; # OK |
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69 | |
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70 | $shout = `echo $arg`; # Insecure, $shout now tainted |
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71 | |
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72 | unlink $data, $arg; # Insecure |
73 | umask $arg; # Insecure |
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74 | |
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75 | exec "echo $arg"; # Insecure |
76 | exec "echo", $arg; # Secure (doesn't use the shell) |
77 | exec "sh", '-c', $arg; # Considered secure, alas! |
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78 | |
79 | If you try to do something insecure, you will get a fatal error saying |
80 | something like "Insecure dependency" or "Insecure PATH". Note that you |
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81 | can still write an insecure B<system> or B<exec>, but only by explicitly |
82 | doing something like the last example above. |
83 | |
84 | =head2 Laundering and Detecting Tainted Data |
85 | |
86 | To test whether a variable contains tainted data, and whose use would thus |
87 | trigger an "Insecure dependency" message, you can use the following |
88 | I<is_tainted()> function. |
89 | |
90 | sub is_tainted { |
91 | return ! eval { |
92 | join('',@_), kill 0; |
93 | 1; |
94 | }; |
95 | } |
96 | |
97 | This function makes use of the fact that the presence of tainted data |
98 | anywhere within an expression renders the entire expression tainted. It |
99 | would be inefficient for every operator to test every argument for |
100 | taintedness. Instead, the slightly more efficient and conservative |
101 | approach is used that if any tainted value has been accessed within the |
102 | same expression, the whole expression is considered tainted. |
103 | |
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104 | But testing for taintedness gets you only so far. Sometimes you have just |
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105 | to clear your data's taintedness. The only way to bypass the tainting |
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106 | mechanism is by referencing sub-patterns from a regular expression match. |
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107 | Perl presumes that if you reference a substring using $1, $2, etc., that |
108 | you knew what you were doing when you wrote the pattern. That means using |
109 | a bit of thought--don't just blindly untaint anything, or you defeat the |
110 | entire mechanism. It's better to verify that the variable has only |
111 | good characters (for certain values of "good") rather than checking |
112 | whether it has any bad characters. That's because it's far too easy to |
113 | miss bad characters that you never thought of. |
114 | |
115 | Here's a test to make sure that the data contains nothing but "word" |
116 | characters (alphabetics, numerics, and underscores), a hyphen, an at sign, |
117 | or a dot. |
118 | |
119 | if ($data =~ /^([-\@\w.]+)$/) { |
120 | $data = $1; # $data now untainted |
121 | } else { |
122 | die "Bad data in $data"; # log this somewhere |
123 | } |
124 | |
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125 | This is fairly secure because C</\w+/> doesn't normally match shell |
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126 | metacharacters, nor are dot, dash, or at going to mean something special |
127 | to the shell. Use of C</.+/> would have been insecure in theory because |
128 | it lets everything through, but Perl doesn't check for that. The lesson |
129 | is that when untainting, you must be exceedingly careful with your patterns. |
130 | Laundering data using regular expression is the I<ONLY> mechanism for |
131 | untainting dirty data, unless you use the strategy detailed below to fork |
132 | a child of lesser privilege. |
133 | |
134 | =head2 Cleaning Up Your Path |
135 | |
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136 | For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a |
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137 | known value, and each directory in the path must be non-writable by others |
138 | than its owner and group. You may be surprised to get this message even |
139 | if the pathname to your executable is fully qualified. This is I<not> |
140 | generated because you didn't supply a full path to the program; instead, |
141 | it's generated because you never set your PATH environment variable, or |
142 | you didn't set it to something that was safe. Because Perl can't |
143 | guarantee that the executable in question isn't itself going to turn |
144 | around and execute some other program that is dependent on your PATH, it |
145 | makes sure you set the PATH. |
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146 | |
147 | It's also possible to get into trouble with other operations that don't |
148 | care whether they use tainted values. Make judicious use of the file |
149 | tests in dealing with any user-supplied filenames. When possible, do |
150 | opens and such after setting C<$E<gt> = $E<lt>>. (Remember group IDs, |
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151 | too!) Perl doesn't prevent you from opening tainted filenames for reading, |
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152 | so be careful what you print out. The tainting mechanism is intended to |
153 | prevent stupid mistakes, not to remove the need for thought. |
154 | |
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155 | Perl does not call the shell to expand wild cards when you pass B<system> |
156 | and B<exec> explicit parameter lists instead of strings with possible shell |
157 | wildcards in them. Unfortunately, the B<open>, B<glob>, and |
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158 | back-tick functions provide no such alternate calling convention, so more |
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159 | subterfuge will be required. |
160 | |
161 | Perl provides a reasonably safe way to open a file or pipe from a setuid |
162 | or setgid program: just create a child process with reduced privilege who |
163 | does the dirty work for you. First, fork a child using the special |
164 | B<open> syntax that connects the parent and child by a pipe. Now the |
165 | child resets its ID set and any other per-process attributes, like |
166 | environment variables, umasks, current working directories, back to the |
167 | originals or known safe values. Then the child process, which no longer |
168 | has any special permissions, does the B<open> or other system call. |
169 | Finally, the child passes the data it managed to access back to the |
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170 | parent. Because the file or pipe was opened in the child while running |
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171 | under less privilege than the parent, it's not apt to be tricked into |
172 | doing something it shouldn't. |
173 | |
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174 | Here's a way to do back-ticks reasonably safely. Notice how the B<exec> is |
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175 | not called with a string that the shell could expand. This is by far the |
176 | best way to call something that might be subjected to shell escapes: just |
177 | never call the shell at all. By the time we get to the B<exec>, tainting |
178 | is turned off, however, so be careful what you call and what you pass it. |
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179 | |
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180 | use English; |
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181 | die unless defined $pid = open(KID, "-|"); |
182 | if ($pid) { # parent |
183 | while (<KID>) { |
184 | # do something |
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185 | } |
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186 | close KID; |
187 | } else { |
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188 | $EUID = $UID; |
189 | $EGID = $GID; # XXX: initgroups() not called |
190 | $ENV{PATH} = "/bin:/usr/bin"; |
191 | exec 'myprog', 'arg1', 'arg2'; |
192 | die "can't exec myprog: $!"; |
193 | } |
194 | |
195 | A similar strategy would work for wildcard expansion via C<glob>. |
196 | |
197 | Taint checking is most useful when although you trust yourself not to have |
198 | written a program to give away the farm, you don't necessarily trust those |
199 | who end up using it not to try to trick it into doing something bad. This |
200 | is the kind of security checking that's useful for setuid programs and |
201 | programs launched on someone else's behalf, like CGI programs. |
202 | |
203 | This is quite different, however, from not even trusting the writer of the |
204 | code not to try to do something evil. That's the kind of trust needed |
205 | when someone hands you a program you've never seen before and says, "Here, |
206 | run this." For that kind of safety, check out the Safe module, |
207 | included standard in the Perl distribution. This module allows the |
208 | programmer to set up special compartments in which all system operations |
209 | are trapped and namespace access is carefully controlled. |
210 | |
211 | =head2 Security Bugs |
212 | |
213 | Beyond the obvious problems that stem from giving special privileges to |
214 | systems as flexible as scripts, on many versions of Unix, setuid scripts |
215 | are inherently insecure right from the start. The problem is a race |
216 | condition in the kernel. Between the time the kernel opens the file to |
217 | see which interpreter to run and when the (now-setuid) interpreter turns |
218 | around and reopens the file to interpret it, the file in question may have |
219 | changed, especially if you have symbolic links on your system. |
220 | |
221 | Fortunately, sometimes this kernel "feature" can be disabled. |
222 | Unfortunately, there are two ways to disable it. The system can simply |
223 | outlaw scripts with the setuid bit set, which doesn't help much. |
224 | Alternately, it can simply ignore the setuid bit on scripts. If the |
225 | latter is true, Perl can emulate the setuid and setgid mechanism when it |
226 | notices the otherwise useless setuid/gid bits on Perl scripts. It does |
227 | this via a special executable called B<suidperl> that is automatically |
228 | invoked for you if it's needed. |
229 | |
230 | However, if the kernel setuid script feature isn't disabled, Perl will |
231 | complain loudly that your setuid script is insecure. You'll need to |
232 | either disable the kernel setuid script feature, or put a C wrapper around |
233 | the script. A C wrapper is just a compiled program that does nothing |
234 | except call your Perl program. Compiled programs are not subject to the |
235 | kernel bug that plagues setuid scripts. Here's a simple wrapper, written |
236 | in C: |
237 | |
238 | #define REAL_PATH "/path/to/script" |
239 | main(ac, av) |
240 | char **av; |
241 | { |
242 | execv(REAL_PATH, av); |
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243 | } |
244 | |
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245 | Compile this wrapper into a binary executable and then make I<it> rather |
246 | than your script setuid or setgid. |
247 | |
248 | See the program B<wrapsuid> in the F<eg> directory of your Perl |
249 | distribution for a convenient way to do this automatically for all your |
250 | setuid Perl programs. It moves setuid scripts into files with the same |
251 | name plus a leading dot, and then compiles a wrapper like the one above |
252 | for each of them. |
253 | |
254 | In recent years, vendors have begun to supply systems free of this |
255 | inherent security bug. On such systems, when the kernel passes the name |
256 | of the setuid script to open to the interpreter, rather than using a |
257 | pathname subject to meddling, it instead passes I</dev/fd/3>. This is a |
258 | special file already opened on the script, so that there can be no race |
259 | condition for evil scripts to exploit. On these systems, Perl should be |
260 | compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure> |
261 | program that builds Perl tries to figure this out for itself, so you |
262 | should never have to specify this yourself. Most modern releases of |
263 | SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. |
264 | |
265 | Prior to release 5.003 of Perl, a bug in the code of B<suidperl> could |
266 | introduce a security hole in systems compiled with strict POSIX |
267 | compliance. |