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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 |
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21 | someone else, such as a CGI script. Once taint mode is on, it's on for |
22 | the remainder of your script. |
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23 | |
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24 | While in this mode, Perl takes special precautions called I<taint |
25 | checks> to prevent both obvious and subtle traps. Some of these checks |
26 | are reasonably simple, such as verifying that path directories aren't |
27 | writable by others; careful programmers have always used checks like |
28 | these. Other checks, however, are best supported by the language itself, |
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29 | and it is these checks especially that contribute to making a set-id Perl |
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30 | program more secure than the corresponding C program. |
31 | |
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32 | You may not use data derived from outside your program to affect |
33 | something else outside your program--at least, not by accident. All |
34 | command line arguments, environment variables, locale information (see |
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35 | L<perllocale>), results of certain system calls (readdir(), |
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36 | readlink(), the variable of shmread(), the messages returned by |
37 | msgrcv(), the password, gcos and shell fields returned by the |
38 | getpwxxx() calls), and all file input are marked as "tainted". |
39 | Tainted data may not be used directly or indirectly in any command |
40 | that invokes a sub-shell, nor in any command that modifies files, |
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41 | directories, or processes, B<with the following exceptions>: |
42 | |
43 | =over 4 |
44 | |
45 | =item * |
46 | |
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47 | Arguments to C<print> and C<syswrite> are B<not> checked for taintedness. |
48 | |
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49 | =item * |
50 | |
51 | Symbolic methods |
52 | |
53 | $obj->$method(@args); |
54 | |
55 | and symbolic sub references |
56 | |
57 | &{$foo}(@args); |
58 | $foo->(@args); |
59 | |
60 | are not checked for taintedness. This requires extra carefulness |
61 | unless you want external data to affect your control flow. Unless |
62 | you carefully limit what these symbolic values are, people are able |
63 | to call functions B<outside> your Perl code, such as POSIX::system, |
64 | in which case they are able to run arbitrary external code. |
65 | |
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66 | =back |
67 | |
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68 | The value of an expression containing tainted data will itself be |
69 | tainted, even if it is logically impossible for the tainted data to |
70 | affect the value. |
71 | |
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72 | Because taintedness is associated with each scalar value, some |
73 | elements of an array can be tainted and others not. |
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74 | |
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75 | For example: |
76 | |
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77 | $arg = shift; # $arg is tainted |
78 | $hid = $arg, 'bar'; # $hid is also tainted |
79 | $line = <>; # Tainted |
8ebc5c01 |
80 | $line = <STDIN>; # Also tainted |
81 | open FOO, "/home/me/bar" or die $!; |
82 | $line = <FOO>; # Still tainted |
a0d0e21e |
83 | $path = $ENV{'PATH'}; # Tainted, but see below |
425e5e39 |
84 | $data = 'abc'; # Not tainted |
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85 | |
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86 | system "echo $arg"; # Insecure |
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87 | system "/bin/echo", $arg; # Considered insecure |
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88 | # (Perl doesn't know about /bin/echo) |
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89 | system "echo $hid"; # Insecure |
90 | system "echo $data"; # Insecure until PATH set |
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91 | |
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92 | $path = $ENV{'PATH'}; # $path now tainted |
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93 | |
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94 | $ENV{'PATH'} = '/bin:/usr/bin'; |
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95 | delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; |
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96 | |
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97 | $path = $ENV{'PATH'}; # $path now NOT tainted |
98 | system "echo $data"; # Is secure now! |
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99 | |
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100 | open(FOO, "< $arg"); # OK - read-only file |
101 | open(FOO, "> $arg"); # Not OK - trying to write |
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102 | |
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103 | open(FOO,"echo $arg|"); # Not OK |
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104 | open(FOO,"-|") |
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105 | or exec 'echo', $arg; # Also not OK |
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106 | |
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107 | $shout = `echo $arg`; # Insecure, $shout now tainted |
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108 | |
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109 | unlink $data, $arg; # Insecure |
110 | umask $arg; # Insecure |
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111 | |
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112 | exec "echo $arg"; # Insecure |
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113 | exec "echo", $arg; # Insecure |
114 | exec "sh", '-c', $arg; # Very insecure! |
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115 | |
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116 | @files = <*.c>; # insecure (uses readdir() or similar) |
117 | @files = glob('*.c'); # insecure (uses readdir() or similar) |
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118 | |
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119 | # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would |
120 | # have used an external program to do the filename expansion; but in |
121 | # either case the result is tainted since the list of filenames comes |
122 | # from outside of the program. |
123 | |
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124 | $bad = ($arg, 23); # $bad will be tainted |
125 | $arg, `true`; # Insecure (although it isn't really) |
126 | |
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127 | If you try to do something insecure, you will get a fatal error saying |
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128 | something like "Insecure dependency" or "Insecure $ENV{PATH}". |
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129 | |
130 | =head2 Laundering and Detecting Tainted Data |
131 | |
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132 | To test whether a variable contains tainted data, and whose use would |
133 | thus trigger an "Insecure dependency" message, you can use the |
134 | tainted() function of the Scalar::Util module, available in your |
135 | nearby CPAN mirror, and included in Perl starting from the release 5.8.0. |
136 | Or you may be able to use the following I<is_tainted()> function. |
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137 | |
138 | sub is_tainted { |
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139 | return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 }; |
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140 | } |
141 | |
142 | This function makes use of the fact that the presence of tainted data |
143 | anywhere within an expression renders the entire expression tainted. It |
144 | would be inefficient for every operator to test every argument for |
145 | taintedness. Instead, the slightly more efficient and conservative |
146 | approach is used that if any tainted value has been accessed within the |
147 | same expression, the whole expression is considered tainted. |
148 | |
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149 | But testing for taintedness gets you only so far. Sometimes you have just |
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150 | to clear your data's taintedness. The only way to bypass the tainting |
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151 | mechanism is by referencing subpatterns from a regular expression match. |
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152 | Perl presumes that if you reference a substring using $1, $2, etc., that |
153 | you knew what you were doing when you wrote the pattern. That means using |
154 | a bit of thought--don't just blindly untaint anything, or you defeat the |
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155 | entire mechanism. It's better to verify that the variable has only good |
156 | characters (for certain values of "good") rather than checking whether it |
157 | has any bad characters. That's because it's far too easy to miss bad |
158 | characters that you never thought of. |
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159 | |
160 | Here's a test to make sure that the data contains nothing but "word" |
161 | characters (alphabetics, numerics, and underscores), a hyphen, an at sign, |
162 | or a dot. |
163 | |
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164 | if ($data =~ /^([-\@\w.]+)$/) { |
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165 | $data = $1; # $data now untainted |
166 | } else { |
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167 | die "Bad data in '$data'"; # log this somewhere |
425e5e39 |
168 | } |
169 | |
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170 | This is fairly secure because C</\w+/> doesn't normally match shell |
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171 | metacharacters, nor are dot, dash, or at going to mean something special |
172 | to the shell. Use of C</.+/> would have been insecure in theory because |
173 | it lets everything through, but Perl doesn't check for that. The lesson |
174 | is that when untainting, you must be exceedingly careful with your patterns. |
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175 | Laundering data using regular expression is the I<only> mechanism for |
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176 | untainting dirty data, unless you use the strategy detailed below to fork |
177 | a child of lesser privilege. |
178 | |
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179 | The example does not untaint $data if C<use locale> is in effect, |
180 | because the characters matched by C<\w> are determined by the locale. |
181 | Perl considers that locale definitions are untrustworthy because they |
182 | contain data from outside the program. If you are writing a |
183 | locale-aware program, and want to launder data with a regular expression |
184 | containing C<\w>, put C<no locale> ahead of the expression in the same |
185 | block. See L<perllocale/SECURITY> for further discussion and examples. |
186 | |
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187 | =head2 Switches On the "#!" Line |
188 | |
189 | When you make a script executable, in order to make it usable as a |
190 | command, the system will pass switches to perl from the script's #! |
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191 | line. Perl checks that any command line switches given to a setuid |
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192 | (or setgid) script actually match the ones set on the #! line. Some |
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193 | Unix and Unix-like environments impose a one-switch limit on the #! |
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194 | line, so you may need to use something like C<-wU> instead of C<-w -U> |
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195 | under such systems. (This issue should arise only in Unix or |
196 | Unix-like environments that support #! and setuid or setgid scripts.) |
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197 | |
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198 | =head2 Cleaning Up Your Path |
199 | |
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200 | For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a |
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201 | known value, and each directory in the path must be non-writable by others |
202 | than its owner and group. You may be surprised to get this message even |
203 | if the pathname to your executable is fully qualified. This is I<not> |
204 | generated because you didn't supply a full path to the program; instead, |
205 | it's generated because you never set your PATH environment variable, or |
206 | you didn't set it to something that was safe. Because Perl can't |
207 | guarantee that the executable in question isn't itself going to turn |
208 | around and execute some other program that is dependent on your PATH, it |
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209 | makes sure you set the PATH. |
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210 | |
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211 | The PATH isn't the only environment variable which can cause problems. |
212 | Because some shells may use the variables IFS, CDPATH, ENV, and |
213 | BASH_ENV, Perl checks that those are either empty or untainted when |
214 | starting subprocesses. You may wish to add something like this to your |
215 | setid and taint-checking scripts. |
216 | |
217 | delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer |
218 | |
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219 | It's also possible to get into trouble with other operations that don't |
220 | care whether they use tainted values. Make judicious use of the file |
221 | tests in dealing with any user-supplied filenames. When possible, do |
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222 | opens and such B<after> properly dropping any special user (or group!) |
223 | privileges. Perl doesn't prevent you from opening tainted filenames for reading, |
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224 | so be careful what you print out. The tainting mechanism is intended to |
225 | prevent stupid mistakes, not to remove the need for thought. |
226 | |
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227 | Perl does not call the shell to expand wild cards when you pass B<system> |
228 | and B<exec> explicit parameter lists instead of strings with possible shell |
229 | wildcards in them. Unfortunately, the B<open>, B<glob>, and |
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230 | backtick functions provide no such alternate calling convention, so more |
231 | subterfuge will be required. |
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232 | |
233 | Perl provides a reasonably safe way to open a file or pipe from a setuid |
234 | or setgid program: just create a child process with reduced privilege who |
235 | does the dirty work for you. First, fork a child using the special |
236 | B<open> syntax that connects the parent and child by a pipe. Now the |
237 | child resets its ID set and any other per-process attributes, like |
238 | environment variables, umasks, current working directories, back to the |
239 | originals or known safe values. Then the child process, which no longer |
240 | has any special permissions, does the B<open> or other system call. |
241 | Finally, the child passes the data it managed to access back to the |
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242 | parent. Because the file or pipe was opened in the child while running |
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243 | under less privilege than the parent, it's not apt to be tricked into |
244 | doing something it shouldn't. |
245 | |
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246 | Here's a way to do backticks reasonably safely. Notice how the B<exec> is |
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247 | not called with a string that the shell could expand. This is by far the |
248 | best way to call something that might be subjected to shell escapes: just |
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249 | never call the shell at all. |
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250 | |
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251 | use English '-no_match_vars'; |
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252 | die "Can't fork: $!" unless defined($pid = open(KID, "-|")); |
253 | if ($pid) { # parent |
254 | while (<KID>) { |
255 | # do something |
256 | } |
257 | close KID; |
258 | } else { |
259 | my @temp = ($EUID, $EGID); |
260 | my $orig_uid = $UID; |
261 | my $orig_gid = $GID; |
262 | $EUID = $UID; |
263 | $EGID = $GID; |
264 | # Drop privileges |
265 | $UID = $orig_uid; |
266 | $GID = $orig_gid; |
267 | # Make sure privs are really gone |
268 | ($EUID, $EGID) = @temp; |
269 | die "Can't drop privileges" |
270 | unless $UID == $EUID && $GID eq $EGID; |
271 | $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH. |
272 | # Consider sanitizing the environment even more. |
273 | exec 'myprog', 'arg1', 'arg2' |
274 | or die "can't exec myprog: $!"; |
275 | } |
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276 | |
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277 | A similar strategy would work for wildcard expansion via C<glob>, although |
278 | you can use C<readdir> instead. |
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279 | |
280 | Taint checking is most useful when although you trust yourself not to have |
281 | written a program to give away the farm, you don't necessarily trust those |
282 | who end up using it not to try to trick it into doing something bad. This |
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283 | is the kind of security checking that's useful for set-id programs and |
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284 | programs launched on someone else's behalf, like CGI programs. |
285 | |
286 | This is quite different, however, from not even trusting the writer of the |
287 | code not to try to do something evil. That's the kind of trust needed |
288 | when someone hands you a program you've never seen before and says, "Here, |
289 | run this." For that kind of safety, check out the Safe module, |
290 | included standard in the Perl distribution. This module allows the |
291 | programmer to set up special compartments in which all system operations |
292 | are trapped and namespace access is carefully controlled. |
293 | |
294 | =head2 Security Bugs |
295 | |
296 | Beyond the obvious problems that stem from giving special privileges to |
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297 | systems as flexible as scripts, on many versions of Unix, set-id scripts |
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298 | are inherently insecure right from the start. The problem is a race |
299 | condition in the kernel. Between the time the kernel opens the file to |
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300 | see which interpreter to run and when the (now-set-id) interpreter turns |
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301 | around and reopens the file to interpret it, the file in question may have |
302 | changed, especially if you have symbolic links on your system. |
303 | |
304 | Fortunately, sometimes this kernel "feature" can be disabled. |
305 | Unfortunately, there are two ways to disable it. The system can simply |
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306 | outlaw scripts with any set-id bit set, which doesn't help much. |
307 | Alternately, it can simply ignore the set-id bits on scripts. If the |
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308 | latter is true, Perl can emulate the setuid and setgid mechanism when it |
309 | notices the otherwise useless setuid/gid bits on Perl scripts. It does |
310 | this via a special executable called B<suidperl> that is automatically |
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311 | invoked for you if it's needed. |
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312 | |
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313 | However, if the kernel set-id script feature isn't disabled, Perl will |
314 | complain loudly that your set-id script is insecure. You'll need to |
315 | either disable the kernel set-id script feature, or put a C wrapper around |
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316 | the script. A C wrapper is just a compiled program that does nothing |
317 | except call your Perl program. Compiled programs are not subject to the |
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318 | kernel bug that plagues set-id scripts. Here's a simple wrapper, written |
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319 | in C: |
320 | |
321 | #define REAL_PATH "/path/to/script" |
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322 | main(ac, av) |
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323 | char **av; |
324 | { |
325 | execv(REAL_PATH, av); |
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326 | } |
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327 | |
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328 | Compile this wrapper into a binary executable and then make I<it> rather |
329 | than your script setuid or setgid. |
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330 | |
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331 | In recent years, vendors have begun to supply systems free of this |
332 | inherent security bug. On such systems, when the kernel passes the name |
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333 | of the set-id script to open to the interpreter, rather than using a |
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334 | pathname subject to meddling, it instead passes I</dev/fd/3>. This is a |
335 | special file already opened on the script, so that there can be no race |
336 | condition for evil scripts to exploit. On these systems, Perl should be |
337 | compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure> |
338 | program that builds Perl tries to figure this out for itself, so you |
339 | should never have to specify this yourself. Most modern releases of |
340 | SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. |
341 | |
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342 | Prior to release 5.6.1 of Perl, bugs in the code of B<suidperl> could |
343 | introduce a security hole. |
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344 | |
345 | =head2 Protecting Your Programs |
346 | |
347 | There are a number of ways to hide the source to your Perl programs, |
348 | with varying levels of "security". |
349 | |
350 | First of all, however, you I<can't> take away read permission, because |
351 | the source code has to be readable in order to be compiled and |
352 | interpreted. (That doesn't mean that a CGI script's source is |
353 | readable by people on the web, though.) So you have to leave the |
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354 | permissions at the socially friendly 0755 level. This lets |
355 | people on your local system only see your source. |
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356 | |
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357 | Some people mistakenly regard this as a security problem. If your program does |
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358 | insecure things, and relies on people not knowing how to exploit those |
359 | insecurities, it is not secure. It is often possible for someone to |
360 | determine the insecure things and exploit them without viewing the |
361 | source. Security through obscurity, the name for hiding your bugs |
362 | instead of fixing them, is little security indeed. |
363 | |
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364 | You can try using encryption via source filters (Filter::* from CPAN, |
365 | or Filter::Util::Call and Filter::Simple since Perl 5.8). |
366 | But crackers might be able to decrypt it. You can try using the byte |
367 | code compiler and interpreter described below, but crackers might be |
368 | able to de-compile it. You can try using the native-code compiler |
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369 | described below, but crackers might be able to disassemble it. These |
370 | pose varying degrees of difficulty to people wanting to get at your |
371 | code, but none can definitively conceal it (this is true of every |
372 | language, not just Perl). |
373 | |
374 | If you're concerned about people profiting from your code, then the |
375 | bottom line is that nothing but a restrictive licence will give you |
376 | legal security. License your software and pepper it with threatening |
377 | statements like "This is unpublished proprietary software of XYZ Corp. |
378 | Your access to it does not give you permission to use it blah blah |
379 | blah." You should see a lawyer to be sure your licence's wording will |
380 | stand up in court. |
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381 | |
0d7c09bb |
382 | =head2 Unicode |
383 | |
384 | Unicode is a new and complex technology and one may easily overlook |
385 | certain security pitfalls. See L<perluniintro> for an overview and |
386 | L<perlunicode> for details, and L<perlunicode/"Security Implications |
387 | of Unicode"> for security implications in particular. |
388 | |
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389 | =head2 Algorithmic Complexity Attacks |
390 | |
391 | Certain internal algorithms used in the implementation of Perl can |
392 | be attacked by choosing the input carefully to consume large amounts |
393 | of either time or space or both. This can lead into the so-called |
394 | I<Denial of Service> (DoS) attacks. |
395 | |
396 | =over 4 |
397 | |
398 | =item * |
399 | |
400 | Hash Function - the algorithm used to "order" hash elements has been |
401 | changed several times during the development of Perl, mainly to be |
402 | reasonably fast. In Perl 5.8.1 also the security aspect was taken |
403 | into account. |
404 | |
405 | In Perls before 5.8.1 one could rather easily generate data that as |
406 | hash keys would cause Perl to consume large amounts of time because |
407 | internal structure of hashes would badly degenerate. In Perl 5.8.1 |
408 | the hash function is randomly perturbed by a pseudorandom seed which |
409 | makes generating such naughty hash keys harder. |
410 | See L<perlrun/PERL_HASH_SEED> for more information. |
411 | |
412 | The random perturbation is done by default but if one wants for some |
413 | reason emulate the old behaviour one can set the environment variable |
414 | PERL_HASH_SEED to zero (or any other integer). One possible reason |
415 | for wanting to emulate the old behaviour is that in the new behaviour |
416 | consecutive runs of Perl will order hash keys differently, which may |
417 | confuse some applications (like Data::Dumper: the outputs of two |
418 | different runs are no more identical). |
419 | |
7b3f7037 |
420 | B<Perl has never guaranteed any ordering of the hash keys>, and the |
421 | ordering has already changed several times during the lifetime of |
422 | Perl 5. Also, the ordering of hash keys has always been, and |
423 | continues to be, affected by the insertion order. |
424 | |
425 | Also note that while the order of the hash elements might be |
426 | randomised, this "pseudoordering" should B<not> be used for |
427 | applications like shuffling a list randomly (use List::Util::shuffle() |
428 | for that, see L<List::Util>, a standard core module since Perl 5.8.0; |
429 | or the CPAN module Algorithm::Numerical::Shuffle), or for generating |
430 | permutations (use e.g. the CPAN modules Algorithm::Permute or |
431 | Algorithm::FastPermute), or for any cryptographic applications. |
432 | |
504f80c1 |
433 | =item * |
434 | |
435 | Regular expressions - Perl's regular expression engine is so called |
436 | NFA (Non-Finite Automaton), which among other things means that it can |
437 | rather easily consume large amounts of both time and space if the |
438 | regular expression may match in several ways. Careful crafting of the |
439 | regular expressions can help but quite often there really isn't much |
440 | one can do (the book "Mastering Regular Expressions" is required |
441 | reading, see L<perlfaq2>). Running out of space manifests itself by |
442 | Perl running out of memory. |
443 | |
444 | =item * |
445 | |
446 | Sorting - the quicksort algorithm used in Perls before 5.8.0 to |
447 | implement the sort() function is very easy to trick into misbehaving |
448 | so that it consumes a lot of time. Nothing more is required than |
449 | resorting a list already sorted. Starting from Perl 5.8.0 a different |
450 | sorting algorithm, mergesort, is used. Mergesort is insensitive to |
451 | its input data, so it cannot be similarly fooled. |
452 | |
453 | =back |
454 | |
455 | See L<http://www.cs.rice.edu/~scrosby/hash/> for more information, |
456 | and any computer science text book on the algorithmic complexity. |
457 | |
5a964f20 |
458 | =head1 SEE ALSO |
459 | |
460 | L<perlrun> for its description of cleaning up environment variables. |