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1 | =head1 NAME |
2 | |
3 | perlsec - Perl security |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | Perl is designed to make it easy to write secure setuid and setgid |
8 | scripts. Unlike shells, which are based on multiple substitution |
9 | passes on each line of the script, Perl uses a more conventional |
10 | evaluation scheme with fewer hidden "gotchas". Additionally, since the |
11 | language has more built-in functionality, it has to rely less upon |
12 | external (and possibly untrustworthy) programs to accomplish its |
13 | purposes. |
14 | |
15 | Beyond the obvious problems that stem from giving special privileges to |
16 | such flexible systems as scripts, on many operating systems, setuid |
17 | scripts are inherently insecure right from the start. This is because |
18 | that between the time that the kernel opens up the file to see what to |
19 | run, and when the now setuid interpreter it ran turns around and reopens |
20 | the file so it can interpret it, things may have changed, especially if |
21 | you have symbolic links on your system. |
22 | |
23 | Fortunately, sometimes this kernel "feature" can be disabled. |
24 | Unfortunately, there are two ways to disable it. The system can simply |
25 | outlaw scripts with the setuid bit set, which doesn't help much. |
26 | Alternately, it can simply ignore the setuid bit on scripts. If the |
27 | latter is true, Perl can emulate the setuid and setgid mechanism when it |
28 | notices the otherwise useless setuid/gid bits on Perl scripts. It does |
29 | this via a special executable called B<suidperl> that is automatically |
30 | invoked for you if it's needed. |
31 | |
32 | If, however, the kernel setuid script feature isn't disabled, Perl will |
33 | complain loudly that your setuid script is insecure. You'll need to |
34 | either disable the kernel setuid script feature, or put a C wrapper around |
35 | the script. See the program B<wrapsuid> in the F<eg> directory of your |
36 | Perl distribution for how to go about doing this. |
37 | |
38 | There are some systems on which setuid scripts are free of this inherent |
39 | security bug. For example, recent releases of Solaris are like this. On |
40 | such systems, when the kernel passes the name of the setuid script to open |
41 | to the interpreter, rather than using a pathname subject to mettling, it |
42 | instead passes /dev/fd/3. This is a special file already opened on the |
43 | script, so that there can be no race condition for evil scripts to |
44 | exploit. On these systems, Perl should be compiled with |
45 | C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure> program that builds |
46 | Perl tries to figure this out for itself. |
47 | |
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48 | When executing a setuid script, or when you have turned on taint checking |
49 | explicitly using the B<-T> flag, Perl takes special precautions to |
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50 | prevent you from falling into any obvious traps. (In some ways, a Perl |
51 | script is more secure than the corresponding C program.) Any command line |
52 | argument, environment variable, or input is marked as "tainted", and may |
53 | not be used, directly or indirectly, in any command that invokes a |
54 | subshell, or in any command that modifies files, directories, or |
55 | processes. Any variable that is set within an expression that has |
56 | previously referenced a tainted value also becomes tainted (even if it is |
57 | logically impossible for the tainted value to influence the variable). |
58 | For example: |
59 | |
60 | $foo = shift; # $foo is tainted |
61 | $bar = $foo,'bar'; # $bar is also tainted |
62 | $xxx = <>; # Tainted |
63 | $path = $ENV{'PATH'}; # Tainted, but see below |
64 | $abc = 'abc'; # Not tainted |
65 | |
66 | system "echo $foo"; # Insecure |
67 | system "/bin/echo", $foo; # Secure (doesn't use sh) |
68 | system "echo $bar"; # Insecure |
69 | system "echo $abc"; # Insecure until PATH set |
70 | |
71 | $ENV{'PATH'} = '/bin:/usr/bin'; |
72 | $ENV{'IFS'} = '' if $ENV{'IFS'} ne ''; |
73 | |
74 | $path = $ENV{'PATH'}; # Not tainted |
75 | system "echo $abc"; # Is secure now! |
76 | |
77 | open(FOO,"$foo"); # OK |
78 | open(FOO,">$foo"); # Not OK |
79 | |
80 | open(FOO,"echo $foo|"); # Not OK, but... |
81 | open(FOO,"-|") || exec 'echo', $foo; # OK |
82 | |
83 | $zzz = `echo $foo`; # Insecure, zzz tainted |
84 | |
85 | unlink $abc,$foo; # Insecure |
86 | umask $foo; # Insecure |
87 | |
88 | exec "echo $foo"; # Insecure |
89 | exec "echo", $foo; # Secure (doesn't use sh) |
90 | exec "sh", '-c', $foo; # Considered secure, alas |
91 | |
92 | The taintedness is associated with each scalar value, so some elements |
93 | of an array can be tainted, and others not. |
94 | |
95 | If you try to do something insecure, you will get a fatal error saying |
96 | something like "Insecure dependency" or "Insecure PATH". Note that you |
97 | can still write an insecure system call or exec, but only by explicitly |
98 | doing something like the last example above. You can also bypass the |
99 | tainting mechanism by referencing subpatterns--Perl presumes that if |
100 | you reference a substring using $1, $2, etc, you knew what you were |
101 | doing when you wrote the pattern: |
102 | |
103 | $ARGV[0] =~ /^-P(\w+)$/; |
104 | $printer = $1; # Not tainted |
105 | |
106 | This is fairly secure since C<\w+> doesn't match shell metacharacters. |
107 | Use of C</.+/> would have been insecure, but Perl doesn't check for that, |
108 | so you must be careful with your patterns. This is the I<ONLY> mechanism |
109 | for untainting user supplied filenames if you want to do file operations |
110 | on them (unless you make C<$E<gt>> equal to C<$E<lt>> ). |
111 | |
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112 | For "Insecure $ENV{PATH}" messages, you need to set C<$ENV{'PATH'}> to a known |
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113 | value, and each directory in the path must be non-writable by the world. |
114 | A frequently voiced gripe is that you can get this message even |
115 | if the pathname to an executable is fully qualified. But Perl can't |
116 | know that the executable in question isn't going to execute some other |
117 | program depending on the PATH. |
118 | |
119 | It's also possible to get into trouble with other operations that don't |
120 | care whether they use tainted values. Make judicious use of the file |
121 | tests in dealing with any user-supplied filenames. When possible, do |
122 | opens and such after setting C<$E<gt> = $E<lt>>. (Remember group IDs, |
123 | too!) Perl doesn't prevent you from opening tainted filenames for reading, |
124 | so be careful what you print out. The tainting mechanism is intended to |
125 | prevent stupid mistakes, not to remove the need for thought. |
126 | |
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127 | This gives us a reasonably safe way to open a file or pipe: just reset the |
128 | id set to the original IDs. Here's a way to do backticks reasonably |
129 | safely. Notice how the exec() is not called with a string that the shell |
130 | could expand. By the time we get to the exec(), tainting is turned off, |
131 | however, so be careful what you call and what you pass it. |
132 | |
133 | die unless defined $pid = open(KID, "-|"); |
134 | if ($pid) { # parent |
135 | while (<KID>) { |
136 | # do something |
137 | } |
138 | close KID; |
139 | } else { |
140 | $> = $<; |
141 | $) = $(; # BUG: initgroups() not called |
142 | exec 'program', 'arg1', 'arg2'; |
143 | die "can't exec program: $!"; |
144 | } |
145 | |
146 | For those even more concerned about safety, see the I<Safe> and I<Safe CGI> |
147 | modules at a CPAN site near you. See L<perlmod> for a list of CPAN sites. |