1=head1 NAME 2 3perlsec - Perl security 4 5=head1 DESCRIPTION 6 7Perl is designed to make it easy to program securely even when running 8with extra privileges, like setuid or setgid programs. Unlike most 9command line shells, which are based on multiple substitution passes on 10each line of the script, Perl uses a more conventional evaluation scheme 11with fewer hidden snags. Additionally, because the language has more 12builtin functionality, it can rely less upon external (and possibly 13untrustworthy) programs to accomplish its purposes. 14 15Perl automatically enables a set of special security checks, called I<taint 16mode>, when it detects its program running with differing real and effective 17user or group IDs. The setuid bit in Unix permissions is mode 04000, the 18setgid bit mode 02000; either or both may be set. You can also enable taint 19mode explicitly by using the B<-T> command line flag. This flag is 20I<strongly> suggested for server programs and any program run on behalf of 21someone else, such as a CGI script. Once taint mode is on, it's on for 22the remainder of your script. 23 24While in this mode, Perl takes special precautions called I<taint 25checks> to prevent both obvious and subtle traps. Some of these checks 26are reasonably simple, such as verifying that path directories aren't 27writable by others; careful programmers have always used checks like 28these. Other checks, however, are best supported by the language itself, 29and it is these checks especially that contribute to making a set-id Perl 30program more secure than the corresponding C program. 31 32You may not use data derived from outside your program to affect 33something else outside your program--at least, not by accident. All 34command line arguments, environment variables, locale information (see 35L<perllocale>), results of certain system calls (readdir(), 36readlink(), the variable of shmread(), the messages returned by 37msgrcv(), the password, gcos and shell fields returned by the 38getpwxxx() calls), and all file input are marked as "tainted". 39Tainted data may not be used directly or indirectly in any command 40that invokes a sub-shell, nor in any command that modifies files, 41directories, or processes, B<with the following exceptions>: 42 43=over 4 44 45=item * 46 47If you pass a list of arguments to either C<system> or C<exec>, 48the elements of that list are B<not> checked for taintedness. 49 50=item * 51 52Arguments to C<print> and C<syswrite> are B<not> checked for taintedness. 53 54=back 55 56Any variable set to a value 57derived from tainted data will itself be tainted, even if it is 58logically impossible for the tainted data to alter the variable. 59Because taintedness is associated with each scalar value, some 60elements of an array can be tainted and others not. 61 62For example: 63 64 $arg = shift; # $arg is tainted 65 $hid = $arg, 'bar'; # $hid is also tainted 66 $line = <>; # Tainted 67 $line = <STDIN>; # Also tainted 68 open FOO, "/home/me/bar" or die $!; 69 $line = <FOO>; # Still tainted 70 $path = $ENV{'PATH'}; # Tainted, but see below 71 $data = 'abc'; # Not tainted 72 73 system "echo $arg"; # Insecure 74 system "/bin/echo", $arg; # Secure (doesn't use sh) 75 system "echo $hid"; # Insecure 76 system "echo $data"; # Insecure until PATH set 77 78 $path = $ENV{'PATH'}; # $path now tainted 79 80 $ENV{'PATH'} = '/bin:/usr/bin'; 81 delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; 82 83 $path = $ENV{'PATH'}; # $path now NOT tainted 84 system "echo $data"; # Is secure now! 85 86 open(FOO, "< $arg"); # OK - read-only file 87 open(FOO, "> $arg"); # Not OK - trying to write 88 89 open(FOO,"echo $arg|"); # Not OK, but... 90 open(FOO,"-|") 91 or exec 'echo', $arg; # OK 92 93 $shout = `echo $arg`; # Insecure, $shout now tainted 94 95 unlink $data, $arg; # Insecure 96 umask $arg; # Insecure 97 98 exec "echo $arg"; # Insecure 99 exec "echo", $arg; # Secure (doesn't use the shell) 100 exec "sh", '-c', $arg; # Considered secure, alas! 101 102 @files = <*.c>; # insecure (uses readdir() or similar) 103 @files = glob('*.c'); # insecure (uses readdir() or similar) 104 105If you try to do something insecure, you will get a fatal error saying 106something like "Insecure dependency" or "Insecure $ENV{PATH}". Note that you 107can still write an insecure B<system> or B<exec>, but only by explicitly 108doing something like the "considered secure" example above. 109 110=head2 Laundering and Detecting Tainted Data 111 112To test whether a variable contains tainted data, and whose use would thus 113trigger an "Insecure dependency" message, check your nearby CPAN mirror 114for the F<Taint.pm> module, which should become available around November 1151997. Or you may be able to use the following I<is_tainted()> function. 116 117 sub is_tainted { 118 return ! eval { 119 join('',@_), kill 0; 120 1; 121 }; 122 } 123 124This function makes use of the fact that the presence of tainted data 125anywhere within an expression renders the entire expression tainted. It 126would be inefficient for every operator to test every argument for 127taintedness. Instead, the slightly more efficient and conservative 128approach is used that if any tainted value has been accessed within the 129same expression, the whole expression is considered tainted. 130 131But testing for taintedness gets you only so far. Sometimes you have just 132to clear your data's taintedness. The only way to bypass the tainting 133mechanism is by referencing subpatterns from a regular expression match. 134Perl presumes that if you reference a substring using $1, $2, etc., that 135you knew what you were doing when you wrote the pattern. That means using 136a bit of thought--don't just blindly untaint anything, or you defeat the 137entire mechanism. It's better to verify that the variable has only good 138characters (for certain values of "good") rather than checking whether it 139has any bad characters. That's because it's far too easy to miss bad 140characters that you never thought of. 141 142Here's a test to make sure that the data contains nothing but "word" 143characters (alphabetics, numerics, and underscores), a hyphen, an at sign, 144or a dot. 145 146 if ($data =~ /^([-\@\w.]+)$/) { 147 $data = $1; # $data now untainted 148 } else { 149 die "Bad data in $data"; # log this somewhere 150 } 151 152This is fairly secure because C</\w+/> doesn't normally match shell 153metacharacters, nor are dot, dash, or at going to mean something special 154to the shell. Use of C</.+/> would have been insecure in theory because 155it lets everything through, but Perl doesn't check for that. The lesson 156is that when untainting, you must be exceedingly careful with your patterns. 157Laundering data using regular expression is the I<only> mechanism for 158untainting dirty data, unless you use the strategy detailed below to fork 159a child of lesser privilege. 160 161The example does not untaint $data if C<use locale> is in effect, 162because the characters matched by C<\w> are determined by the locale. 163Perl considers that locale definitions are untrustworthy because they 164contain data from outside the program. If you are writing a 165locale-aware program, and want to launder data with a regular expression 166containing C<\w>, put C<no locale> ahead of the expression in the same 167block. See L<perllocale/SECURITY> for further discussion and examples. 168 169=head2 Switches On the "#!" Line 170 171When you make a script executable, in order to make it usable as a 172command, the system will pass switches to perl from the script's #! 173line. Perl checks that any command line switches given to a setuid 174(or setgid) script actually match the ones set on the #! line. Some 175Unix and Unix-like environments impose a one-switch limit on the #! 176line, so you may need to use something like C<-wU> instead of C<-w -U> 177under such systems. (This issue should arise only in Unix or 178Unix-like environments that support #! and setuid or setgid scripts.) 179 180=head2 Cleaning Up Your Path 181 182For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a 183known value, and each directory in the path must be non-writable by others 184than its owner and group. You may be surprised to get this message even 185if the pathname to your executable is fully qualified. This is I<not> 186generated because you didn't supply a full path to the program; instead, 187it's generated because you never set your PATH environment variable, or 188you didn't set it to something that was safe. Because Perl can't 189guarantee that the executable in question isn't itself going to turn 190around and execute some other program that is dependent on your PATH, it 191makes sure you set the PATH. 192 193The PATH isn't the only environment variable which can cause problems. 194Because some shells may use the variables IFS, CDPATH, ENV, and 195BASH_ENV, Perl checks that those are either empty or untainted when 196starting subprocesses. You may wish to add something like this to your 197setid and taint-checking scripts. 198 199 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer 200 201It's also possible to get into trouble with other operations that don't 202care whether they use tainted values. Make judicious use of the file 203tests in dealing with any user-supplied filenames. When possible, do 204opens and such B<after> properly dropping any special user (or group!) 205privileges. Perl doesn't prevent you from opening tainted filenames for reading, 206so be careful what you print out. The tainting mechanism is intended to 207prevent stupid mistakes, not to remove the need for thought. 208 209Perl does not call the shell to expand wild cards when you pass B<system> 210and B<exec> explicit parameter lists instead of strings with possible shell 211wildcards in them. Unfortunately, the B<open>, B<glob>, and 212backtick functions provide no such alternate calling convention, so more 213subterfuge will be required. 214 215Perl provides a reasonably safe way to open a file or pipe from a setuid 216or setgid program: just create a child process with reduced privilege who 217does the dirty work for you. First, fork a child using the special 218B<open> syntax that connects the parent and child by a pipe. Now the 219child resets its ID set and any other per-process attributes, like 220environment variables, umasks, current working directories, back to the 221originals or known safe values. Then the child process, which no longer 222has any special permissions, does the B<open> or other system call. 223Finally, the child passes the data it managed to access back to the 224parent. Because the file or pipe was opened in the child while running 225under less privilege than the parent, it's not apt to be tricked into 226doing something it shouldn't. 227 228Here's a way to do backticks reasonably safely. Notice how the B<exec> is 229not called with a string that the shell could expand. This is by far the 230best way to call something that might be subjected to shell escapes: just 231never call the shell at all. 232 233 use English; 234 die "Can't fork: $!" unless defined($pid = open(KID, "-|")); 235 if ($pid) { # parent 236 while (<KID>) { 237 # do something 238 } 239 close KID; 240 } else { 241 my @temp = ($EUID, $EGID); 242 my $orig_uid = $UID; 243 my $orig_gid = $GID; 244 $EUID = $UID; 245 $EGID = $GID; 246 # Drop privileges 247 $UID = $orig_uid; 248 $GID = $orig_gid; 249 # Make sure privs are really gone 250 ($EUID, $EGID) = @temp; 251 die "Can't drop privileges" 252 unless $UID == $EUID && $GID eq $EGID; 253 $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH. 254 # Consider sanitizing the environment even more. 255 exec 'myprog', 'arg1', 'arg2' 256 or die "can't exec myprog: $!"; 257 } 258 259A similar strategy would work for wildcard expansion via C<glob>, although 260you can use C<readdir> instead. 261 262Taint checking is most useful when although you trust yourself not to have 263written a program to give away the farm, you don't necessarily trust those 264who end up using it not to try to trick it into doing something bad. This 265is the kind of security checking that's useful for set-id programs and 266programs launched on someone else's behalf, like CGI programs. 267 268This is quite different, however, from not even trusting the writer of the 269code not to try to do something evil. That's the kind of trust needed 270when someone hands you a program you've never seen before and says, "Here, 271run this." For that kind of safety, check out the Safe module, 272included standard in the Perl distribution. This module allows the 273programmer to set up special compartments in which all system operations 274are trapped and namespace access is carefully controlled. 275 276=head2 Security Bugs 277 278Beyond the obvious problems that stem from giving special privileges to 279systems as flexible as scripts, on many versions of Unix, set-id scripts 280are inherently insecure right from the start. The problem is a race 281condition in the kernel. Between the time the kernel opens the file to 282see which interpreter to run and when the (now-set-id) interpreter turns 283around and reopens the file to interpret it, the file in question may have 284changed, especially if you have symbolic links on your system. 285 286Fortunately, sometimes this kernel "feature" can be disabled. 287Unfortunately, there are two ways to disable it. The system can simply 288outlaw scripts with any set-id bit set, which doesn't help much. 289Alternately, it can simply ignore the set-id bits on scripts. If the 290latter is true, Perl can emulate the setuid and setgid mechanism when it 291notices the otherwise useless setuid/gid bits on Perl scripts. It does 292this via a special executable called B<suidperl> that is automatically 293invoked for you if it's needed. 294 295However, if the kernel set-id script feature isn't disabled, Perl will 296complain loudly that your set-id script is insecure. You'll need to 297either disable the kernel set-id script feature, or put a C wrapper around 298the script. A C wrapper is just a compiled program that does nothing 299except call your Perl program. Compiled programs are not subject to the 300kernel bug that plagues set-id scripts. Here's a simple wrapper, written 301in C: 302 303 #define REAL_PATH "/path/to/script" 304 main(ac, av) 305 char **av; 306 { 307 execv(REAL_PATH, av); 308 } 309 310Compile this wrapper into a binary executable and then make I<it> rather 311than your script setuid or setgid. 312 313In recent years, vendors have begun to supply systems free of this 314inherent security bug. On such systems, when the kernel passes the name 315of the set-id script to open to the interpreter, rather than using a 316pathname subject to meddling, it instead passes I</dev/fd/3>. This is a 317special file already opened on the script, so that there can be no race 318condition for evil scripts to exploit. On these systems, Perl should be 319compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure> 320program that builds Perl tries to figure this out for itself, so you 321should never have to specify this yourself. Most modern releases of 322SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. 323 324Prior to release 5.6.1 of Perl, bugs in the code of B<suidperl> could 325introduce a security hole. 326 327=head2 Protecting Your Programs 328 329There are a number of ways to hide the source to your Perl programs, 330with varying levels of "security". 331 332First of all, however, you I<can't> take away read permission, because 333the source code has to be readable in order to be compiled and 334interpreted. (That doesn't mean that a CGI script's source is 335readable by people on the web, though.) So you have to leave the 336permissions at the socially friendly 0755 level. This lets 337people on your local system only see your source. 338 339Some people mistakenly regard this as a security problem. If your program does 340insecure things, and relies on people not knowing how to exploit those 341insecurities, it is not secure. It is often possible for someone to 342determine the insecure things and exploit them without viewing the 343source. Security through obscurity, the name for hiding your bugs 344instead of fixing them, is little security indeed. 345 346You can try using encryption via source filters (Filter::* from CPAN). 347But crackers might be able to decrypt it. You can try using the 348byte code compiler and interpreter described below, but crackers might 349be able to de-compile it. You can try using the native-code compiler 350described below, but crackers might be able to disassemble it. These 351pose varying degrees of difficulty to people wanting to get at your 352code, but none can definitively conceal it (this is true of every 353language, not just Perl). 354 355If you're concerned about people profiting from your code, then the 356bottom line is that nothing but a restrictive licence will give you 357legal security. License your software and pepper it with threatening 358statements like "This is unpublished proprietary software of XYZ Corp. 359Your access to it does not give you permission to use it blah blah 360blah." You should see a lawyer to be sure your licence's wording will 361stand up in court. 362 363=head1 SEE ALSO 364 365L<perlrun> for its description of cleaning up environment variables. 366