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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 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 builtin functionality, it can rely less upon external (and possibly 13 untrustworthy) programs to accomplish its purposes. 14 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 19 mode explicitly by using the B<-T> command line flag. This flag is 20 I<strongly> suggested for server programs and any program run on behalf of 21 someone else, such as a CGI script. Once taint mode is on, it's on for 22 the remainder of your script. 23 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, 29 and it is these checks especially that contribute to making a set-id Perl 30 program more secure than the corresponding C program. 31 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 35 L<perllocale>), results of certain system calls (C<readdir()>, 36 C<readlink()>, the variable of C<shmread()>, the messages returned by 37 C<msgrcv()>, the password, gcos and shell fields returned by the 38 C<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, 41 directories, or processes, B<with the following exceptions>: 42 43 =over 4 44 45 =item * 46 47 Arguments to C<print> and C<syswrite> are B<not> checked for taintedness. 48 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 66 =item * 67 68 Hash keys are B<never> tainted. 69 70 =back 71 72 For efficiency reasons, Perl takes a conservative view of 73 whether data is tainted. If an expression contains tainted data, 74 any subexpression may be considered tainted, even if the value 75 of the subexpression is not itself affected by the tainted data. 76 77 Because taintedness is associated with each scalar value, some 78 elements of an array or hash can be tainted and others not. 79 The keys of a hash are B<never> tainted. 80 81 For example: 82 83 $arg = shift; # $arg is tainted 84 $hid = $arg, 'bar'; # $hid is also tainted 85 $line = <>; # Tainted 86 $line = <STDIN>; # Also tainted 87 open FOO, "/home/me/bar" or die $!; 88 $line = <FOO>; # Still tainted 89 $path = $ENV{'PATH'}; # Tainted, but see below 90 $data = 'abc'; # Not tainted 91 92 system "echo $arg"; # Insecure 93 system "/bin/echo", $arg; # Considered insecure 94 # (Perl doesn't know about /bin/echo) 95 system "echo $hid"; # Insecure 96 system "echo $data"; # Insecure until PATH set 97 98 $path = $ENV{'PATH'}; # $path now tainted 99 100 $ENV{'PATH'} = '/bin:/usr/bin'; 101 delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; 102 103 $path = $ENV{'PATH'}; # $path now NOT tainted 104 system "echo $data"; # Is secure now! 105 106 open(FOO, "< $arg"); # OK - read-only file 107 open(FOO, "> $arg"); # Not OK - trying to write 108 109 open(FOO,"echo $arg|"); # Not OK 110 open(FOO,"-|") 111 or exec 'echo', $arg; # Also not OK 112 113 $shout = `echo $arg`; # Insecure, $shout now tainted 114 115 unlink $data, $arg; # Insecure 116 umask $arg; # Insecure 117 118 exec "echo $arg"; # Insecure 119 exec "echo", $arg; # Insecure 120 exec "sh", '-c', $arg; # Very insecure! 121 122 @files = <*.c>; # insecure (uses readdir() or similar) 123 @files = glob('*.c'); # insecure (uses readdir() or similar) 124 125 # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would 126 # have used an external program to do the filename expansion; but in 127 # either case the result is tainted since the list of filenames comes 128 # from outside of the program. 129 130 $bad = ($arg, 23); # $bad will be tainted 131 $arg, `true`; # Insecure (although it isn't really) 132 133 If you try to do something insecure, you will get a fatal error saying 134 something like "Insecure dependency" or "Insecure $ENV{PATH}". 135 136 The exception to the principle of "one tainted value taints the whole 137 expression" is with the ternary conditional operator C<?:>. Since code 138 with a ternary conditional 139 140 $result = $tainted_value ? "Untainted" : "Also untainted"; 141 142 is effectively 143 144 if ( $tainted_value ) { 145 $result = "Untainted"; 146 } else { 147 $result = "Also untainted"; 148 } 149 150 it doesn't make sense for C<$result> to be tainted. 151 152 =head2 Laundering and Detecting Tainted Data 153 154 To test whether a variable contains tainted data, and whose use would 155 thus trigger an "Insecure dependency" message, you can use the 156 C<tainted()> function of the Scalar::Util module, available in your 157 nearby CPAN mirror, and included in Perl starting from the release 5.8.0. 158 Or you may be able to use the following C<is_tainted()> function. 159 160 sub is_tainted { 161 return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 }; 162 } 163 164 This function makes use of the fact that the presence of tainted data 165 anywhere within an expression renders the entire expression tainted. It 166 would be inefficient for every operator to test every argument for 167 taintedness. Instead, the slightly more efficient and conservative 168 approach is used that if any tainted value has been accessed within the 169 same expression, the whole expression is considered tainted. 170 171 But testing for taintedness gets you only so far. Sometimes you have just 172 to clear your data's taintedness. Values may be untainted by using them 173 as keys in a hash; otherwise the only way to bypass the tainting 174 mechanism is by referencing subpatterns from a regular expression match. 175 Perl presumes that if you reference a substring using $1, $2, etc., that 176 you knew what you were doing when you wrote the pattern. That means using 177 a bit of thought--don't just blindly untaint anything, or you defeat the 178 entire mechanism. It's better to verify that the variable has only good 179 characters (for certain values of "good") rather than checking whether it 180 has any bad characters. That's because it's far too easy to miss bad 181 characters that you never thought of. 182 183 Here's a test to make sure that the data contains nothing but "word" 184 characters (alphabetics, numerics, and underscores), a hyphen, an at sign, 185 or a dot. 186 187 if ($data =~ /^([-\@\w.]+)$/) { 188 $data = $1; # $data now untainted 189 } else { 190 die "Bad data in '$data'"; # log this somewhere 191 } 192 193 This is fairly secure because C</\w+/> doesn't normally match shell 194 metacharacters, nor are dot, dash, or at going to mean something special 195 to the shell. Use of C</.+/> would have been insecure in theory because 196 it lets everything through, but Perl doesn't check for that. The lesson 197 is that when untainting, you must be exceedingly careful with your patterns. 198 Laundering data using regular expression is the I<only> mechanism for 199 untainting dirty data, unless you use the strategy detailed below to fork 200 a child of lesser privilege. 201 202 The example does not untaint C<$data> if C<use locale> is in effect, 203 because the characters matched by C<\w> are determined by the locale. 204 Perl considers that locale definitions are untrustworthy because they 205 contain data from outside the program. If you are writing a 206 locale-aware program, and want to launder data with a regular expression 207 containing C<\w>, put C<no locale> ahead of the expression in the same 208 block. See L<perllocale/SECURITY> for further discussion and examples. 209 210 =head2 Switches On the "#!" Line 211 212 When you make a script executable, in order to make it usable as a 213 command, the system will pass switches to perl from the script's #! 214 line. Perl checks that any command line switches given to a setuid 215 (or setgid) script actually match the ones set on the #! line. Some 216 Unix and Unix-like environments impose a one-switch limit on the #! 217 line, so you may need to use something like C<-wU> instead of C<-w -U> 218 under such systems. (This issue should arise only in Unix or 219 Unix-like environments that support #! and setuid or setgid scripts.) 220 221 =head2 Taint mode and @INC 222 223 When the taint mode (C<-T>) is in effect, the "." directory is removed 224 from C<@INC>, and the environment variables C<PERL5LIB> and C<PERLLIB> 225 are ignored by Perl. You can still adjust C<@INC> from outside the 226 program by using the C<-I> command line option as explained in 227 L<perlrun>. The two environment variables are ignored because 228 they are obscured, and a user running a program could be unaware that 229 they are set, whereas the C<-I> option is clearly visible and 230 therefore permitted. 231 232 Another way to modify C<@INC> without modifying the program, is to use 233 the C<lib> pragma, e.g.: 234 235 perl -Mlib=/foo program 236 237 The benefit of using C<-Mlib=/foo> over C<-I/foo>, is that the former 238 will automagically remove any duplicated directories, while the later 239 will not. 240 241 Note that if a tainted string is added to C<@INC>, the following 242 problem will be reported: 243 244 Insecure dependency in require while running with -T switch 245 246 =head2 Cleaning Up Your Path 247 248 For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to 249 a known value, and each directory in the path must be absolute and 250 non-writable by others than its owner and group. You may be surprised to 251 get this message even if the pathname to your executable is fully 252 qualified. This is I<not> generated because you didn't supply a full path 253 to the program; instead, it's generated because you never set your PATH 254 environment variable, or you didn't set it to something that was safe. 255 Because Perl can't guarantee that the executable in question isn't itself 256 going to turn around and execute some other program that is dependent on 257 your PATH, it makes sure you set the PATH. 258 259 The PATH isn't the only environment variable which can cause problems. 260 Because some shells may use the variables IFS, CDPATH, ENV, and 261 BASH_ENV, Perl checks that those are either empty or untainted when 262 starting subprocesses. You may wish to add something like this to your 263 setid and taint-checking scripts. 264 265 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer 266 267 It's also possible to get into trouble with other operations that don't 268 care whether they use tainted values. Make judicious use of the file 269 tests in dealing with any user-supplied filenames. When possible, do 270 opens and such B<after> properly dropping any special user (or group!) 271 privileges. Perl doesn't prevent you from opening tainted filenames for reading, 272 so be careful what you print out. The tainting mechanism is intended to 273 prevent stupid mistakes, not to remove the need for thought. 274 275 Perl does not call the shell to expand wild cards when you pass C<system> 276 and C<exec> explicit parameter lists instead of strings with possible shell 277 wildcards in them. Unfortunately, the C<open>, C<glob>, and 278 backtick functions provide no such alternate calling convention, so more 279 subterfuge will be required. 280 281 Perl provides a reasonably safe way to open a file or pipe from a setuid 282 or setgid program: just create a child process with reduced privilege who 283 does the dirty work for you. First, fork a child using the special 284 C<open> syntax that connects the parent and child by a pipe. Now the 285 child resets its ID set and any other per-process attributes, like 286 environment variables, umasks, current working directories, back to the 287 originals or known safe values. Then the child process, which no longer 288 has any special permissions, does the C<open> or other system call. 289 Finally, the child passes the data it managed to access back to the 290 parent. Because the file or pipe was opened in the child while running 291 under less privilege than the parent, it's not apt to be tricked into 292 doing something it shouldn't. 293 294 Here's a way to do backticks reasonably safely. Notice how the C<exec> is 295 not called with a string that the shell could expand. This is by far the 296 best way to call something that might be subjected to shell escapes: just 297 never call the shell at all. 298 299 use English '-no_match_vars'; 300 die "Can't fork: $!" unless defined($pid = open(KID, "-|")); 301 if ($pid) { # parent 302 while (<KID>) { 303 # do something 304 } 305 close KID; 306 } else { 307 my @temp = ($EUID, $EGID); 308 my $orig_uid = $UID; 309 my $orig_gid = $GID; 310 $EUID = $UID; 311 $EGID = $GID; 312 # Drop privileges 313 $UID = $orig_uid; 314 $GID = $orig_gid; 315 # Make sure privs are really gone 316 ($EUID, $EGID) = @temp; 317 die "Can't drop privileges" 318 unless $UID == $EUID && $GID eq $EGID; 319 $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH. 320 # Consider sanitizing the environment even more. 321 exec 'myprog', 'arg1', 'arg2' 322 or die "can't exec myprog: $!"; 323 } 324 325 A similar strategy would work for wildcard expansion via C<glob>, although 326 you can use C<readdir> instead. 327 328 Taint checking is most useful when although you trust yourself not to have 329 written a program to give away the farm, you don't necessarily trust those 330 who end up using it not to try to trick it into doing something bad. This 331 is the kind of security checking that's useful for set-id programs and 332 programs launched on someone else's behalf, like CGI programs. 333 334 This is quite different, however, from not even trusting the writer of the 335 code not to try to do something evil. That's the kind of trust needed 336 when someone hands you a program you've never seen before and says, "Here, 337 run this." For that kind of safety, check out the Safe module, 338 included standard in the Perl distribution. This module allows the 339 programmer to set up special compartments in which all system operations 340 are trapped and namespace access is carefully controlled. 341 342 =head2 Security Bugs 343 344 Beyond the obvious problems that stem from giving special privileges to 345 systems as flexible as scripts, on many versions of Unix, set-id scripts 346 are inherently insecure right from the start. The problem is a race 347 condition in the kernel. Between the time the kernel opens the file to 348 see which interpreter to run and when the (now-set-id) interpreter turns 349 around and reopens the file to interpret it, the file in question may have 350 changed, especially if you have symbolic links on your system. 351 352 Fortunately, sometimes this kernel "feature" can be disabled. 353 Unfortunately, there are two ways to disable it. The system can simply 354 outlaw scripts with any set-id bit set, which doesn't help much. 355 Alternately, it can simply ignore the set-id bits on scripts. If the 356 latter is true, Perl can emulate the setuid and setgid mechanism when it 357 notices the otherwise useless setuid/gid bits on Perl scripts. It does 358 this via a special executable called F<suidperl> that is automatically 359 invoked for you if it's needed. 360 361 However, if the kernel set-id script feature isn't disabled, Perl will 362 complain loudly that your set-id script is insecure. You'll need to 363 either disable the kernel set-id script feature, or put a C wrapper around 364 the script. A C wrapper is just a compiled program that does nothing 365 except call your Perl program. Compiled programs are not subject to the 366 kernel bug that plagues set-id scripts. Here's a simple wrapper, written 367 in C: 368 369 #define REAL_PATH "/path/to/script" 370 main(ac, av) 371 char **av; 372 { 373 execv(REAL_PATH, av); 374 } 375 376 Compile this wrapper into a binary executable and then make I<it> rather 377 than your script setuid or setgid. 378 379 In recent years, vendors have begun to supply systems free of this 380 inherent security bug. On such systems, when the kernel passes the name 381 of the set-id script to open to the interpreter, rather than using a 382 pathname subject to meddling, it instead passes I</dev/fd/3>. This is a 383 special file already opened on the script, so that there can be no race 384 condition for evil scripts to exploit. On these systems, Perl should be 385 compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F<Configure> 386 program that builds Perl tries to figure this out for itself, so you 387 should never have to specify this yourself. Most modern releases of 388 SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. 389 390 Prior to release 5.6.1 of Perl, bugs in the code of F<suidperl> could 391 introduce a security hole. 392 393 =head2 Protecting Your Programs 394 395 There are a number of ways to hide the source to your Perl programs, 396 with varying levels of "security". 397 398 First of all, however, you I<can't> take away read permission, because 399 the source code has to be readable in order to be compiled and 400 interpreted. (That doesn't mean that a CGI script's source is 401 readable by people on the web, though.) So you have to leave the 402 permissions at the socially friendly 0755 level. This lets 403 people on your local system only see your source. 404 405 Some people mistakenly regard this as a security problem. If your program does 406 insecure things, and relies on people not knowing how to exploit those 407 insecurities, it is not secure. It is often possible for someone to 408 determine the insecure things and exploit them without viewing the 409 source. Security through obscurity, the name for hiding your bugs 410 instead of fixing them, is little security indeed. 411 412 You can try using encryption via source filters (Filter::* from CPAN, 413 or Filter::Util::Call and Filter::Simple since Perl 5.8). 414 But crackers might be able to decrypt it. You can try using the byte 415 code compiler and interpreter described below, but crackers might be 416 able to de-compile it. You can try using the native-code compiler 417 described below, but crackers might be able to disassemble it. These 418 pose varying degrees of difficulty to people wanting to get at your 419 code, but none can definitively conceal it (this is true of every 420 language, not just Perl). 421 422 If you're concerned about people profiting from your code, then the 423 bottom line is that nothing but a restrictive licence will give you 424 legal security. License your software and pepper it with threatening 425 statements like "This is unpublished proprietary software of XYZ Corp. 426 Your access to it does not give you permission to use it blah blah 427 blah." You should see a lawyer to be sure your licence's wording will 428 stand up in court. 429 430 =head2 Unicode 431 432 Unicode is a new and complex technology and one may easily overlook 433 certain security pitfalls. See L<perluniintro> for an overview and 434 L<perlunicode> for details, and L<perlunicode/"Security Implications 435 of Unicode"> for security implications in particular. 436 437 =head2 Algorithmic Complexity Attacks 438 439 Certain internal algorithms used in the implementation of Perl can 440 be attacked by choosing the input carefully to consume large amounts 441 of either time or space or both. This can lead into the so-called 442 I<Denial of Service> (DoS) attacks. 443 444 =over 4 445 446 =item * 447 448 Hash Function - the algorithm used to "order" hash elements has been 449 changed several times during the development of Perl, mainly to be 450 reasonably fast. In Perl 5.8.1 also the security aspect was taken 451 into account. 452 453 In Perls before 5.8.1 one could rather easily generate data that as 454 hash keys would cause Perl to consume large amounts of time because 455 internal structure of hashes would badly degenerate. In Perl 5.8.1 456 the hash function is randomly perturbed by a pseudorandom seed which 457 makes generating such naughty hash keys harder. 458 See L<perlrun/PERL_HASH_SEED> for more information. 459 460 The random perturbation is done by default but if one wants for some 461 reason emulate the old behaviour one can set the environment variable 462 PERL_HASH_SEED to zero (or any other integer). One possible reason 463 for wanting to emulate the old behaviour is that in the new behaviour 464 consecutive runs of Perl will order hash keys differently, which may 465 confuse some applications (like Data::Dumper: the outputs of two 466 different runs are no more identical). 467 468 B<Perl has never guaranteed any ordering of the hash keys>, and the 469 ordering has already changed several times during the lifetime of 470 Perl 5. Also, the ordering of hash keys has always been, and 471 continues to be, affected by the insertion order. 472 473 Also note that while the order of the hash elements might be 474 randomised, this "pseudoordering" should B<not> be used for 475 applications like shuffling a list randomly (use List::Util::shuffle() 476 for that, see L<List::Util>, a standard core module since Perl 5.8.0; 477 or the CPAN module Algorithm::Numerical::Shuffle), or for generating 478 permutations (use e.g. the CPAN modules Algorithm::Permute or 479 Algorithm::FastPermute), or for any cryptographic applications. 480 481 =item * 482 483 Regular expressions - Perl's regular expression engine is so called NFA 484 (Non-deterministic Finite Automaton), which among other things means that 485 it can rather easily consume large amounts of both time and space if the 486 regular expression may match in several ways. Careful crafting of the 487 regular expressions can help but quite often there really isn't much 488 one can do (the book "Mastering Regular Expressions" is required 489 reading, see L<perlfaq2>). Running out of space manifests itself by 490 Perl running out of memory. 491 492 =item * 493 494 Sorting - the quicksort algorithm used in Perls before 5.8.0 to 495 implement the sort() function is very easy to trick into misbehaving 496 so that it consumes a lot of time. Nothing more is required than 497 resorting a list already sorted. Starting from Perl 5.8.0 a different 498 sorting algorithm, mergesort, is used. Mergesort is insensitive to 499 its input data, so it cannot be similarly fooled. 500 501 =back 502 503 See L<http://www.cs.rice.edu/~scrosby/hash/> for more information, 504 and any computer science textbook on the algorithmic complexity. 505 506 =head1 SEE ALSO 507 508 L<perlrun> for its description of cleaning up environment variables.
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