=head1 DESCRIPTION
-Perl is designed to make it easy to write secure setuid and setgid
-scripts. Unlike shells, which are based on multiple substitution
-passes on each line of the script, Perl uses a more conventional
-evaluation scheme with fewer hidden "gotchas". Additionally, since the
-language has more built-in functionality, it has to rely less upon
-external (and possibly untrustworthy) programs to accomplish its
-purposes.
+Perl is designed to make it easy to program securely even when running
+with extra privileges, like setuid or setgid programs. Unlike most
+command line shells, which are based on multiple substitution passes on
+each line of the script, Perl uses a more conventional evaluation scheme
+with fewer hidden snags. Additionally, because the language has more
+builtin functionality, it can rely less upon external (and possibly
+untrustworthy) programs to accomplish its purposes.
-Beyond the obvious problems that stem from giving special privileges to
-such flexible systems as scripts, on many operating systems, setuid
-scripts are inherently insecure right from the start. This is because
-that between the time that the kernel opens up the file to see what to
-run, and when the now setuid interpreter it ran turns around and reopens
-the file so it can interpret it, things may have changed, especially if
-you have symbolic links on your system.
+Perl automatically enables a set of special security checks, called I<taint
+mode>, when it detects its program running with differing real and effective
+user or group IDs. The setuid bit in Unix permissions is mode 04000, the
+setgid bit mode 02000; either or both may be set. You can also enable taint
+mode explicitly by using the B<-T> command line flag. This flag is
+I<strongly> suggested for server programs and any program run on behalf of
+someone else, such as a CGI script. Once taint mode is on, it's on for
+the remainder of your script.
-Fortunately, sometimes this kernel "feature" can be disabled.
-Unfortunately, there are two ways to disable it. The system can simply
-outlaw scripts with the setuid bit set, which doesn't help much.
-Alternately, it can simply ignore the setuid bit on scripts. If the
-latter is true, Perl can emulate the setuid and setgid mechanism when it
-notices the otherwise useless setuid/gid bits on Perl scripts. It does
-this via a special executable called B<suidperl> that is automatically
-invoked for you if it's needed.
+While in this mode, Perl takes special precautions called I<taint
+checks> to prevent both obvious and subtle traps. Some of these checks
+are reasonably simple, such as verifying that path directories aren't
+writable by others; careful programmers have always used checks like
+these. Other checks, however, are best supported by the language itself,
+and it is these checks especially that contribute to making a set-id Perl
+program more secure than the corresponding C program.
+
+You may not use data derived from outside your program to affect
+something else outside your program--at least, not by accident. All
+command line arguments, environment variables, locale information (see
+L<perllocale>), results of certain system calls (C<readdir()>,
+C<readlink()>, the variable of C<shmread()>, the messages returned by
+C<msgrcv()>, the password, gcos and shell fields returned by the
+C<getpwxxx()> calls), and all file input are marked as "tainted".
+Tainted data may not be used directly or indirectly in any command
+that invokes a sub-shell, nor in any command that modifies files,
+directories, or processes, B<with the following exceptions>:
+
+=over 4
+
+=item *
+
+Arguments to C<print> and C<syswrite> are B<not> checked for taintedness.
+
+=item *
+
+Symbolic methods
+
+ $obj->$method(@args);
+
+and symbolic sub references
+
+ &{$foo}(@args);
+ $foo->(@args);
+
+are not checked for taintedness. This requires extra carefulness
+unless you want external data to affect your control flow. Unless
+you carefully limit what these symbolic values are, people are able
+to call functions B<outside> your Perl code, such as POSIX::system,
+in which case they are able to run arbitrary external code.
+
+=back
+
+For efficiency reasons, Perl takes a conservative view of
+whether data is tainted. If an expression contains tainted data,
+any subexpression may be considered tainted, even if the value
+of the subexpression is not itself affected by the tainted data.
+
+Because taintedness is associated with each scalar value, some
+elements of an array or hash can be tainted and others not.
+The keys of a hash are never tainted.
-If, however, the kernel setuid script feature isn't disabled, Perl will
-complain loudly that your setuid script is insecure. You'll need to
-either disable the kernel setuid script feature, or put a C wrapper around
-the script. See the program B<wrapsuid> in the F<eg> directory of your
-Perl distribution for how to go about doing this.
-
-There are some systems on which setuid scripts are free of this inherent
-security bug. For example, recent releases of Solaris are like this. On
-such systems, when the kernel passes the name of the setuid script to open
-to the interpreter, rather than using a pathname subject to mettling, it
-instead passes /dev/fd/3. This is a special file already opened on the
-script, so that there can be no race condition for evil scripts to
-exploit. On these systems, Perl should be compiled with
-C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B<Configure> program that builds
-Perl tries to figure this out for itself.
-
-When executing a setuid script, or when you have turned on taint checking
-explicitly using the B<-T> flag, Perl takes special precautions to
-prevent you from falling into any obvious traps. (In some ways, a Perl
-script is more secure than the corresponding C program.) Any command line
-argument, environment variable, or input is marked as "tainted", and may
-not be used, directly or indirectly, in any command that invokes a
-subshell, or in any command that modifies files, directories, or
-processes. Any variable that is set within an expression that has
-previously referenced a tainted value also becomes tainted (even if it is
-logically impossible for the tainted value to influence the variable).
For example:
- $foo = shift; # $foo is tainted
- $bar = $foo,'bar'; # $bar is also tainted
- $xxx = <>; # Tainted
+ $arg = shift; # $arg is tainted
+ $hid = $arg, 'bar'; # $hid is also tainted
+ $line = <>; # Tainted
+ $line = <STDIN>; # Also tainted
+ open FOO, "/home/me/bar" or die $!;
+ $line = <FOO>; # Still tainted
$path = $ENV{'PATH'}; # Tainted, but see below
- $abc = 'abc'; # Not tainted
+ $data = 'abc'; # Not tainted
- system "echo $foo"; # Insecure
- system "/bin/echo", $foo; # Secure (doesn't use sh)
- system "echo $bar"; # Insecure
- system "echo $abc"; # Insecure until PATH set
+ system "echo $arg"; # Insecure
+ system "/bin/echo", $arg; # Considered insecure
+ # (Perl doesn't know about /bin/echo)
+ system "echo $hid"; # Insecure
+ system "echo $data"; # Insecure until PATH set
+
+ $path = $ENV{'PATH'}; # $path now tainted
$ENV{'PATH'} = '/bin:/usr/bin';
- $ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
+ delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
+
+ $path = $ENV{'PATH'}; # $path now NOT tainted
+ system "echo $data"; # Is secure now!
+
+ open(FOO, "< $arg"); # OK - read-only file
+ open(FOO, "> $arg"); # Not OK - trying to write
- $path = $ENV{'PATH'}; # Not tainted
- system "echo $abc"; # Is secure now!
+ open(FOO,"echo $arg|"); # Not OK
+ open(FOO,"-|")
+ or exec 'echo', $arg; # Also not OK
- open(FOO,"$foo"); # OK
- open(FOO,">$foo"); # Not OK
+ $shout = `echo $arg`; # Insecure, $shout now tainted
- open(FOO,"echo $foo|"); # Not OK, but...
- open(FOO,"-|") || exec 'echo', $foo; # OK
+ unlink $data, $arg; # Insecure
+ umask $arg; # Insecure
- $zzz = `echo $foo`; # Insecure, zzz tainted
+ exec "echo $arg"; # Insecure
+ exec "echo", $arg; # Insecure
+ exec "sh", '-c', $arg; # Very insecure!
- unlink $abc,$foo; # Insecure
- umask $foo; # Insecure
+ @files = <*.c>; # insecure (uses readdir() or similar)
+ @files = glob('*.c'); # insecure (uses readdir() or similar)
- exec "echo $foo"; # Insecure
- exec "echo", $foo; # Secure (doesn't use sh)
- exec "sh", '-c', $foo; # Considered secure, alas
+ # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would
+ # have used an external program to do the filename expansion; but in
+ # either case the result is tainted since the list of filenames comes
+ # from outside of the program.
-The taintedness is associated with each scalar value, so some elements
-of an array can be tainted, and others not.
+ $bad = ($arg, 23); # $bad will be tainted
+ $arg, `true`; # Insecure (although it isn't really)
If you try to do something insecure, you will get a fatal error saying
-something like "Insecure dependency" or "Insecure PATH". Note that you
-can still write an insecure system call or exec, but only by explicitly
-doing something like the last example above. You can also bypass the
-tainting mechanism by referencing subpatterns--Perl presumes that if
-you reference a substring using $1, $2, etc, you knew what you were
-doing when you wrote the pattern:
-
- $ARGV[0] =~ /^-P(\w+)$/;
- $printer = $1; # Not tainted
-
-This is fairly secure since C<\w+> doesn't match shell metacharacters.
-Use of C</.+/> would have been insecure, but Perl doesn't check for that,
-so you must be careful with your patterns. This is the I<ONLY> mechanism
-for untainting user supplied filenames if you want to do file operations
-on them (unless you make C<$E<gt>> equal to C<$E<lt>> ).
-
-For "Insecure $ENV{PATH}" messages, you need to set C<$ENV{'PATH'}> to a known
-value, and each directory in the path must be non-writable by the world.
-A frequently voiced gripe is that you can get this message even
-if the pathname to an executable is fully qualified. But Perl can't
-know that the executable in question isn't going to execute some other
-program depending on the PATH.
+something like "Insecure dependency" or "Insecure $ENV{PATH}".
+
+The exception to the principle of "one tainted value taints the whole
+expression" is with the ternary conditional operator C<?:>. Since code
+with a ternary conditional
+
+ $result = $tainted_value ? "Untainted" : "Also untainted";
+
+is effectively
+
+ if ( $tainted_value ) {
+ $result = "Untainted";
+ } else {
+ $result = "Also untainted";
+ }
+
+it doesn't make sense for C<$result> to be tainted.
+
+=head2 Laundering and Detecting Tainted Data
+
+To test whether a variable contains tainted data, and whose use would
+thus trigger an "Insecure dependency" message, you can use the
+C<tainted()> function of the Scalar::Util module, available in your
+nearby CPAN mirror, and included in Perl starting from the release 5.8.0.
+Or you may be able to use the following C<is_tainted()> function.
+
+ sub is_tainted {
+ return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
+ }
+
+This function makes use of the fact that the presence of tainted data
+anywhere within an expression renders the entire expression tainted. It
+would be inefficient for every operator to test every argument for
+taintedness. Instead, the slightly more efficient and conservative
+approach is used that if any tainted value has been accessed within the
+same expression, the whole expression is considered tainted.
+
+But testing for taintedness gets you only so far. Sometimes you have just
+to clear your data's taintedness. Values may be untainted by using them
+as keys in a hash; otherwise the only way to bypass the tainting
+mechanism is by referencing subpatterns from a regular expression match.
+Perl presumes that if you reference a substring using $1, $2, etc., that
+you knew what you were doing when you wrote the pattern. That means using
+a bit of thought--don't just blindly untaint anything, or you defeat the
+entire mechanism. It's better to verify that the variable has only good
+characters (for certain values of "good") rather than checking whether it
+has any bad characters. That's because it's far too easy to miss bad
+characters that you never thought of.
+
+Here's a test to make sure that the data contains nothing but "word"
+characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
+or a dot.
+
+ if ($data =~ /^([-\@\w.]+)$/) {
+ $data = $1; # $data now untainted
+ } else {
+ die "Bad data in '$data'"; # log this somewhere
+ }
+
+This is fairly secure because C</\w+/> doesn't normally match shell
+metacharacters, nor are dot, dash, or at going to mean something special
+to the shell. Use of C</.+/> would have been insecure in theory because
+it lets everything through, but Perl doesn't check for that. The lesson
+is that when untainting, you must be exceedingly careful with your patterns.
+Laundering data using regular expression is the I<only> mechanism for
+untainting dirty data, unless you use the strategy detailed below to fork
+a child of lesser privilege.
+
+The example does not untaint C<$data> if C<use locale> is in effect,
+because the characters matched by C<\w> are determined by the locale.
+Perl considers that locale definitions are untrustworthy because they
+contain data from outside the program. If you are writing a
+locale-aware program, and want to launder data with a regular expression
+containing C<\w>, put C<no locale> ahead of the expression in the same
+block. See L<perllocale/SECURITY> for further discussion and examples.
+
+=head2 Switches On the "#!" Line
+
+When you make a script executable, in order to make it usable as a
+command, the system will pass switches to perl from the script's #!
+line. Perl checks that any command line switches given to a setuid
+(or setgid) script actually match the ones set on the #! line. Some
+Unix and Unix-like environments impose a one-switch limit on the #!
+line, so you may need to use something like C<-wU> instead of C<-w -U>
+under such systems. (This issue should arise only in Unix or
+Unix-like environments that support #! and setuid or setgid scripts.)
+
+=head2 Taint mode and @INC
+
+When the taint mode (C<-T>) is in effect, the "." directory is removed
+from C<@INC>, and the environment variables C<PERL5LIB> and C<PERLLIB>
+are ignored by Perl. You can still adjust C<@INC> from outside the
+program by using the C<-I> command line option as explained in
+L<perlrun>. The two environment variables are ignored because
+they are obscured, and a user running a program could be unaware that
+they are set, whereas the C<-I> option is clearly visible and
+therefore permitted.
+
+Another way to modify C<@INC> without modifying the program, is to use
+the C<lib> pragma, e.g.:
+
+ perl -Mlib=/foo program
+
+The benefit of using C<-Mlib=/foo> over C<-I/foo>, is that the former
+will automagically remove any duplicated directories, while the later
+will not.
+
+Note that if a tainted string is added to C<@INC>, the following
+problem will be reported:
+
+ Insecure dependency in require while running with -T switch
+
+=head2 Cleaning Up Your Path
+
+For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to
+a known value, and each directory in the path must be absolute and
+non-writable by others than its owner and group. You may be surprised to
+get this message even if the pathname to your executable is fully
+qualified. This is I<not> generated because you didn't supply a full path
+to the program; instead, it's generated because you never set your PATH
+environment variable, or you didn't set it to something that was safe.
+Because Perl can't guarantee that the executable in question isn't itself
+going to turn around and execute some other program that is dependent on
+your PATH, it makes sure you set the PATH.
+
+The PATH isn't the only environment variable which can cause problems.
+Because some shells may use the variables IFS, CDPATH, ENV, and
+BASH_ENV, Perl checks that those are either empty or untainted when
+starting subprocesses. You may wish to add something like this to your
+setid and taint-checking scripts.
+
+ delete @ENV{qw(IFS CDPATH ENV BASH_ENV)}; # Make %ENV safer
It's also possible to get into trouble with other operations that don't
care whether they use tainted values. Make judicious use of the file
tests in dealing with any user-supplied filenames. When possible, do
-opens and such after setting C<$E<gt> = $E<lt>>. (Remember group IDs,
-too!) Perl doesn't prevent you from opening tainted filenames for reading,
+opens and such B<after> properly dropping any special user (or group!)
+privileges. Perl doesn't prevent you from opening tainted filenames for reading,
so be careful what you print out. The tainting mechanism is intended to
prevent stupid mistakes, not to remove the need for thought.
-This gives us a reasonably safe way to open a file or pipe: just reset the
-id set to the original IDs. Here's a way to do backticks reasonably
-safely. Notice how the exec() is not called with a string that the shell
-could expand. By the time we get to the exec(), tainting is turned off,
-however, so be careful what you call and what you pass it.
-
- die unless defined $pid = open(KID, "-|");
- if ($pid) { # parent
- while (<KID>) {
- # do something
- }
- close KID;
- } else {
- $> = $<;
- $) = $(; # BUG: initgroups() not called
- exec 'program', 'arg1', 'arg2';
- die "can't exec program: $!";
- }
-
-For those even more concerned about safety, see the I<Safe> and I<Safe CGI>
-modules at a CPAN site near you. See L<perlmod> for a list of CPAN sites.
+Perl does not call the shell to expand wild cards when you pass C<system>
+and C<exec> explicit parameter lists instead of strings with possible shell
+wildcards in them. Unfortunately, the C<open>, C<glob>, and
+backtick functions provide no such alternate calling convention, so more
+subterfuge will be required.
+
+Perl provides a reasonably safe way to open a file or pipe from a setuid
+or setgid program: just create a child process with reduced privilege who
+does the dirty work for you. First, fork a child using the special
+C<open> syntax that connects the parent and child by a pipe. Now the
+child resets its ID set and any other per-process attributes, like
+environment variables, umasks, current working directories, back to the
+originals or known safe values. Then the child process, which no longer
+has any special permissions, does the C<open> or other system call.
+Finally, the child passes the data it managed to access back to the
+parent. Because the file or pipe was opened in the child while running
+under less privilege than the parent, it's not apt to be tricked into
+doing something it shouldn't.
+
+Here's a way to do backticks reasonably safely. Notice how the C<exec> is
+not called with a string that the shell could expand. This is by far the
+best way to call something that might be subjected to shell escapes: just
+never call the shell at all.
+
+ use English '-no_match_vars';
+ die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
+ if ($pid) { # parent
+ while (<KID>) {
+ # do something
+ }
+ close KID;
+ } else {
+ my @temp = ($EUID, $EGID);
+ my $orig_uid = $UID;
+ my $orig_gid = $GID;
+ $EUID = $UID;
+ $EGID = $GID;
+ # Drop privileges
+ $UID = $orig_uid;
+ $GID = $orig_gid;
+ # Make sure privs are really gone
+ ($EUID, $EGID) = @temp;
+ die "Can't drop privileges"
+ unless $UID == $EUID && $GID eq $EGID;
+ $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH.
+ # Consider sanitizing the environment even more.
+ exec 'myprog', 'arg1', 'arg2'
+ or die "can't exec myprog: $!";
+ }
+
+A similar strategy would work for wildcard expansion via C<glob>, although
+you can use C<readdir> instead.
+
+Taint checking is most useful when although you trust yourself not to have
+written a program to give away the farm, you don't necessarily trust those
+who end up using it not to try to trick it into doing something bad. This
+is the kind of security checking that's useful for set-id programs and
+programs launched on someone else's behalf, like CGI programs.
+
+This is quite different, however, from not even trusting the writer of the
+code not to try to do something evil. That's the kind of trust needed
+when someone hands you a program you've never seen before and says, "Here,
+run this." For that kind of safety, check out the Safe module,
+included standard in the Perl distribution. This module allows the
+programmer to set up special compartments in which all system operations
+are trapped and namespace access is carefully controlled.
+
+=head2 Security Bugs
+
+Beyond the obvious problems that stem from giving special privileges to
+systems as flexible as scripts, on many versions of Unix, set-id scripts
+are inherently insecure right from the start. The problem is a race
+condition in the kernel. Between the time the kernel opens the file to
+see which interpreter to run and when the (now-set-id) interpreter turns
+around and reopens the file to interpret it, the file in question may have
+changed, especially if you have symbolic links on your system.
+
+Fortunately, sometimes this kernel "feature" can be disabled.
+Unfortunately, there are two ways to disable it. The system can simply
+outlaw scripts with any set-id bit set, which doesn't help much.
+Alternately, it can simply ignore the set-id bits on scripts. If the
+latter is true, Perl can emulate the setuid and setgid mechanism when it
+notices the otherwise useless setuid/gid bits on Perl scripts. It does
+this via a special executable called F<suidperl> that is automatically
+invoked for you if it's needed.
+
+However, if the kernel set-id script feature isn't disabled, Perl will
+complain loudly that your set-id script is insecure. You'll need to
+either disable the kernel set-id script feature, or put a C wrapper around
+the script. A C wrapper is just a compiled program that does nothing
+except call your Perl program. Compiled programs are not subject to the
+kernel bug that plagues set-id scripts. Here's a simple wrapper, written
+in C:
+
+ #define REAL_PATH "/path/to/script"
+ main(ac, av)
+ char **av;
+ {
+ execv(REAL_PATH, av);
+ }
+
+Compile this wrapper into a binary executable and then make I<it> rather
+than your script setuid or setgid.
+
+In recent years, vendors have begun to supply systems free of this
+inherent security bug. On such systems, when the kernel passes the name
+of the set-id script to open to the interpreter, rather than using a
+pathname subject to meddling, it instead passes I</dev/fd/3>. This is a
+special file already opened on the script, so that there can be no race
+condition for evil scripts to exploit. On these systems, Perl should be
+compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F<Configure>
+program that builds Perl tries to figure this out for itself, so you
+should never have to specify this yourself. Most modern releases of
+SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.
+
+Prior to release 5.6.1 of Perl, bugs in the code of F<suidperl> could
+introduce a security hole.
+
+=head2 Protecting Your Programs
+
+There are a number of ways to hide the source to your Perl programs,
+with varying levels of "security".
+
+First of all, however, you I<can't> take away read permission, because
+the source code has to be readable in order to be compiled and
+interpreted. (That doesn't mean that a CGI script's source is
+readable by people on the web, though.) So you have to leave the
+permissions at the socially friendly 0755 level. This lets
+people on your local system only see your source.
+
+Some people mistakenly regard this as a security problem. If your program does
+insecure things, and relies on people not knowing how to exploit those
+insecurities, it is not secure. It is often possible for someone to
+determine the insecure things and exploit them without viewing the
+source. Security through obscurity, the name for hiding your bugs
+instead of fixing them, is little security indeed.
+
+You can try using encryption via source filters (Filter::* from CPAN,
+or Filter::Util::Call and Filter::Simple since Perl 5.8).
+But crackers might be able to decrypt it. You can try using the byte
+code compiler and interpreter described below, but crackers might be
+able to de-compile it. You can try using the native-code compiler
+described below, but crackers might be able to disassemble it. These
+pose varying degrees of difficulty to people wanting to get at your
+code, but none can definitively conceal it (this is true of every
+language, not just Perl).
+
+If you're concerned about people profiting from your code, then the
+bottom line is that nothing but a restrictive licence will give you
+legal security. License your software and pepper it with threatening
+statements like "This is unpublished proprietary software of XYZ Corp.
+Your access to it does not give you permission to use it blah blah
+blah." You should see a lawyer to be sure your licence's wording will
+stand up in court.
+
+=head2 Unicode
+
+Unicode is a new and complex technology and one may easily overlook
+certain security pitfalls. See L<perluniintro> for an overview and
+L<perlunicode> for details, and L<perlunicode/"Security Implications
+of Unicode"> for security implications in particular.
+
+=head2 Algorithmic Complexity Attacks
+
+Certain internal algorithms used in the implementation of Perl can
+be attacked by choosing the input carefully to consume large amounts
+of either time or space or both. This can lead into the so-called
+I<Denial of Service> (DoS) attacks.
+
+=over 4
+
+=item *
+
+Hash Function - the algorithm used to "order" hash elements has been
+changed several times during the development of Perl, mainly to be
+reasonably fast. In Perl 5.8.1 also the security aspect was taken
+into account.
+
+In Perls before 5.8.1 one could rather easily generate data that as
+hash keys would cause Perl to consume large amounts of time because
+internal structure of hashes would badly degenerate. In Perl 5.8.1
+the hash function is randomly perturbed by a pseudorandom seed which
+makes generating such naughty hash keys harder.
+See L<perlrun/PERL_HASH_SEED> for more information.
+
+The random perturbation is done by default but if one wants for some
+reason emulate the old behaviour one can set the environment variable
+PERL_HASH_SEED to zero (or any other integer). One possible reason
+for wanting to emulate the old behaviour is that in the new behaviour
+consecutive runs of Perl will order hash keys differently, which may
+confuse some applications (like Data::Dumper: the outputs of two
+different runs are no more identical).
+
+B<Perl has never guaranteed any ordering of the hash keys>, and the
+ordering has already changed several times during the lifetime of
+Perl 5. Also, the ordering of hash keys has always been, and
+continues to be, affected by the insertion order.
+
+Also note that while the order of the hash elements might be
+randomised, this "pseudoordering" should B<not> be used for
+applications like shuffling a list randomly (use List::Util::shuffle()
+for that, see L<List::Util>, a standard core module since Perl 5.8.0;
+or the CPAN module Algorithm::Numerical::Shuffle), or for generating
+permutations (use e.g. the CPAN modules Algorithm::Permute or
+Algorithm::FastPermute), or for any cryptographic applications.
+
+=item *
+
+Regular expressions - Perl's regular expression engine is so called
+NFA (Non-Finite Automaton), which among other things means that it can
+rather easily consume large amounts of both time and space if the
+regular expression may match in several ways. Careful crafting of the
+regular expressions can help but quite often there really isn't much
+one can do (the book "Mastering Regular Expressions" is required
+reading, see L<perlfaq2>). Running out of space manifests itself by
+Perl running out of memory.
+
+=item *
+
+Sorting - the quicksort algorithm used in Perls before 5.8.0 to
+implement the sort() function is very easy to trick into misbehaving
+so that it consumes a lot of time. Nothing more is required than
+resorting a list already sorted. Starting from Perl 5.8.0 a different
+sorting algorithm, mergesort, is used. Mergesort is insensitive to
+its input data, so it cannot be similarly fooled.
+
+=back
+
+See L<http://www.cs.rice.edu/~scrosby/hash/> for more information,
+and any computer science text book on the algorithmic complexity.
+
+=head1 SEE ALSO
+
+L<perlrun> for its description of cleaning up environment variables.
https://perl5.git.perl.org / perl5.git / blobdiff