Perl is designed to make it easy to program securely even when runningwith extra privileges, like setuid or setgid programs. Unlike mostcommand line shells, which are based on multiple substitution passes oneach line of the script, Perl uses a more conventional evaluation schemewith fewer hidden snags. Additionally, because the language has morebuiltin functionality, it can rely less upon external (and possiblyuntrustworthy) programs to accomplish its purposes.

SECURITY VULNERABILITY CONTACT INFORMATION

If you believe you have found a security vulnerability in Perl, please [email protected] with details. This points to a closedsubscription, unarchived mailing list. Please only use this address forsecurity issues in the Perl core, not for modules independently distributed onCPAN.

SECURITY MECHANISMS AND CONCERNS

Taint mode

Perl automatically enables a set of special security checks, called taintmode, when it detects its program running with differing real and effectiveuser or group IDs. The setuid bit in Unix permissions is mode 04000, thesetgid bit mode 02000; either or both may be set. You can also enable taintmode explicitly by using the -T command line flag. This flag isstrongly suggested for server programs and any program run on behalf ofsomeone else, such as a CGI script. Once taint mode is on, it's on forthe remainder of your script.

While in this mode, Perl takes special precautions called taintchecks to prevent both obvious and subtle traps. Some of these checksare reasonably simple, such as verifying that path directories aren'twritable by others; careful programmers have always used checks likethese. Other checks, however, are best supported by the language itself,and it is these checks especially that contribute to making a set-id Perlprogram more secure than the corresponding C program.

You may not use data derived from outside your program to affectsomething else outside your program--at least, not by accident. Allcommand line arguments, environment variables, locale information (seeperllocale), results of certain system calls (readdir(),readlink(), the variable of shmread(), the messages returned bymsgrcv(), the password, gcos and shell fields returned by thegetpwxxx() calls), and all file input are marked as "tainted".Tainted data may not be used directly or indirectly in any commandthat invokes a sub-shell, nor in any command that modifies files,directories, or processes, with the following exceptions:

Arguments to print and syswrite are not checked for taintedness.
Symbolic methods
```
 $obj->$method(@args);
 
```
and symbolic sub references
```
 &{$foo}(@args);
  $foo->(@args);
 
```
are not checked for taintedness. This requires extra carefulnessunless you want external data to affect your control flow. Unlessyou carefully limit what these symbolic values are, people are ableto call functions outside your Perl code, such as POSIX::system,in which case they are able to run arbitrary external code.
Hash keys are never tainted.

For efficiency reasons, Perl takes a conservative view ofwhether data is tainted. If an expression contains tainted data,any subexpression may be considered tainted, even if the valueof the subexpression is not itself affected by the tainted data.

Because taintedness is associated with each scalar value, someelements of an array or hash can be tainted and others not.The keys of a hash are never tainted.

For example:

 $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
  $data = 'abc';# Not tainted
 
  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';
  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
 
  open(FOO,"echo $arg|");# Not OK
  open(FOO,"-|")
 or exec 'echo', $arg;# Also not OK
 
  $shout = `echo $arg`;# Insecure, $shout now tainted
 
  unlink $data, $arg;# Insecure
  umask $arg;# Insecure
 
  exec "echo $arg";# Insecure
  exec "echo", $arg;# Insecure
  exec "sh", '-c', $arg;# Very insecure!
 
  @files = <*.c>;# insecure (uses readdir() or similar)
  @files = glob('*.c');# insecure (uses readdir() or similar)
 
  # 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.
 
  $bad = ($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 sayingsomething like "Insecure dependency" or "Insecure $ENV{PATH}".

The exception to the principle of "one tainted value taints the wholeexpression" is with the ternary conditional operator ?:. Since codewith 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 $result to be tainted.

Laundering and Detecting Tainted Data

To test whether a variable contains tainted data, and whose use wouldthus trigger an "Insecure dependency" message, you can use thetainted() function of the Scalar::Util module, available in yournearby CPAN mirror, and included in Perl starting from the release 5.8.0.Or you may be able to use the following is_tainted() function.

 sub is_tainted {
  local $@;   # Don't pollute caller's value.
  return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
  }

This function makes use of the fact that the presence of tainted dataanywhere within an expression renders the entire expression tainted. Itwould be inefficient for every operator to test every argument fortaintedness. Instead, the slightly more efficient and conservativeapproach is used that if any tainted value has been accessed within thesame expression, the whole expression is considered tainted.

But testing for taintedness gets you only so far. Sometimes you have justto clear your data's taintedness. Values may be untainted by using themas keys in a hash; otherwise the only way to bypass the taintingmechanism is by referencing subpatterns from a regular expression match.Perl presumes that if you reference a substring using $1, $2, etc., thatyou knew what you were doing when you wrote the pattern. That means usinga bit of thought--don't just blindly untaint anything, or you defeat theentire mechanism. It's better to verify that the variable has only goodcharacters (for certain values of "good") rather than checking whether ithas any bad characters. That's because it's far too easy to miss badcharacters 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 /\w+/ doesn't normally match shellmetacharacters, nor are dot, dash, or at going to mean something specialto the shell. Use of /.+/ would have been insecure in theory becauseit lets everything through, but Perl doesn't check for that. The lessonis that when untainting, you must be exceedingly careful with your patterns.Laundering data using regular expression is the only mechanism foruntainting dirty data, unless you use the strategy detailed below to forka child of lesser privilege.

The example does not untaint $data if use locale is in effect,because the characters matched by \w are determined by the locale.Perl considers that locale definitions are untrustworthy because theycontain data from outside the program. If you are writing alocale-aware program, and want to launder data with a regular expressioncontaining \w, put no locale ahead of the expression in the sameblock. See SECURITY in perllocale for further discussion and examples.

Switches On the "#!" Line

When you make a script executable, in order to make it usable as acommand, 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. SomeUnix and Unix-like environments impose a one-switch limit on the #!line, so you may need to use something like -wU instead of -w -Uunder such systems. (This issue should arise only in Unix orUnix-like environments that support #! and setuid or setgid scripts.)

Taint mode and @INC

When the taint mode (-T) is in effect, the "." directory is removedfrom @INC, and the environment variables PERL5LIB and PERLLIBare ignored by Perl. You can still adjust @INC from outside theprogram by using the -I command line option as explained inperlrun. The two environment variables are ignored becausethey are obscured, and a user running a program could be unaware thatthey are set, whereas the -I option is clearly visible andtherefore permitted.

Another way to modify @INC without modifying the program, is to usethe lib pragma, e.g.:

  perl -Mlib=/foo program

The benefit of using -Mlib=/foo over -I/foo, is that the formerwill automagically remove any duplicated directories, while the laterwill not.

Note that if a tainted string is added to @INC, the followingproblem will be reported:

  Insecure dependency in require while running with -T switch

Cleaning Up Your Path

For "Insecure $ENV{PATH}" messages, you need to set $ENV{'PATH'} toa known value, and each directory in the path must be absolute andnon-writable by others than its owner and group. You may be surprised toget this message even if the pathname to your executable is fullyqualified. This is not generated because you didn't supply a full pathto the program; instead, it's generated because you never set your PATHenvironment variable, or you didn't set it to something that was safe.Because Perl can't guarantee that the executable in question isn't itselfgoing to turn around and execute some other program that is dependent onyour 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, andBASH_ENV, Perl checks that those are either empty or untainted whenstarting subprocesses. You may wish to add something like this to yoursetid 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'tcare whether they use tainted values. Make judicious use of the filetests in dealing with any user-supplied filenames. When possible, doopens and such 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 toprevent stupid mistakes, not to remove the need for thought.

Perl does not call the shell to expand wild cards when you pass systemand exec explicit parameter lists instead of strings with possible shellwildcards in them. Unfortunately, the open, glob, andbacktick functions provide no such alternate calling convention, so moresubterfuge will be required.

Perl provides a reasonably safe way to open a file or pipe from a setuidor setgid program: just create a child process with reduced privilege whodoes the dirty work for you. First, fork a child using the specialopen syntax that connects the parent and child by a pipe. Now thechild resets its ID set and any other per-process attributes, likeenvironment variables, umasks, current working directories, back to theoriginals or known safe values. Then the child process, which no longerhas any special permissions, does the open or other system call.Finally, the child passes the data it managed to access back to theparent. Because the file or pipe was opened in the child while runningunder less privilege than the parent, it's not apt to be tricked intodoing something it shouldn't.

Here's a way to do backticks reasonably safely. Notice how the exec isnot called with a string that the shell could expand. This is by far thebest way to call something that might be subjected to shell escapes: justnever 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 glob, althoughyou can use readdir instead.

Taint checking is most useful when although you trust yourself not to havewritten a program to give away the farm, you don't necessarily trust thosewho end up using it not to try to trick it into doing something bad. Thisis the kind of security checking that's useful for set-id programs andprograms launched on someone else's behalf, like CGI programs.

This is quite different, however, from not even trusting the writer of thecode not to try to do something evil. That's the kind of trust neededwhen someone hands you a program you've never seen before and says, "Here,run this." For that kind of safety, you might want to check out the Safemodule, included standard in the Perl distribution. This module allows theprogrammer to set up special compartments in which all system operationsare trapped and namespace access is carefully controlled. Safe shouldnot be considered bullet-proof, though: it will not prevent the foreigncode to set up infinite loops, allocate gigabytes of memory, or evenabusing perl bugs to make the host interpreter crash or behave inunpredictable ways. In any case it's better avoided completely if you'rereally concerned about security.

Security Bugs

Beyond the obvious problems that stem from giving special privileges tosystems as flexible as scripts, on many versions of Unix, set-id scriptsare inherently insecure right from the start. The problem is a racecondition in the kernel. Between the time the kernel opens the file tosee which interpreter to run and when the (now-set-id) interpreter turnsaround and reopens the file to interpret it, the file in question may havechanged, 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 simplyoutlaw scripts with any set-id bit set, which doesn't help much.Alternately, it can simply ignore the set-id bits on scripts.

However, if the kernel set-id script feature isn't disabled, Perl willcomplain loudly that your set-id script is insecure. You'll need toeither disable the kernel set-id script feature, or put a C wrapper aroundthe script. A C wrapper is just a compiled program that does nothingexcept call your Perl program. Compiled programs are not subject to thekernel bug that plagues set-id scripts. Here's a simple wrapper, writtenin 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 it ratherthan your script setuid or setgid.

In recent years, vendors have begun to supply systems free of thisinherent security bug. On such systems, when the kernel passes the nameof the set-id script to open to the interpreter, rather than using apathname subject to meddling, it instead passes /dev/fd/3. This is aspecial file already opened on the script, so that there can be no racecondition for evil scripts to exploit. On these systems, Perl should becompiled with -DSETUID_SCRIPTS_ARE_SECURE_NOW. The Configureprogram that builds Perl tries to figure this out for itself, so youshould never have to specify this yourself. Most modern releases ofSysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.

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 can't take away read permission, becausethe source code has to be readable in order to be compiled andinterpreted. (That doesn't mean that a CGI script's source isreadable by people on the web, though.) So you have to leave thepermissions 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 doesinsecure things, and relies on people not knowing how to exploit thoseinsecurities, it is not secure. It is often possible for someone todetermine the insecure things and exploit them without viewing thesource. Security through obscurity, the name for hiding your bugsinstead 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 bytecode compiler and interpreter described below, but crackers might beable to de-compile it. You can try using the native-code compilerdescribed below, but crackers might be able to disassemble it. Thesepose varying degrees of difficulty to people wanting to get at yourcode, but none can definitively conceal it (this is true of everylanguage, not just Perl).

If you're concerned about people profiting from your code, then thebottom line is that nothing but a restrictive license will give youlegal security. License your software and pepper it with threateningstatements like "This is unpublished proprietary software of XYZ Corp.Your access to it does not give you permission to use it blah blahblah." You should see a lawyer to be sure your license's wording willstand up in court.

Unicode

Unicode is a new and complex technology and one may easily overlookcertain security pitfalls. See perluniintro for an overview andperlunicode for details, and Security Implications of Unicode in perlunicode for security implications in particular.

Algorithmic Complexity Attacks

Certain internal algorithms used in the implementation of Perl canbe attacked by choosing the input carefully to consume large amountsof either time or space or both. This can lead into the so-calledDenial of Service (DoS) attacks.

Hash Function - the algorithm used to "order" hash elements has beenchanged several times during the development of Perl, mainly to bereasonably fast. In Perl 5.8.1 also the security aspect was takeninto account.
In Perls before 5.8.1 one could rather easily generate data that ashash keys would cause Perl to consume large amounts of time becauseinternal structure of hashes would badly degenerate. In Perl 5.8.1the hash function is randomly perturbed by a pseudorandom seed whichmakes generating such naughty hash keys harder.See PERL_HASH_SEED in perlrun for more information.
In Perl 5.8.1 the random perturbation was done by default, but as of5.8.2 it is only used on individual hashes if the internals detect theinsertion of pathological data. If one wants for some reason emulate theold behaviour (and expose oneself to DoS attacks) one can set theenvironment variable PERL_HASH_SEED to zero to disable the protection(or any other integer to force a known perturbation, rather than random). One possible reason for wanting to emulate the old behaviour is that in thenew behaviour consecutive runs of Perl will order hash keys differently,which may confuse some applications (like Data::Dumper: the outputs of twodifferent runs are no longer identical).
Perl has never guaranteed any ordering of the hash keys, and theordering has already changed several times during the lifetime ofPerl 5. Also, the ordering of hash keys has always been, andcontinues to be, affected by the insertion order.
Also note that while the order of the hash elements might berandomised, this "pseudoordering" should not be used forapplications like shuffling a list randomly (use List::Util::shuffle()for that, see List::Util, a standard core module since Perl 5.8.0;or the CPAN module Algorithm::Numerical::Shuffle), or for generatingpermutations (use e.g. the CPAN modules Algorithm::Permute orAlgorithm::FastPermute), or for any cryptographic applications.
Regular expressions - Perl's regular expression engine is so called NFA(Non-deterministic Finite Automaton), which among other things means thatit can rather easily consume large amounts of both time and space if theregular expression may match in several ways. Careful crafting of theregular expressions can help but quite often there really isn't muchone can do (the book "Mastering Regular Expressions" is requiredreading, see perlfaq2). Running out of space manifests itself byPerl running out of memory.
Sorting - the quicksort algorithm used in Perls before 5.8.0 toimplement the sort() function is very easy to trick into misbehavingso that it consumes a lot of time. Starting from Perl 5.8.0 a differentsorting algorithm, mergesort, is used by default. Mergesort cannotmisbehave on any input.

See http://www.cs.rice.edu/~scrosby/hash/ for more information,and any computer science textbook on algorithmic complexity.

Perl security

Daftar Isi

NAME

DESCRIPTION