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diff --git a/contrib/perl5/pod/perlsec.pod b/contrib/perl5/pod/perlsec.pod new file mode 100644 index 0000000..0b22acd --- /dev/null +++ b/contrib/perl5/pod/perlsec.pod @@ -0,0 +1,351 @@ +=head1 NAME + +perlsec - Perl security + +=head1 DESCRIPTION + +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. + +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. + +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 (readdir, readlink, +the gecos field of getpw* 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<Important exception>: If you pass +a list of arguments to either C<system> or C<exec>, the elements of +that list are B<NOT> checked for taintedness.) Any variable set +to a value derived from tainted data will itself be tainted, +even if it is logically impossible for the tainted data +to alter the variable. Because taintedness is associated with each +scalar value, some elements of an array can be tainted and others not. + +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; # Secure (doesn't use sh) + 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, but... + open(FOO,"-|") + or exec 'echo', $arg; # OK + + $shout = `echo $arg`; # Insecure, $shout now tainted + + unlink $data, $arg; # Insecure + umask $arg; # Insecure + + exec "echo $arg"; # Insecure + exec "echo", $arg; # Secure (doesn't use the shell) + exec "sh", '-c', $arg; # Considered secure, alas! + + @files = <*.c>; # Always insecure (uses csh) + @files = glob('*.c'); # Always insecure (uses csh) + +If you try to do something insecure, you will get a fatal error saying +something like "Insecure dependency" or "Insecure $ENV{PATH}". Note that you +can still write an insecure B<system> or B<exec>, but only by explicitly +doing something like the "considered secure" example above. + +=head2 Laundering and Detecting Tainted Data + +To test whether a variable contains tainted data, and whose use would thus +trigger an "Insecure dependency" message, check your nearby CPAN mirror +for the F<Taint.pm> module, which should become available around November +1997. Or you may be able to use the following I<is_tainted()> function. + + sub is_tainted { + return ! eval { + join('',@_), kill 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. 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 $data if C<use locale> is in effect, +because the characters matched by C<\w> are determined by the locale. +Perl considers that locale definitions are untrustworthy because they +contain data from outside the program. If you are writing a +locale-aware program, and want to launder data with a regular expression +containing C<\w>, put C<no locale> ahead of the expression in the same +block. See L<perllocale/SECURITY> for further discussion and examples. + +=head2 Switches On the "#!" Line + +When you make a script executable, in order to make it usable as a +command, the system will pass switches to perl from the script's #! +line. Perl checks that any command line switches given to a setuid +(or setgid) script actually match the ones set on the #! line. Some +Unix and Unix-like environments impose a one-switch limit on the #! +line, so you may need to use something like C<-wU> instead of C<-w -U> +under such systems. (This issue should arise only in Unix or +Unix-like environments that support #! and setuid or setgid scripts.) + +=head2 Cleaning Up Your Path + +For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a +known value, and each directory in the path must be 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 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. + +Perl does not call the shell to expand wild cards when you pass B<system> +and B<exec> explicit parameter lists instead of strings with possible shell +wildcards in them. Unfortunately, the B<open>, B<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 +B<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 B<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 B<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; + die "Can't fork: $!" unless defined $pid = open(KID, "-|"); + if ($pid) { # parent + while (<KID>) { + # do something + } + close KID; + } else { + my @temp = ($EUID, $EGID); + $EUID = $UID; + $EGID = $GID; # initgroups() also called! + # Make sure privs are really gone + ($EUID, $EGID) = @temp; + die "Can't drop privileges" + unless $UID == $EUID && $GID eq $EGID; + $ENV{PATH} = "/bin:/usr/bin"; + 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 B<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. + +See the program B<wrapsuid> in the F<eg> directory of your Perl +distribution for a convenient way to do this automatically for all your +setuid Perl programs. It moves setuid scripts into files with the same +name plus a leading dot, and then compiles a wrapper like the one above +for each of them. + +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 B<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 5.003 of Perl, a bug in the code of B<suidperl> could +introduce a security hole in systems compiled with strict POSIX +compliance. + +=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). +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. + +=head1 SEE ALSO + +L<perlrun> for its description of cleaning up environment variables. |
