-
=head1 NAME
perlsec - Perl security
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 the B<-T> command line flag. This flag is
+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.
-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,
+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 not blindly using the PATH inherited from one's parent
+process. Other checks, however, are best supported by the language itself,
and it is these checks especially that contribute to making a setuid 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, and file input are marked as "tainted".
-Tainted data may not be used directly or indirectly in any command that
-invokes a subshell, nor in any command that modifies files, directories,
-or processes. Any variable set within an expression that has previously
-referenced a tainted value itself becomes tainted, even if it is logically
-impossible for the tainted value to influence the variable. Because
-taintedness is associated with each scalar value, some elements of an
-array can be tainted and others not.
+arguments, environment variables, locale information (see L<perllocale>),
+and 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. Any variable set
+within an expression that has previously referenced a tainted value itself
+becomes tainted, even if it is logically impossible for the tainted value
+to influence 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
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 only gets you so far. Sometimes you just have
+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.
+mechanism is by referencing sub-patterns 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.
+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,
die "Bad data in $data"; # log this somewhere
}
-This is fairly secure since C</\w+/> doesn't normally match shell
+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
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 $ENV{PATH}" messages, you need to set C<$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.
+For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a
+known value. 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. 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.
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
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
+back-tick 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
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. Since the file or pipe was opened in the child while running
+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
+Here's a way to do back-ticks 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. By the time we get to the B<exec>, tainting
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