MAN page from RedHat Other perl-5.004-0.1.i386.rpm

PERLSEC

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

NAME

perlsec - Perl security

DESCRIPTION

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.

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 (seethe perllocale manpage), 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 anycommand that invokes a sub-shell, nor in any command that modifiesfiles, directories, or processes. Any variable setto a value derived from tainted data will itself be tainted,even if it is logically impossible for the tainted datato alter the variable. Because taintedness is associated with eachscalar 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 sayingsomething like ``Insecure dependency'' or ``Insecure PATH''. Note that youcan still write an insecure system or exec, but only by explicitlydoing something like the last example above.

Laundering and Detecting Tainted Data

To test whether a variable contains tainted data, and whose use would thustrigger an ``Insecure dependency'' message, check your nearby CPAN mirrorfor the Taint.pm module, which should become available around November1997. Or you may be able to use the following is_tainted() function.

    sub is_tainted {        return ! eval {            join('',@_), kill 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. 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 the SECURITY entry in the perllocale manpage 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.)

Cleaning Up Your Path

For ``Insecure $ENV{PATH}'' messages, you need to set $ENV{'PATH'} to aknown value, and each directory in the path must be non-writable by othersthan its owner and group. You may be surprised to get this message evenif the pathname to your executable is fully qualified. This is notgenerated because you didn't supply a full path to the program; instead,it's generated because you never set your PATH environment variable, oryou didn't set it to something that was safe. Because Perl can'tguarantee that the executable in question isn't itself going to turnaround and execute some other program that is dependent on your PATH, itmakes sure you set the PATH.

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;    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 = $GID;    # XXX: initgroups() not called        # Make sure privs are really gone        ($EUID, $EGID) = @temp;        die "Can't drop privileges" unless            $UID == $EUID and            $GID eq $EGID;      # String test        $ENV{PATH} = "/bin:/usr/bin";        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, check out the Safe module,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.

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. If thelatter is true, Perl can emulate the setuid and setgid mechanism when itnotices the otherwise useless setuid/gid bits on Perl scripts. It doesthis via a special executable called suidperl that is automaticallyinvoked for you if it's needed.

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.

See the program wrapsuid in the eg directory of your Perldistribution for a convenient way to do this automatically for all yoursetuid Perl programs. It moves setuid scripts into files with the samename plus a leading dot, and then compiles a wrapper like the one abovefor each of them.

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.

Prior to release 5.003 of Perl, a bug in the code of suidperl couldintroduce a security hole in systems compiled with strict POSIXcompliance.

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.

Some people 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).But crackers might be able to decrypt it. You can try using thebyte code compiler and interpreter described below, but crackers mightbe able 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 licence 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 licence's wording willstand up in court.

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