Polymorphism Script
by xdroop
#!/usr/local/bin/perl
# A script to demonstrate polymorphism.
# 
# Written 23 May 2000 by xdroop
#
# This script is a demonstration only.  Be careful with it.
#  I deny any responsibility for any variants or decendants
#  (including the demonstration itself after being run once).
#
# This script was written after the polymorphic variant of
#  the ILOVEYOU Outlook .vbs worm appeared.  The media reported
#  that the polymorphism demonstrated was merely writing itself
#  along with about 100 random comment characters interleaved
#  inside it.  That got me thinking -- every time it ran, it
#  would increase its size, and would quickly become too large
#  to spread effectively.
#
# A better strategy is to remove _all_ the existing comments,
#  then sprinkle random lines of comments every so often
#  as the script re-wrote itself.
#
# You see, the way the majority of these email virus detectors work is
#  they scan attachments looking for "signatures" -- that is, a known
#  sequence of characters at a known offset from the beginning or end
#  of a file.  By varying the number and length of the comments, any
#  character constants tend to move around, making signaturing a hit-or-miss
#  proposition at best.
#
# The next defense isn't exactly polymorphism.  It attempts
#  to make the script shorter and harder to read by attaching
#  short lines together.  This will only work so often; eventually,
#  all the lines in the script will be of a length that the
#  script will refuse to attach any more together.
#
# By combining these techniques, you end up with a script that
#  hovers around a certain size when run repeatedly -- but isn't
#  of a predictable size.
#
# The third defense was an off-the-cuff idea.  To make the
#  script even harder to read and follow, why not rename all
#  the variables and subroutine names as they were hit?
#
# Finally, the script has a couple of long string constants which are used
#  to select the characters permitted in random comments and mangled
#  variable/subroutine names, and the script re-writes these constants each
#  time it is run.
#
# Put together, this script looks remarkably like line noise
#  after run a couple of times...
#
# There are lots of ways that the code could be improved.  The
#  script has only been tested on itself, so can't be counted on
#  to morph abstract perl code.  The loadArray subroutine in particular
#  can't handle any characters which have special meaning in regexps.
#  I didn't do this to prove how elite I am, nor to show how hot a
#  coder I was.  I am neither elite nor a hot coder.  I am merely a
#  system administrator who fought both Melissa and ILOVEYOU and found
#  the common defenses lacking.  The ideas contained herein are interesting
#  problems, and the idea of defending against them is a similarly
#  interesting problem.  I like interesting problems.
#
# The reason why I did this in perl is because right now, the script is
#  completely harmless.  There is no trivial way to turn it into a world
#  eating email virus, although it could be extended to Trojan fairly
#  trivially.  This is merely a technology demonstrator.  Someone suitably
#  clever could write it up in .vbs -- I can't, since I don't know .vbs.
#
# Consider the ramifications if ILOVEYOU had the following improvements
#  in it's original release:
#   - select subject line at random from an email already in victim's inbox
#     or use a blank subject line
#   - a polymorpher similar to this
#
#  This theoretical virus would have eaten the outlook world alive, since
#   none of the immediate defenses (attachment signaturing, blocking known
#   carrier subject lines) would have worked.  As it was, the common
#   subject line block for Melissa could be defeated if the infected
#   victim computer used a different encoding for their text.  We saw
#   numerous examples of where a brazilian system would send an email
#   with the literal subject line
#     Subject: =?iso-8859-1?Q?Important_Message_From_$BRAZILIAN_VICTIM?=
#   ...and there was no way to make sendmail see that as an infected message.
#
#  ALL attachments would have to be denied, a prospect which
#   wouldn't be terrible to most security concious administrators.
#   VBS would have to be squashed in Outlook for once and for all.

# The script was written for perl 5.005_03 and has been run under
#  linux and solaris.  My guess is that it will run on any unix-like OS.
#  It may run on windows -- you'll definitely need to change the 'cp'
#  command to something Windows can do for you -- but I don't really
#  care, since I don't run Windows...
#
# Greetz to cyclone and dr dave -- two kindly gents from the old country.
#  Know where your $FUZZY_SQUEEKY_PINK_THING is, guys?  Didn't think so.
#
# don't worry about all the comments in the code -- running the
#  script once will fix that :)
#
# this is our name.
$BASENAME = $0;
$BASENAME =~ s|\\|/|g;
if ($BASENAME =~ m|(.*/)(.*)|)
{
  $BASENAME = $2;
}
# this is the maximum lines since comment
$mlsc=int(rand(4));
# this is the maximum line length, in characters
$mll=75;
# this is a list of variable names we don't want mangled.
#  note that this isn't an exaustive list, just enough
#  to make the script mangle itself.
@reserved=("0","","2","_","1",);
# this is a list of characters for use in fake comments.
#  since comments have more spaces than anything else,
#  there are lots in the source string.
$commentSource='`1234567890 -=~!@# %^&_ qwerty uiop QWERTYUIO P{}|a sdfghj kl;ASDFGHJ KL:"zx cv bnm,. /ZXC VBNM<> ';
# this is a list of characters  for use in variable and subroutine
#  names.  It is different because there are no spaces and things in subroutine
#  names (duh!)
$secondSource='1234567890qwertyuiopasdfghjklzxcvbnmMNBVCXZASDFGHJKLPOIUYTREWQ_';
# Load the selector arrays.  See the comment with the sub definition.
@a=&loadArray($commentSource);
@b=&loadArray($secondSource);
# if we exist
if (-e $BASENAME)
{
        # make a copy of ourselves
        `cp $BASENAME old-$BASENAME`;
        # open the files, complain if we can't
        open(IN,"<old-$BASENAME") or &die($!);
        open(OUT,">$BASENAME") or &die($!);
        # we haven't seen any comments yet
        $lsc=0;
        # for each line in the script
        while(<IN>)
        {
                # remove \n
                chop;
                # remove leading whitespace
                s/^\s*//;
                # don't bother if there is nothing left
                next if (!$_);
                # if we are not looking at a comment (\043 is the char code for #)
                if (!/^\w*\043/)
                {
                        # if we have not seen a comment in a while
                        if ($lsc > $mlsc)
                        {
                                # print the line being constructed and reset variables
                                print OUT "$output\n";
                                undef $output;
                                $n=&nc();
                                print OUT "$n\n";
                                $lsc=0;
                                $mlsc=int(rand(4));
                        }
                        # it's been another line since we saw a comment
                        $lsc++;
                        # go change all the variable and subroutine names
                        $r=&tokenizer($_);
                        # a clumsy bit of code to see if our candidate line is too long
                        $c_out="$output$r";
                        if (length($c_out) > $mll)
                        {
                                # it is too long, print the current line and then
                                # stick the new stuff in the holding area
                                print OUT "$output\n";
                                $output=$r;
                        }
                        else
                        {
                                # it isn't too long, so glue 'em together
                                $output=$output . $r;
                        }
                        # go do it again
                        next;
                }
                else
                {
                        # right, this is a comment
                        # so, if we have a she-bang (like #!/usr/local/bin/perl)
                        if (/^\#\!/)
                        {
                                # If we have not printed one already, just print it
                                if (!$SHEBANG)
                                {
                                        print OUT "$_\n";
                                        $SHEBANG=1;
                                }
                        }
                }
                # if we get here, it is a comment line that doesn't have
                # a she-bang, so it gets discarded by inaction.  Back to the
                # top of the while loop!
        }
        # print the stored output line since we are done
        print OUT "$output\n" if ($output);
        # we are done
        close OUT;
}
# sub nc generates random comments
# forgive the variable names, it was written before the
#  variable name mangler was written.
sub nc
{
        local($s,$r,$i,$c,$l);
        # store the length of the array with the comment characters
        #  (from right at the top)
        $l=@a;
        # $i is the index we'll generate randomly
        # $c is the number of characters already placed in the comment
        # $s is the string we are building, its a comment so put a # in it
        $i=0;$c=0;$s="\043";
        # $r is the actual length of the comment we'll build
        $r=int(rand(75))+1;
        # while we are not done
        while ($r > $c)
        {
                $c++;
                # pick a character
                $i=int(rand($l));
                # glue it on
                $s=$s.$a[$i];
        }
        return $s;
}
# sub tokenizer was originally going to be a dragon-book
# tokenizer, and I even had a basic rough out going, but
# then I realized that I could just use regular expressions
# to check to see what I had.
sub tokenizer
{
        # the string to mangle
        local ($string)=shift @_;
        # $return is the string we will return
        # $char is a holding area while we loop through things
        local ($return,$char);
        # while we still have string to work with
        while($string=~/^(.)/)
        {
                # grab the result of the match
                $char=$1;
                # strip the held character off the front of the string
                $string=~s/^.//;
                # check for trigger states
                if($char eq "\$" or $char eq "\@" or $char eq "\%" or $char eq "\&" or $char eq "s" or $char eq "\047")
                {
                        # right, we think we have something worth mangling.
                        # First thing to check is whether we are dealing with one of
                        #  our two possible "signature" strings -- $x and $z. If
                        #  we are, we can re-order the strings at random so that there
                        #  is no signature.
                        if ($char eq "\047")
                        {
                                # try to pick the rest of the string out
                                if($string =~ /^(.*)\047/)
                                {
                                        $candidate = $1;
                                        # check to see if it is one of the signature strings.
                                        if ($candidate eq $commentSource or $candidate eq $secondSource)
                                        {
                                                # scramble it
                                                $scrambled=&scramble($candidate);
                                                # ...now tack it on the output string and clean up the
                                                # source string.
                                                $return=$return.$char.$scrambled;
                                                $string=~s/.*\047//;
                                        }
                                }
                                # if we get here, we are in one of two cases: either we have
                                #  a string which isn't a signature, or we have a string which
                                #  isn't a string (probably a hit from the messy code, above).
                                #  In both cases, we need to slap the remaining " character
                                #  on the output string (either to close the string we just
                                #  rewrote, or to pass the beginning of our harmless string
                                #  on through) -- and then kick out of this trigger state
                                #  detector to the top of while loop.
                                $return=$return.$char;
                                next;
                        }
                        # special handling: subroutines.  With every other trigger
                        #  you can just glue the trigger on the return string, but
                        #  the subroutine trigger is three characters long.  So we
                        #  check for the whole trigger, then doctor both the source
                        #  and return strings so that they will work with the mangler
                        #  code written for the other trigger states.
                        if ($char eq "s")
                        {
                                # if this is a 'sub'
                                if ($string =~ /^ub /)
                                {
                                        # put the characters s,u,b, space intothe return string
                                        $return=$return."s"."ub ";
                                        # hack it off the source string
                                        $string=~s/^ub //;
                                }
                                else
                                {
                                        # ok, it isn't a subroutine, false alarm, glue it on
                                        # the return string and go back to thetop of the loop.
                                        $return=$return.$char;
                                        next;
                                }
                        }
                        else
                        {
                                # it isn't a sub, but it is one of the other trigger
                                #  states -- we're good, glue the trigger on the return
                                #  string.
                                $return=$return.$char;
                        }
                        # zap the name of the target from last time (important!)
                        undef $varname;
                        # clumsy loop time.  Grab the next character and if itisn't
                        #  a non-name character, glue it on the variable name and
                        #  hack it off the source string.
                        $string=~s/^(.)//;
                        $char=$1;
                        while ($char =~ /[a-zA-Z0-9_]/)
                        {
                                $varname=$varname.$char;
                                $string=~s/^(.)//;
                                $char=$1;
                        }
                        # assume that the variable name isn't a reserved name
                        $OK=1;
                        # check each reserved name.  If it matches our name we
                        #  just built, we can't mangle it.
                        foreach $name (@reserved)
                        {
                                $OK=0 if ($name eq $varname);
                        }
                        if ($OK)
                        {
                                # let's go mangle it!  If we have not see this name before...
                                if(!$lookup{$varname})
                                {
                                        # we go create a new name.
                                        $lookup{$varname}=&getNewVarName();
                                }
                                # and now, the mangling.
                                $varname=$lookup{$varname};
                        }
                        # glue the mangled (or not) varname on the output string.
                        $return=$return.$varname;
                        # we are still holding a character from the last loop,
                        #  glue it back on the input string and we go again.
                        $string=$char.$string;
                        next;
                }
                # we don't have a trigger state.  Just glue it on the output string.
                $return=$return.$char;
        }
        # we are out of input string, return it.
        return $return;
}
# sub getNewVarName generates the new variable/subroutine names.
sub getNewVarName
{
        # $name is the name we are building
        # $count is the number of characters we still have to add
        # $index is the index into the array of acceptable characters
        # $alength is a place to hold the length of the array
        local ($name,$count,$index,$alength);
        # hold the length
        $alength=@b;
        # determine how many characters to use -- between 3 and 8
        $count=int(rand(6))+3;
        # while we are not done
        while ($count > 0)
        {
                # another character
                $count--;
                # ok, if this is the first character in the name, we can't
                #  use any of the special characters (which in this context
                #  means 0-9 and _) because they have special meaning.  So
                #  we loop through the randomizer until we get one that isn't
                #  special.
                if (length($name) < 1)
                {
                        while($b[$index] =~ /[0-9_]/)
                        {
                                $index=int(rand($alength));
                        }
                        # got a character, use it
                        $name=$b[$index];
                        # back to the top of the loop with ya!
                        next;
                }
                # pick a card any card
                $index=int(rand($alength));
                # glue it on
                $name=$name.$b[$index];
        }
        # return it to the breathless masses
        return $name;
}
#
# scramble the supplied string so that it is different.
sub scramble
{
        local ($string,$scrambled,$count,$char,$number);
        # $string is the the input string.
        $string = pop (@_);
        # $count is the number of characters in our string.
        $count=length($string);
        # $scrambled is the scrambled string
        # $char is the character we are currently dealing with
        # $number is our random number between 0 and $count.
        while ($count)
        {
                $number=int(rand($count));
                $string=~m/^.{$number}(.)/;
                $char=$1;
                $string=~s/$char//;
                $scrambled=$scrambled.$char;
                $count--;
        }
        return $scrambled;
}
sub loadArray
{
        # here's an opportunity for improvement.  I use the arrays
        #  to store single characters to make random selection
        #  easier.  Clumsy, yet effective.
        local ($string,$char,@array);
        $string=pop(@_);
        undef @array;
        while ($string)
        {
                $char = chop $string;
                push (@array,$char);
        }
        return @array;
}
#
# A (braindead) undertaker.  These two are from my
# template that I use for all my perl scripts.
sub die
{
  local ($gripe);
  $gripe = pop(@_);
  &warn("fatal:$gripe");
  exit 1;
}
#
# A (braindead) friend for our undertaker.
sub warn
{
  local ($gripe);
  $gripe = pop(@_);
  print STDERR "$BASENAME:$gripe\n";
}
Code polymorphism.pl
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