PERVERT WORLD WIDE
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  Almost   all   trojans   and  viruses  are detected using simple signatures.
  Which   means      that  simple  crc is calculated on the entire file, or on
  some parts of the code being checked.

  There  are  thousands  of  simple signatures already stored in the antiviral
  databases.  Each  signature  is  equivalent  to  hours  of  an aver's work.

  Using  simple  length  disassembler  and  some  simple rules, it is possible
  to   analyze   an  arbitrary   executable  file and change some instructions
  in  it,  so  that it will run the same as before, but file's checksum   will
  be changed.

  This  means  that   antivirus will no longer be able to identify these files
  by using the previous checksums.

  A  tool    called   "Code   Pervertor"  was  written  some years ago. It can
  analyze  a PE  file  and  swap a few equivalent  instructions, such as "test
  eax, eax" with "or eax, eax" and vice versa.

  Another   similar   process  is  "diversification",  which  means the random
  changing  of   some   data  offsets  within  all  system  DLLs and services.
  Diversification  complicates  exploitation  based on fixed address usage and
  will probably soon be implemented as a security measure.

  Now   imagine  that  some  worm "perverted" and "diversified" all executable
  files  it  found  on  a  machines  over the net.  It is likely that the same
  vulnerable  machines  will  also contain trojans.  So when all these trojans
  become unique, what avers will do?

  There are two methods of detecting such a modified files.

  First   method  is to  modify  files  before  analyzing,  the  same as "code
  pervertors"  do,  but  without  the  randomization.   For  example,  if some
  instructions  can be  interchanged  with each other, perform one-way changes
  only, for  example  replace  all  "or eax, eax" with "test eax, eax", but not
  vice versa.

  This   method   has  tons  of  negative aspects: there can be many different
  methods     of     file     modification,   but   some   of   them  can   be
  irreversible.

  The   second   method  consists  of  re-writing  all   checksum   algorithms
  and  recalculating   all  the signatures.  The new checksum algorithm should
  become   invariant   to   simple   modifications   such  as  swapping  equal
  or interchangable instructions with each other.

  This method is something like image recognition, where the new algorithm can
  return an equal result for many different data inputs.

  This  method  also  has a serious disadvantage.  If someone introduced a new
  file  modification method, the checksum algorithm will have to be once again
  changed and all the antiviral signatures recalculated.

  A  few  hundred infected machines with automatic "pervertors" will catch all
  the  new  just-released  worms  and  viruses  and  modify  'em "on the fly",
  automatically spawning new variants.


                                                                    2004.03.01