A brand new way to fool TBScan
by
Automag/VLAD

       Today I worked on some features for Antipode:
       I wanted it to infect a file during a scan by AV software so I added
       the usual int 21h 3Dh (open) infection.  It already infected the files
       under McAfee's SCAN so I added the 21h 6Ch (extended open) infection and
       F-PROT became a vector but I was surprised that TBSCAN didn't infect my
       test files (5 byte .COM just 3 NOPs and an int 20h).  I took SoftICE and
       traced some code and was really surprised as TBSCAN didn't open any file
       in my directory!

       I thought that Frans had found a brand new way to open files ?  Taking
       int 21h as a breakpoint I found that TBSCAN just used the Find-Next
       function.  I was dispirited, how would I use TBSCAN as a vector ?
       Scanning another directory I was suprised to find TBSCAN used Int 21h 3Dh!

       Rebooting, I tried to scan my directory again and now TBSCAN only opened
       two files, both were infected (627 bytes long) while the others were
       skipped (5 bytes long).

       So here is the trick:
       TBSCAN does not 'waste' any time with tiny files, it just skips them.
       Let's imagine an algorithm...

          Int 21 4Bh entry point:
               if (file_to_be_executed='TBSCAN.EXE')
               then TBSCAN_FLAG=1
          end

          int 21 4E entry point:
               if (TBSCAN_FLAG=1)
               then
               {
               if DTA.FILEEXT=COM
               then DTA.SIZE=0
               }
          end

          int 21 4C entry point:
               TBSCAN_FLAG=0
          end

       with such an int 21h, TBSCAN won't scan any COM file :)

       That's done and tested and TBSCAN doesn't scan any file :)
       The scanning is just a bit too fast as it scanned 726 executables in six
       seconds :)  Now Frans can say that TBSCAN is the fastest scanner ever !

       But anyway TBAV is the best AV program I have ever used...
       So greets to Frans Veldman...